The Embryology of a Scorpion (Euscorpius italicus) (2024)

Malcolm Laurie

Malcolm Laurie

The Scorpion is interesting not only as being the lowest, and, as far as we know, the oldest type of air-breathing Arachnid, but also as being exceptional among Arthropods in that the whole development takes place within the body of the female—in the ovarian tubes. The only other instances of this with which I am acquainted are Phrynus, which is also viviparous, and Sphœrogyna ventricosa, one of the Acarina in which the young are born sexually mature.

I may fitly here express my thanks to Professor Ray Lankester not only for the suggestion that I should work at this interesting subject, and for the generous way in which he has provided me with material, but even more for his continual and invaluable assistance and advice while the work has been in progress.

Johannes Müller¹ gave a short description, with five or six figures, of the development of Buthus. Owing to its brevity and the absence of any attempt to ascertain the internal arrangement, his paper is of little value except from an historical point of view.

Duvernoy² gives also only a few figures of Buthus and of another form, probably Euscorpius. He describes at some length a cord (baguette) which he says passes from the appendix of the follicle in Buthus to the mouth of the embryo, and which Müller had compared to an umbilical cord. I hope to be able in a future paper to give a detailed account of this and other curious points in the development of Buthus. The chief value of Duvernoy’s paper was that he reconciled the contradictory descriptions of the ovary which had been given by Müller and Rathke.³ While doing this he makes a rather serious mistake in describing the ovum of Buthus as occupying the whole of the diverticulum of the ovarian tube, instead of only a small space at the top.

The next writer on this subject is Léon Dufour,⁴ who gives an elaborate description with numerous figures of the anatomy of the adult. His description of the embryo is, however, very brief and his figures unsatisfactory.

Elias Metschnikoff¹ is the only writer who has treated of the development of the Scorpion with any degree of fulness. He gives a detailed account of the whole development, and his paper, which deals chiefly with the surface views and optical sections, contains a large amount of accurate and laborious observation. It is the classic on this subject, and up to 1886 no attempt was made to add to it or supersede it.

In 1886 Kowalevsky and Schulgin² published a short account of the development of Androctonus ornatus. Unfortunately their paper has no figures, which detracts much from its value. I find reason to differ from them on a few points, but it is quite possible that this may be due to our having worked on different genera.

The only other paper on this subject which I am acquainted with is by G. H. Parker,³ who treats at some length of the development of the central and lateral eyes. I had worked at this point before the appearance of his paper, and on the whole agree with his conclusions. These are briefly that the lateral eyes are monostichous, being formed from the hypodermis without invagin*tion. The median eyes, on the other hand, are formed by invagin*tion, and are therefore three-layered, all the layers being derived from the hypodermis. The retina is the second layer, the third being reduced to a post-retinal membrane. The material at Mr. Parker’s disposal did not enable him to go back to the commencement of the formation of the central eyes and their connection with the cerebral invagin*tions.

The ovary consists, as is well known, of three longitudinal tubes connected by transverse anastomoses, so as to form eight quadrilateral meshes. The oviducts arise from the lateral angles of the two foremost meshes and run forward to open together on the genital operculum. The ovary appears to be embedded in the liver, the chief mass of which lies dorsal to it; this is not really the case, for, though lobes of the liver pass through the meshes of the ovarian network they do not unite on its ventral side. Both the longitudinal and transverse tubes bear ova, which project from their outer surface as oval bodies each attached by a short pedicle and measuring when ripe about 1·2 mm. in length and ‘83 mm. in breadth. Ova in all stages of development are present on the ovarian tubes at the same time, and there are in addition the corpora lutea (v. p. 111).

The microscopic structure of the ovarian tubes is shown in Pl. XIII, fig. 2. They are there seen to be formed of two layers surrounding an irregular lumen. The outer layer, o. l., which is purely skeletal in function, consists of irregularly polygonal cells, with circular nuclei and strongly marked cell outlines. The contents of these cells are highly refractive. Towards the inside of this layer the cells become flattened so as to form a distinct, cellular, limiting layer. The inner layer, which surrounds the lumen of the tube, is formed of very long and thin columnar cells, with oval nuclei and clear, faintly granular protoplasm. The nuclei are for the most part confined to a central zone, leaving a large part of the outer and a smaller part of the inner ends of the cells clear. It is from this inner layer of cells that the ova and their follicles are formed; it is, in fact, the germinal epithelium.

The first sign of the formation of an ovum is that one of the cells of the inner layer of the ovarian tube begins to increase in size (fig. 1, ov.). It contains finely granular protoplasm, a large and distinct oval nucleus, and a darkly staining nucleolus. There is at first no sign of its presence on the outside of the ovarian tube. As it increases in size, however, it pushes its way, at the head of a column of cells, towards the outside. The outer layer of the ovarian tube becomes very thin, but remains as a membrane containing few, if any, nuclei (fig. 2, fol′.). By the time the ovum is about ·04 mm. in length (fig. 2) it has passed completely through the outer layer and is visible as a small protuberance on the surface of the ovary. It remains connected to the inner layer of the tube by a column of cells which is somewhat expanded over the base of the ovum. The nucleus has not increased in size in proportion to the growth of the cell.

The nuclei of the cells of the column which connects the ovum to the inner layer of the ovarian tube next arrange themselves so as to leave a clear space down the centre of the column (Pl. XIII, fig. 3, mi.). They also grow round the ovum so as to form a follicle (fig. 3, fol.) one cell thick. The cells of this follicle rapidly become flattened and their nuclei become smaller. The cells which remain clustered at the base of the ovum (fig. 3, ger′.) on the other hand increase in size, and shortly after the stage represented in fig. 3, which is a drawing of an ovum of about ·1 mm. in diameter, they begin to secrete the yolk of which the greater part of the ripe egg is composed. The outer layer of the ovarian tube can still be traced as a thin and apparently structureless membrane (fig. 3, fol′.) surrounding the egg outside the cellular follicle. The nucleus has increased in size and is now a distinct oval body with a round, granular nucleolus.

In fig. 4 is shown a longitudinal section of an egg of about ·4 mm. in length and ·28 mm. in breadth. A considerable quantity of yolk is now present in the form of spheres ranging in size from mere granules up to as much as ·025 mm. These spheres are clear, hom*ogeneous, sharply defined bodies showing no internal structure except that there is, in the larger ones, a collection of granules at one point near the outside. Round the nucleus the yolk-spheres are small, and round the margin of the egg the protoplasm is coarsely granular, the rest of the space being filled up with the larger spheres.

The nucleus (fig. 4, n.), which retains its central position, is large (·05 mm.) but indistinct in outline and is probably breaking down as I have been unable to find any trace of it in eggs larger than that here figured. The nucleolus (fig. 4, n′.), which is situated towards one side of the nucleus, is also large, staining darkly with carmine and showing a very distinct circular outline. It often contains one large, clear, circular vesicle and a number of smaller ones.

The whole egg is surrounded by a distinct, rather thick vitelline membrane (fig. 4, v. m.). No trace of pores or any other structure was made out. Outside the vitelline membrane the egg is surrounded, except at the base, by the follicle in which the two layers (fol. and fol′.) of the ovarian tube can still be traced. The cells of the inner layer of the follicle are now flattened and small. The large yolk-forming cells at the base of the egg (ger′.) have increased in size and arranged themselves in a circle the centre of which is occupied by a prolongation of the ovarian tube (mi.). The egg is only separated from this prolongation of the lumen by the vitelline membrane. The spermatozoa are thus enabled to reach and fertilize the egg while it is still in its follicle.

Pl. XIII, fig. 5, shows the base of a ripe egg attached to the ovarian tube. The pedicle has become shortened and its lumen has increased very much in size. The yolk-forming cells have degenerated, their flattened nuclei (ger′.) being, however, still distinguishable, and the follicle has become much thinner owing to the growth of the egg. The egg itself is a mass of tightly compressed yolk-spheres, among which I have in vain sought for the nucleus. It is probable, however, that the nucleus and the greater part of the protoplasm migrate to the base of the egg as segmentation commences there.

The yolk (Pl. XIII, fig. 6) consists of spheres, ranging up to ·2 mm. in diameter. They are not hom*ogeneous, but contain spherical or prismatic bodies, which stain darkly with borax carmine. These bodies are very large in the smaller yolk-spheres, which contain one, two, or more of them. In the larger spheres they are much more numerous and much smaller. Many of the spheres show round holes as if the darkly staining bodies had dropped out. It may be, however, that these cavities contained a fatty or oily substance, which has been dissolved out in the course of embedding and mounting.

The only structures remaining to be described in connection with the ovary are the corpora lutea mentioned above (fig. 7). These are irregularly shaped bodies of about ·12 mm. in diameter, showing a slight tendency to radiate structure, and containing a considerable number of nuclei, which are scattered about without any definite arrangement. They project from the surface of the ovarian tubes, and are evidently the collapsed remains of the follicles after the egg has passed out. I was confirmed in my idea that these were corpora lutea by their resemblance to the structures described by v. Siebold¹ in the ovary of Apus. They differ from these latter, however, in not containing fluid.

Formation of Blastoderm

The egg is fertilized in the follicle, from which it does not begin to pass out until the end of this period. It then passes into the ovarian tube in which it undergoes the rest of its development, the young when born being exactly like the parent in form. Kowalevsky and Schulgin² state that the egg in Androctonus is not fertilized until it has entirely left the follicle, and passed into the ovarian, tube, or, as he calls it, uterus. I can hardly believe this to be the case, but it is quite possible that it leaves the follicle at an earlier stage in Androctonus than in Euscorpius.

I have not, unfortunately, been able to observe the processes of fertilization and the formation of the first segmentation-spheres. I should think it probable that the greater part of the protoplasm with the nucleus collects at the base of the egg. The youngest stage in my possession is shown in surface view in Pl. XIV, fig. 8, and in section in fig. 9. The blastoderm forms a circular patch about ·2 mm. in diameter, lying on the surface of the yolk at the end of the egg nearest to the micropyle, and consists of about twenty large cells, those in the centre measuring about ·03 mm. in diameter. In section (Pl. XIV, fig. 9) it is seen to be a single layer, the cells of which are about ·023 mm. thick in the centre. Round the margin the cells are wedge-shaped so that the blastoderm lies flush with the surface of the yolk. The cell-contents are coarsely granular, rather more so towards the lower side. The nuclei are large, round and granular with distinct outlines.

The yolk-spheres under the blastoderm appear to be breaking down. The blastoderm and yolk are closely surrounded by the structureless vitelline membrane (v. m.). This stage seems to be a little younger than that figured in Metschnikoff’s paper in Pl. XIV, fig. 6.

In the next stage (Pl. XIV, fig. 10) the blastoderm is somewhat larger, measuring ·23 mm. in diameter. The blastoderm is now almo.st twice as thick (·045 mm.). Some of the cells are columnar, and occupy the whole depth of the blastoderm, but the majority have divided in a plane parallel to the surface, so that it is in places two or even three cells deep. The nuclei vary in shape, those in the columnar cells being oval.

In the next stage (Pl. XIV, fig. 11) the blastoderm, now ·3 mm. in diameter, is formed of an irregular mass of cells showing as yet no trace of arrangement into layers. The cells are comparatively small with well-marked outlines and large nuclei. Round the margin of the blastoderm the cells form a single layer on the surface of the yolk, but in the centre the blastoderm is five or six cells thick, and the cells push their way in between the yolk-spheres to which some of the cells attach themselves. These cells, which attach themselves to yolk-spheres, lose their definite outline and take, as far as I have been able to ascertain, no part in the further growth of the embryo. There is no doubt that these yolk-cells are derived from the blastoderm in this and the next stages, and do not arise in the yolk by any process of free cell-formation. Kowalevsky is also of this opinion. The yolk in the Scorpion’s egg shows no sign of segmenting as does that of the Spider. The yolk of the Spider’s egg seems¹ to represent the hypoblast, and takes an active part in the building up of the embryo; that of the Scorpion, on the other hand, remains throughout development an inert mass of food-material. This fundamental difference in the segmentation makes any comparison of the early stages of these two groups impossible, and would seem to point to an independent origin for their abundance of food-material. If the segmentation in Scorpions is a modification of the centrolecithal type, as would seem probable from the modes of segmentation in other groups of the Arachnida, it is a very extreme one, and almost all trace of its origin has been lost.

It is difficult to get good sections at this stage as the blastoderm is often humped up at the end of the egg and compressed by the ovarian tube into which it is beginning to pass. In one, and only one, series of sections I have seen what appeared to be a longitudinal groove in the blastoderm. This primitive groove is figured by Metschnikoff (Pl. XVII, figs. 2 and 3), but he may have been misled by the edges of the serous membrane which is growing up and might easily give the appearance of a groove in surface view. If the primitive groove exists, which I am inclined to doubt, as the appearance in my sections may have been due to shrinking, it is a very temporary structure. Towards the posterior end of the blastoderm the cells are proliferating and forming what I shall call the primitive thickening. From this primitive thickening is formed the mass of hypoblast which is found later on in the tail-segment.

It would seem to represent a modified invagin*tion, and is comparable to the primitive streak in the chick. I was at first inclined to call this the primitive cumulus, but considering the fundamental differences between Scorpions and Spiders, and also the fact that, while Balfour¹ places what he calls the primitive cumulus at the posterior end of the embyro, Locy² gives the same name to a thickening at the anterior end, it seemed better to avoid a term which might suggest erroneous hom*ologies.

A layer of cells (fig. 12, pr. hy.) is seen to be forming under the rest of the blastoderm, though not yet extending to its edges. This is well marked in the next stage, and forms the greater part of the primitive hypoblast or hypomesoblast. It would seem to be simply split off from the epiblast. I have seen no appearance of a “down-sinking “of cells to form the hypoblast, such as is described by Kowalevsky and Schulgin;³ but, without the help of figures, it is not easy to be certain of their exact meaning. Whether this “down-sinking “is supposed to take place over the whole blastoderm or only at the primitive thickening is not clear from their description.

Round the edges of the blastoderm a single layer of large cells (fig. 12, s. m′.) is seen to be spreading a little way over the surface of the yolk. These peripheral cells, which are at present continuous with the epiblast, form later on the continuation of the serous membrane. This serous membrane, or outer layer of the amnion, is seen growing up as a single layer of cells from the edges of the blastoderm (Pl. XIV, fig. 12, 5. m.). It spreads over the surface of the blastoderm from all sides, and its edges ultimately meet and fuse in the middle line. At this stage the edges have not yet come together, and the cells of the layer are still small and similar in appearance to those of the rest of the blastoderm.

The yolk is broken down to a considerable extent, and the cells in it (fig. 12, y. c.) are numerous. Their nuclei are very large and granular, and of irregular shapes. The cell-outlines have entirely vanished, the cells being swollen up by an enormous quantity of yolk-stuff. According to Kowalevsky and Schulgin these cells are capable of amœboid movements. Cells continue to be added from the under surface of the blastoderm to those already in the yolk up to the end of this stage. Their function—of breaking down the yolk—is carried on at a later period by the hypoblast.

In the next stage the blastoderm (Pl. XIV, fig. 13) has assumed an oval form, the thickened part or ventral plate measuring ·35 mm. in length and ·25 mm. in breadth, though the peripheral cells extend some way beyond this. I have not been able, either in surface view or section, to find any trace of the primitive groove, and imagine that, if ever present, it has filled up. The primitive thickening (fig. 14, pr. t.) is better developed than in the last stage, and the single layer of primitive hypoblast (figs. 14 and 15, pr. hy.) is now quite definite and extends a little way beyond the thick part of the blastoderm, and forms a layer (hy′.) of cells under the peripheral cells. These last (s. m′.) extend a good deal further than in the last stage. The serous membrane (s. m.) is now completed over the surface of the ventral plate.

In the next stage the embryo, of which fig. 16 shows a longitudinal section, consists of two somites—those which will afterwards bear the cheliceræ and chelæ—in addition to the head- and tail-segments. The head- and tail-segments are large, and a third somite is beginning to be formed from the tail. The first somite is smaller than the second, and not as yet very distinctly marked off from the head. It does not become fully separated from the head until a much later stage (eight somites). Except for this curious delay in the formation of the first, all the somites are formed and separated in regular succession from the tail-segment.

The epiblast has undergone little change since the last stage, except that it is somewhat thinner between the somites than in them. It is beginning to grow up at the edges over the surface of the ventral plate as a single layer of flat cells to form the inner embryonic membrane—the amnion proper (fig. 16, am.). This amnion never loses its connection with the epiblast as the serous membrane has now done, but remains attached to its edges and only extends round the egg as the epiblast extends.

The most important change in this stage is the formation of the mesoblast (mes.). This layer is formed under the whole ventral plate by a multiplication of the cells of the primitive hypoblast, from which it is in places not yet distinguishable. The mesoblast extends across the whole ventral plate from side to side, and is much thicker in the somites than between them.

The serous membrane (s. m.) has, as mentioned above, now lost all connection with the blastoderm, and is continued round about two thirds of the egg by the “peripheral cells,” which are now beginning to separate from the egg and form a definite membrane. The cells of the serous membrane are becoming large and flat.

The hypoblast extends a little way beyond the ventral plate, forming a single layer of cells (Ay.) in the periphery of the yolk immediately under the serous membrane.

By the time the embryo has reached a stage with three somites completely formed (Pl. XIV, fig. 17) most of the changes which were going on in the last stage are completed. The amnion has entirely closed over the embryo (fig. 18, am.), though its cells have not yet attained their characteristic form. The mesoblast (mes.) is entirely separated from the hypoblast, and remains henceforth a distinct and independent layer. The hypoblast (hy.) is now a single layer, extending under the whole ventral plate, except in the tail-segment, where it consists of a spherical mass. This hypoblastic mass in the tail-segment is the direct product of the primitive thickening. The hypoblast extends somewhat further round the egg than the other layers, as is diagrammatically shown in fig. 19.

The description given above of the mode of formation of the serous membrane and amnion differs very considerably from that of Kowalevsky and Schulgin. They describe it as a fold, the outer layer of which forms the serous membrane while the inner forms the amnion. This is probably the more primitive mode of origin for these structures, and the mode described above for E. italicus is probably derived from it either by a hastening of the formation of the serous membrane or a retardation of that of the amnion. I am unable to confirm their statement that mesoderm cells are present between the two layers.

This period covers the rest of the time before the appendages begin to form. The egg has by this time entirely passed into the ovarian tube. It has also increased considerably in size, but I am unable to say whether this is due in any degree to absorption of fluid or whether it is entirely due to internal changes.

In the first stage belonging to this period which I have examined (Pl. XV, fig. 20) the embryo consists of nine somites. The first of these—that which will bear the cheliceræ, is much smaller than the others, and is seen in section to be not yet fully separated from the head. The second somite, which will bear the chelæ, is larger than those following it. The next four are the ambulatory, and the seventh will bear the genital operculum. A slight groove (n. g.) runs down the middle line of the body; this is chiefly due to the mesoblast having divided into two longitudinal bands (figs. 21 and 22, mes.).

The epiblast is moderately thick in the somites, and is beginning to grow as a single layer round the rest of the egg (fig. 21, ep′.), carrying the amnion with it. By this stage it has extended almost as far as has the hypoblast. The cells in the middle line show a more definite arrangement than the rest of the epiblast. This is preparatory to the formation of the neural groove. The cells of the amnion (am.) have developed their characteristic nuclei—spindle-shaped in section—’and form a well-marked thin membrane lying close over the embryo.

The mesoblast (figs. 21 and 22, mes.) shows most impor-tant changes. As mentioned above, it has now separated into two longitudinal bands. This separation does not extend into the tail-segment (fig. 23, mes.), where the mesoblast remains as a solid mass of cells somewhat thinner in the middle line. The cœlomic spaces are now formed by a splitting of the mesoblast in the somites. They are best seen in the posterior somites (fig. 21, cœ.), where the mesoblast is thin and forms only a single layer on each side of the cœlomic space. Further forward (fig. 22) the mesoblast is thicker and the cœlomic space is not so well marked.

The hypoblast has undergone very little change. It is still visible in the tail as a solid mass (fig. S3, hy. m.), and spreads under the ventral plate and a little way beyond its margin as a single layer (figs. 21–23, hy.). The cells of this single layer have large oval nuclei which stain less darkly than those of the epi- and meso-blast. These nuclei are somewhat widely separated from each other, and the cells seem to contain a considerable amount of food-stuff.

The serous membrane (figs. 21–23, s. m.) is by this time quite separate from the egg all round. It has attained its final structure, the nuclei being enormously large (·05 mm.), flat, and at a considerable distance from each other. As far as my observations go I can confirm Blochmann’s statement¹ that the nuclei of the serous membrane divide directly without forming any karyokinetic figures. As the serous membrane plays a purely passive part in the future development it will not be necessary to refer to it again.

In the next stage (Pl. XV, fig. 24), which is the last before the formation of the appendages, the embryo consists of nine somites. The first is very much smaller than the others, while on the second, which is the largest, a trace of the appendages is just visible. The first six somites are clearly distinguished from those further back, owing to their sloping backwards and outwards, while the posterior ones are at right angles to the axis of the embryo.

A distinct groove, the neural groove (n. g.), runs down the middle line and extends some distance into the head-segment. It is due to a thinning of the epiblast in the middle line (figs. 25 and 26, n. g.). The ventral nervous system is formed by a thickening of the epiblast along each side of this groove.

The epiblast now spreads as a single layer beyond the hypoblast (ep′.) and extends over nearly half the yolk, carrying the amnion with it. This is diagrammatically shown in fig. 27. In the head-segment (fig. 25) the epiblast is irregularly grooved and thickened. This is the commencement of the formation of the cerebral ganglion. In the thoracic somites (fig. 26) the epiblast is very thick and solid at the corners (ap.) where the appendages are about to appear. It is also somewhat solid just at each side of the neural groove (n. th.). This is the commencement of the thickening which will form the ventral nervous system.

The mesoblast is a thin layer in the head-segment (fig. 24, mes.), but shows the cœlomic space (cœ.) distinctly. This development of a head cœlom does not, of course, as Balfour has pointed out, necessarily indicate that the head-segment is equivalent to a body somite. In the body somites (fig. 26) the mesoblast is pretty thick and the cœlomic space is almost entirely closed up. The mesoblast does not extend across the middle line or beyond the limits of the ventral plate.

The hypoblast (figs. 25, 26, hy.) shows no change from the last stage but remains as a single layer, except in the tailsegment, where the hypoblastic mass is distinctly visible.

As the next stage shows the commencement of a large number of new structures, the ventral nervous system, the appendages, &c., it seems advisable to give a short summary of what has taken place so far.

(1) The blastoderm commences as a single saucer-shaped layer of cells at one end of the egg (Stage A).

(2) These multiply and form a thick mass (Stages B, C).

(3) The serous membrane grows up from the edges of the blastoderm over its surface as a single layer of cells, and is continued round the yolk by the peripheral cells (Stages D—F).

(4) The hypo-mesoblast is formed partly as a single layer of cells split off from the under surface of the blastoderm and partly, at the tail end, as a thick mass, the primitive thickening, which probably represents an invagin*tion. Before and up to this stage cells pass from the blastoderm into the yolk (Stage D).

(5) The mesoblast is formed as a layer several cells thick, extending right across the blastoderm. The hypoblast remains, after the formation of the mesoblast, as a single layer, except in the region of the primitive thickening, where it is a spherical mass (Stage E).

(6) The amnion is formed as a single layer of cells growing up from the edges of the epiblast, with which it retains its connection. The serous membrane has by this time lost all connection with the blastoderm, and spreads round the greater part of the yolk (Stage F).

The embryo by this time consists of three somites and the large head- and tail-segments. The somites are formed from the tail in regular succession.

(7) The mesoblast divides into two longitudinal bands, and cœlomic spaces are formed in the somites and in the head (Stage G).

(8) The epiblast and amnion begin to spread round the egg beyond the limits of the ventral plate (Stage G).

(9) The neural groove is formed by a thinning of the epiblast in the middle line (Stage H).

(10) The epiblast in the head-segment begins to thicken to form the cerebral nervous system (Stage H).

The first stage of this third period shows—as mentioned above—the commencement of some of the most important structures. The embryo, of which a surface view is given in Pl. XV, fig. 28, now consists of twelve somites in addition to the head- and tail-segments. These somites are no longer separate thickenings as in the last stage, but have grown close up to one another, and are marked off by narrow grooves. The epiblast extends as a single layer all round the egg. The longitudinal neural groove is well marked and extends the whole length of the body with the exception of the tail-segment.

The first six somites bear appendages, i.e. the cheliceræ, chelæ, and four pairs of walking legs. These appendages are simple outgrowths, and are, with the exception of the first two pairs, of approximately equal size. The cheliceræ are much smaller, and the chelæ somewhat larger than the other appendages. The appendages are an outpushing of the epiblast and the outer layer of mesoblast or somatopleure (Pl. XVI, fig. 31). They are hollow, the spaces being prolongations of the cœlomic pouches. There is at this stage no sign of appendages on the somites behind those bearing the walking legs.

The embryo has a strong dorsal flexure so that the cephalic segment curves round the end of the egg. This is best seen in longitudinal section (Pl. XVI, fig. 29). The anterior margin of the cephalic segment is deeply cleft in the middle line, the segment being thus divided into two lobes. The lobes are in much the same state as in the last stage, and show no signs of the cerebral invagin*tion from which a greater part of the brain is formed. In the middle line, and a very short way behind the bottom of the cleft, is a circular raised area with a pit in its centre (Pl. XV, fig. 28, st.). This pit is the stomodæum. It is seen in section in Pl. XVI, fig. 29, and is a simple inpushing of the epiblast.

The ventral nervous system consists of a pair of thickened bands of epiblast running the whole length of the body on each side of the neural groove (Pl. XV, fig. 28). The bands are cut up into blocks by the grooves which separate the somites. The epiblast is not evenly thickened, but the nuclei are arranged so as to present a wavy outline. This is characteristic of the formation of nerve-tissue in this animal, and was well seen in the cerebral lobes in the last stage (Pl. XV, fig. 25). The small ganglia of the cheliceral somite are well seen at this stage (fig. 28, g. I).

The tail-segment, from which the six caudal somites have yet to be formed, has begun to be pushed out (Pl. XVI, fig. 29). The epiblast in this region is very thick, and the cavity of the outpushing is lined by a thick layer of hypoblast, which is the “hypoblastic mass” of earlier stages (fig. 29, hy. m.).

Besides this mass in the tail-segment the hypoblast extends as a single layer round the whole egg (Pl. XVI, fig. 29, hy.). Along the ventral side the cells of this layer are close together, hut towards the sides and back they become more scattered, and are to a great extent involved in the yolk. It is from the mass in the tail-segment that the mesenteron is chiefly formed. The hypoblast along the ventral surface also takes some part in its formation, but that round the sides and back is not involved, though it aids in the formation of the great digestive gland or liver.

The mesoblastic bands (Pl. XVI, fig. 31) are not yet united across the middle line. The cœlomic spaces (Pl. XVI, figs. 30 and 31) are well marked and quite separate for each segment. Those in the first six somites are prolonged into the appendages. The somatopleure is several cells thick; the splanchnopleure, on the contrary, consists of a single layer of cells. The mesoblast in the cephalic segment is thinner than in the body somites, and the cœlomic space is narrower.

The thoracic appendages have increased very much in size, and the cheliceræ and chelæ are both bifurcated at the extremity. A section through the base of one of the ambulatory appendages (Pl. XVI, fig. 33) shows a well-developed process extending inwards towards the middle line. This is undoubtedly the sternocoxal process, which is present on the second, third, and fourth appendages of the adult. Lankester¹ characterises the presence of this process as a very important point of resemblance between the thoracic appendages of Limulus and Scorpio. It is therefore interesting to find it at this early stage present on all four pairs of ambulatory appendages. A series of sections through the base of the fifth appendage, i. e. third ambulatory (Pl. XVI, fig. 34, a—h), shows the first stage of another structure characteristic of Limulus and the Arachnids—the coxal gland. This consists of a simple tube opening to the exterior at the base of the fifth appendage (fig. 34 a), and running forwards through the mesoblast to open in fig. 34 h into the cœlomic space. There can be no doubt that it is a nephridium. Gulland’s researches³ on the coxal gland in the young Limulus point to the same conclusion. I have been unable to find traces of nephridia in any other somites, unless, indeed, the genital tubes are partly nephridial. The six abdominal segments also bear appendages (Pl. XVI, figs. 32 and 35). These appear on surface view much more prominent than they really are owing to their white colour, which is due to the greater thickness of cells. In section (Pl. XVI, fig. 35) they are seen to project very slightly, and to be formed by a thickening of the epiblast and somatopleure, but with no definite outpushing such as there is in the thoracic appendages. The first pair of these appendages—the genital opercula—is very small, and concealed by the last pair of walking legs. The other five pairs—the pectines and four pairs of lung-books—are all of approximately equal size and structure. I have been unable to find the smallest trace of appendages on the somites behind these, i.e. somites 13‱17, and do not believe they exist.

The cephalic segment is not so deeply cleft as in the last stage, and the mouth has shifted posteriorly so that now it lies between the bases of the cheliceræ. In the centre of each cephalic lobe is seen a dark spot (fig. 32, ce. in.). These spots are the cerebral invagin*tions. They begin in a somewhat earlier stage (Pl. XVI, fig. 36) as a pair of small inpushings. These extend rapidly backwards and meet in the middle line, their two lumens becoming continuous. This is seen in Pl. XVI, fig. 37 A-D, in which four transverse sections through this region are figured. Owing to the strong cephalic flexure in this stage the stomodæum (st.) is also shown in section. The cells, both at the sides of the cephalic lobes and throughout the greater part of the invagin*tions, are rapidly increasing in number to form the cerebral ganglia. Those in the centre of the cerebral lobes remain as a thin layer, and take no part in the brain formation. The cells also on the dorsal side in the middle, where the two invagin*tions have united (Pl. XVI, fig. 37 D, oc.), are more closely packed than the others, and take no part in the formation of the brain. They are the beginning of the retinal layer of the central eyes.

The ventral nervous system is in much the same condition histologically as it was in the last stage. The commencement of its separation from the hypodermis can, however, be seen (Pl. XVI, fig. 35) where the hypodermis is growing over it from each side as a thin layer.

The tail segment is now divided into six somites, and extends forward along the ventral surface of the body, reaching, at this stage, to the third abdominal somite. The epiblast is thickened on the ventral surface to form the nervous system. This is not shown in fig. 35, as the section passes between two thickenings. The cavity of the tail is occupied by a tubular extension of the hypoblast (fig. 35, hy.) surrounded by mesoblast. There is as yet no trace of the proctodæum.

The cœlomic spaces in the thoracic somites have not developed much. Those in the abdominal somites, however (Pl. XVI, fig. 35, cœ.), have extended enormously, and now reach round almost one third of the egg. The mesoblast, except in abdominal appendages, consists of two single layers of cells. In the tail the cœlomic spaces are not yet formed.

The embryo, of which fig. 38 (Pl. XVII) shows a surface view, has by this time made considerable progress in several important points. The thoracic appendages are slightly segmented (Pl. XVII, fig. 39, ap.), though this is not apparent in a surface view. The cheliceræ have moved in towards the middle line, and the mouth is now concealed between their bases. The chelæ are very large, and have their pincers well developed. The coxal gland, which opens at the base of the fifth pair of appendages, is no longer a straight tube, but has become bent on itself, so that a section through it (Pl. XVII, fig. 39, cox.) shows the tube cut in three places. It can still, however, be traced through a series of sections as a simple tube opening into the coelom. The abdominal appendages have undergone great changes. The genital opercula are still simple thickenings of the epi- and meso-blast, but the pectines (Pl. XVII, fig. 40) have become folded in a direction parallel to the long axis of the body, i. e. transverse to their own axis. The most important change is, however, that of the four following abdominal appendages. These (Pl. XVII, fig. 41) are pushed in so as to form shallow cup-shaped cavities. The inpushing is on the posterior part of the appendage, and is directed slightly forwards. This is the commencement of the formation of the lung-book.

The cephalic segment, which is shown in Pl. XVII, fig. 38, extended in the same plane as the ventral surface of the embryo, is no longer so distinctly bilobed as in the last stage. The cerebral invagin*tions (Pl. XVII, figs. 42, a and b, and 43) are much shallower, and have entirely joined together, so that there is now only a single inpushing. This lies just in front of the cheliceræ (Pl. XVII, fig. 38). The brain is being formed from the sides of the inpushing, and shows a very characteristic structure’. The mass of cells is more or less grouped round small circular clear spaces (fig. 43), which give to this part of the brain the appearance of being composed of a number of small vesicles. I have not succeeded in tracing the development of the nerve-fibres, which occupy the centre of the cerebral ganglion (fig. 43). This central portion appears at this stage perfectly transparent and empty.

The retina of the central eyes is still a thickening of the dorsal layer of the cerebral invagin*tion (Pl. XVII, fig. 43, rtn.). It is visible in surface view (fig. 38, oc.) as a white spot on the margin of the invagin*tion. The hypodermis immediately outside it is somewhat thickened, and will in this region form the vitreous layer (fig. 43, vit.).

The ventral nervous system is now completely separated from the hypodermis (figs. 40 and 41, n. c.). The cells are beginning to congregate together to form the ganglia, though the nervecord between the ganglia is still largely cellular. Nerves are seen growing out from the ganglia as thick cords of cells (fig. 40). The ganglia contain a clear space in their centre which later is occupied by a mass of fibres.

The tail (Pl. XVII, fig. 38) has now attained its full number of segments but the sting is not yet formed. The gut extends up almost the whole length of the tail. There is no sign yet of the formation of the proctodæum. The hypoblast in the rest of the body remains as a scattered layer of cells.

The mesoblast has now grown round the body as a double layer, with the cœlomic space between. In the middle line of the back, where the right and left folds of mesoblast meet, there is a somewhat irregular thickening in which both somatopleure and splanchnopleure seem to be involved. From this thickened band, which extends from close behind the brain to the beginning of the tail, the heart is formed. On the ventral side in the thoracic region the mesohlast of the outer layer is broken up into long strings of cells—the muscles—so that the cœlomic space can no longer be very definitely made out.

The stomodæum reaches as far as the back of the cerebral ganglion. This is the limit of its growth, and it remains a closed tube until, at a much later stage, the gut has grown forward and united with it.

The embryo (Pl. XVII, figs. 44 and 45) does not show very much change in surface view. The thoracic appendages are longer and distinctly segmented. They overlap across the middle line and conceal the pectines. The cheliceræ are further forward in relation to the mouth, which can now be seen lying between the bases of the chelæ.

The genital opercula begin to grow out from the body wall and the genital duct begins to be formed. This last (Pl. XVII, fig. 46) is developed in the mesoblast as a tubular portion of the cœlom, but does not open to the exterior up to the time of hatching. It may be nephridial in its nature, but this very late formation of the external aperture is not very favorable to such an hypothesis. The pectines are separated at their outer ends from the body wall. The inpushings for the lunghooks are much deeper, and the cavity, which extends forwards from the opening, is divided up by lamellæ which grow down from its upper end (Pl. XVII, fig. 47). It is in close relation to a space in the mesoblast which contains blood-corpuscles.

The cephalic segment (Pl. XVII, fig. 45) is now rapidly approximating to its final shape. The cerebral ganglion, which is seen from the surface as a four-lobed white mass (fig. 45, ce.), has now lost all connection with the epiblast. The invagin*tion remains, but its sides no longer give rise to nerve-tissue (Pl. XVII, figs. 48 and 49). The thickening for the central eye (figs. 48 and 49, rtn.) is more largely developed, and pigment is deposited in the ends of the cells furthest from the invagin*tion. The eye is plainly visible as a double black spot on the surface. The upper edge of the invagin*tion is growing down to close its orifice. The hypodermis lying immediately above it is clearly marked off from the rest as the vitreous layer (fig. 49, vit.). A considerable space still separates the retina from the vitreous layer.

The lateral eyes now appear for the first time as black spots on what Lankester terms the “optic area,” i. e. the front margin of the head (Pl. XVII, fig. 45, oc.). Their development, as Parker¹ has shown, is strikingly different from that of the central eyes. Each eye, and in this species there are at first three, is formed (fig. 50) by a slightly cup-shaped thickening of the hypodermis. The nuclei of this thickened portion become larger, and pigment soon begins to be deposited at the outer ends of the cells. Fig. 50 a shows a somewhat later stage, in which the cupping of the hypodermis has become flattened out. There is no invagin*tion of any sort, and the eyes are, as Lankester and Bourne¹ described them, monostichous. The ventral nervous system has not undergone much development. It has sunk somewhat deeper and is separated from the hypodermis by the mesoblast.

The tail has now developed its terminal segment—the sting. The cavity of this last is partly occupied by the paired poison gland, apparently formed by inpushing of the hypodermis (Pl. XVIII, fig. 51, p. gl.). Each mass is connected to the superficial hypodermis by a short duct.

The gut extends down the whole length of the tail, and the proctodæum is present in the form of a solid mass of hypodermis cells blocking up its end (Pl. XVIII, fig. 51, proct.). The gut has also begun to grow forward (Pl. XVIII, figs. 52 and 53). In the last abdominal segment it is a complete tube surrounded by a thin layer of mesoblast (fig. 52, int.). It gives rise to two tubular outgrowths from its dorsal side, which are the Malpighian tubes (fig. 52, mlph.). These run first towards the back and then bend forward. There can be no doubt as to their hypoblastic origin in this form, as the proctodæum is not yet formed. They have been already shown to be outgrowths of the mesenterou in some Spiders by Loman,² and also in terrestrial Amphipoda by Spencer.³ Further forward (Pl. XVIII, fig. 53, int.) the gut is simply a semi-cylindrical layer of hypoblast supported by a string of mesoblast and open to the yolk on its dorsal side. In the thorax it has not yet begun to form.

The mesoblast is broken up into strings and bands. The cœlom is still pretty distinct in the abdominal region (Pl. XVIII, fig, 53, cœ.), and the heart is a large thin-walled tube apparently connected with both somatopleure and splanchnopleure. As mentioned above, the genital tube is formed in the somatopleure in the seventh somite and is a portion of the cœlom.

The changes from the last stage up to the time of hatching are not very numerous, though very important. The body attains a structure almost exactly like that of the adult, the appendages being segmented and the whole animal covered by a thin, structureless, highly refracting cuticle. The coxal gland still opens by a small aperture to the exterior at the base of the fifth appendage (Pl. XVIII, fig. 54). This aperture, which is lined for a short distance by the cuticle, leads to a straight duct (fig. 54) lined by cubical cells with round nuclei, which closely resemble the cells of the gland. The gland itself is distinguishable into medullary and cortical portions as described by Professor Lankester¹ in the adult. The tubules have distinct lumens surrounded by a cubical epithelium. The gland and its duct are surrounded by a thin capsule of flat mesoblast cells.

The genital tubes have pushed their way some distance between the lobes of the liver, but they are not yet connected by transverse tubes nor do they open to the exterior. The two layers of which the tube is composed in the adult (v. supra, p. 108) are not yet distinguishable. The pectines approximate very closely to their adult structure.

The ninth, tenth, eleventh, and twelfth appendages are also very similar to those in the adult (Pl. XVIII, fig. 55). The number of lamellæ is not so great, but their structure is very well shown. Each lamella is covered by a thin cuticle, and its cavity is in direct communication with a blood-sinus (fig. 55, bl. s.). The cells which form the lamellæ are very large, especially towards the base of the appendage. Towards the apex they become smaller, and finally pass into a mass of different cells from which more lamellæ are formed as the animal grows. The spaces between the lamellæ (fig. 55, a. c′.) are narrower and in communication with the exterior through the stigma.

The head is now completely formed, the mouth having shifted so as to lie behind the cheliceræ. The invagin*tion which forms the central eyes has closed up. A stage immediately after its closure is shown in Pl. XVIII, fig. 56. Here the lips of the invagin*tions have come together, but not fused. The posterior layer of the invagin*tion is visible as a thin layer of cells (fig. 56, rtn′.), separated from the retina by a narrow space. The vitreous layer (vit.) is distinctly marked as a thickening of the hypodermis on the top of the head, the nuclei in that region being elongated, but there is still a small space separating it from the retina. By the time the embryo is hatched the eye (Pl. XVIII, fig. 57) has lost all connection with the hypodermis at the point where it was invagin*ted. The cells are long and deeply pigmented round their margins. The pigment is not equally abundant throughout the whole length of the cell, but five alternately more and less deeply pigmented zones can be distinguished (fig. 58). The base of the cells is most deeply pigmented, and their superficial ends come next. The nuclei of the retinal cells lie in zone 4, but I have been unable to find any trace of either rhabdomes or phaospheres. I have also not been able to trace any migration of mesoderm cells among the retinal cells. The posterior layer of the invagin*tion can with difficulty be made out in depigmented sections owing to the flatness of its nuclei, and it is absolutely undistinguishable in sections from which the pigment has not been removed. This posterior layer forms the post-retinal membrane of the adult eye. The optic nerve is beginning to grow out from the cerebral ganglion, but has not yet come into connection with the eye. The hypodermis, immediately in front of the eye, is formed of a single layer of large transparent cells with faintly staining oval nuclei (Pl. XVIII, figs. 57 and 58, vit.). This vitreous layer is covered by a thin cuticle exactly like that which covers the rest of the surface of the body. The only sign of the formation of the lens is a slight cupping of the vitreous layer at one point (fig. 58). The hollow formed here is, however, not as yet filled up by any cuticular substance, but the cuticle passes straight over it. Round the area, where the lens will form the hypodermis is deeply pigmented. The cells are much smaller than those of the vitreous layer, and their nuclei are irregular in shape.

The cells of the lateral eyes (Pl. XVIII, fig. 59) are about the same size as those of the median ones. The pigment is not, however, arranged in definite zones, though it is more abundant at the base and at the outer ends of the cells than in the middle. There is a small third lateral eye present, and the hypodermis around and—the lateral eyes being so to to say on the edge of the head—below the eyes is all pigmented. I have been unable to find the nerve to these eyes and think it is probably not yet formed. The cuticle over the lateral eyes is not thickened to form the lens, and I have seen no sign of the peculiar mode of lens-formation described by Parker,¹ i. e. the ends of the perineural cells pushing in front of the retina. It is, of course, possible that this does not take place till later.

The brain and ventral cord have almost attained their adult structure. In the nerve-cord there is a string of cells in the middle line (Pl. XVIII, fig. 60) dorsal to the cords proper, which seems to represent the centre of the neural groove.

The tail is exactly similar to that of the adult, and is carried in the same way curved over the back. The poison-glands are fully formed and surrounded by muscles, but do not occupy so much of the terminal segment as in the adult. The proctodæum is lined by flat cells and has pushed its way almost to the base of the tail. The mesenteron is fully formed only in the hind segments of the body (Pl. XVIII, fig. 60). From the end of the stomodæum to where the last hepatic cæca join it the intestine (fig. 61) is surrounded by a definite cylindrical layer of mesoblast which is continuous with that surrounding the lobes of the liver, but the hypoblast cells lining this cylinder (fig. 61, hy.) are not yet definitely arranged. The nuclei are scattered about in groups for the most part near the outside, and the cells are drawn out into irregular more or less pyramidal form, the apex of the pyramid pointing towards the centre of the tube. There is no definite lumen, the space between the cells being filled up by small yolk-spheres (fig. 61, yk.).

The liver-follicles are much the same in structure as the intestine. They contain, however, a rather larger proportion of yolk. The scattered layer of hypoblast cells, which in the preceding stages surrounded the yolk, takes a large part in their formation. They open into the intestine in pairs by wide ducts.

The Malpighian tubes (Pl. XVIII, fig. 60, mlph.) have not undergone much development. They reach well forward in the body, and open into the intestine in the first caudal segment.

It is evident from the structure of the intestine that the young scorpion does not need food for some time after hatching. The large amount of yolk which still exists must last it for some weeks, or most probably till the next spring. If this is the case embryonic life practically lasts twelve months as the eggs are fertilized in May.

The outer layer of the mesoblast has now for the most part formed itself into muscles. The inner layer is very much complicated, being folded in so as to surround the gut and the lobes of the liver. The spaces between the lobes of the liver, which are undoubtedly the true cœlom, are filled up by a network of trabecular tissue (Pl. XVIII, fig. 60). The heart, pericardium, and blood-vessels are fully formed and contain a considerable number of large nucleated corpuscles.

(1) The thoracic appendages begin as simple outpushings of the body wall containing a portion of the cœlom (Stage I).

They rapidly increase in length and the cheliceræ and chelæ become bifid at their extremities. Sternocoxal processes are present on the third to sixth appendages (Stage K).

The cheliceræ, which were at first behind the mouth, gradually move forward relatively to it till they come to lie in front of it (Stage L).

(2) The coxal gland begins as a simple tube opening to the outside at the base of the fifth pair of appendages, and opening at the other end into the cœlom (Stage K). The tube soon becomes coiled, but the external opening persists until after hatching. It is undoubtely a nephridium,

(3) The abdominal appendages appear as thickenings of the epi- and meso-blast on the seventh to twelfth somites (Stage K). The first pair (genital opercula) does not develop further till a late stage (L).

The second pair (pectines) form a number of short longitudinal ridges on the surface of the abdomen (Stage K). They then separate from the body, the separation beginning at their outer ends (Stage M).

The third to sixth pairs (gill-books) begin to be pushed in (Stage L). The inpushing becomes deeper, and begins to be divided up (Stage M), and by the time the embryo is hatched they have atta ined their adult condition in every respect except size and number of lamellæ.

(4) The cerebral ganglion and central eyes begin as a pair of invagin*tions on the cephalic lobes. These invagin*tions meet in the middle (Stage K). The cerebral ganglion is formed from the sides of the invagin*tions, which rapidly become shallower and unite so as to open in the middle line. The dorsal surface of the invagin*tion becomes thickened to form the retina of the central eyes (Stage L).

The brain becomes entirely separate from the hypodermis, the invagin*tion remaining to form the eyes (Stage M). The invagin*tion closes up and its lumen disappears. The cells of its lower layer form the post-retinal membrane. Those of the upper layer form the retina, and come in contact with the hypodermis on the top of the head, which is thickened in this region to form the vitreous layer. The retinal cells become deeply pigmented (Stage N).

(5) The lateral eyes form as cup-shaped thickenings of the hypodermis in the “optic area,” the cells of which become pigmented. There is no invagin*tion, and they consist of a single layer (Stage M).

(6) The ventral nervous system forms as a pair of thickened segmented bands, one on each side of the neural groove (Stage I). The nerve-cords sink down, a thin layer of hypodermis growing over them. There is at this time (Stage K) a distinct postoral pair of ganglia for the cheliceræ. The cells become aggregated to form ganglia, and the cbeliceral ganglia become fused with the cerebrum.

(7) The tail grows out, lying along the ventral surface of the abdomen. The poison-gland in its terminal segment is formed by a pair of invagin*tions of the epiblast.

(8) The hypoblast consists of an irregular layer under the whole embryo and a solid mass at the tail end (Stage I). As the tail grows the hypoblast grows into it as a tube reaching down to the last somite (Stage K).

The hypoblast forms the gut in the abdominal portion of the body, growing forward in a sling of mesoblast at first as a flat layer, which soon becomes bent round into a cylinder. The Malpighian tubes are formed as outgrowths from the mesenteron in the first post-abdominal somite (Stage M).

The gut does not reach forward to the stomodæum till shortly before hatching, and at this period the portion of it into which the liver-follicles open is not fully formed (Stage N).

(9) The stomodæum is formed early. It lies at first in front of the cheliceræ (Stage I), but soon shifts its position and comes to lie behind them. It extends inwards as far as the back of the brain.

(10) The proctodæum is formed much later than the stomodæum. It is at first a solid plug of cells (Stage M). As it increases in size it appears to replace the hypoblast in the last four somites.

(11) The mesoblast consists at first of a pair of segmented bands with a separate cœlomic space in each somite, and also one in the cephalic segment (Stage I). The cœlomic spaces soon unite, and the mesoblast bands join across the ventral surface. Somewhat later they extend round—the cœlomic space extending with them—and unite in the middle line on the dorsal surface (Stage L). From the thickened band where they have united on the dorsal surface the heart is formed. A portion of the cœlom in the seventh segment becomes separated off to form the genital tubes (Stage M). These do not open to the exterior. The outer layer of the mesoblast forms chiefly the muscles of the body. The inner layer becomes folded so as to surround the liver and intestine, and the cœlomic space becomes partly filled up by trabecular tissue.

The development of this Scorpion, of which I have tried to give an outline above, is interesting in many points. It does not agree closely with any other Arachnid type as yet described, and I have for the present given up all attempts at comparison.

The development of the central and lateral eyes entirely bears out Lankester and Bourne’s description of their structure. It is true that the central eyes are three-layered, but as the retina is the second layer from the surface—the third layer forming only a post-retinal membrane—they may be called diplostichous. The account given above of their development agrees in all essential respects with that of Parker, but, having a larger supply of embryos, I have been able to trace the earlier stages and the connection of the eyes with the cerebral invagin*tion. Their mode of origin resembles very closely Locy’s¹ description of the development of the eyes in Agelena nævia, the chief difference being that in Agelena the optic invagin*tions appear to have no connection with the formation of the brain. Locy does not, however, give a detailed description of the formation of the latter.

The description given above of the development of the lateral eyes also agrees pretty closely with that of Parker. In these, as in the central eyes, Lankester and Bourne’s conclusions are confirmed, and Patten’s³ conclusions as to what the structure of the eyes must be in order to fit in with his theories are shown to be without foundation. The lateral eyes are monostichous, being simply somewhat specialised hypodermis cells.

The mode of formation of the ventral nervous system is exceptional among Invertebrates, resembling rather that of Chordata. The nerve-cord instead of peeling off from the superficial layer of epiblast sinks down bodily, and is covered by a layer of epiblast which grows over it from each side.

The development of the coxal gland leaves, I think, no room to doubt that it is a nephridium. That of the genital tubes is less conclusive, but I should think it probable that they are also, at least in part, nephridial.

The gill-books are undoubtedly appendages comparable to the abdominal appendages of Limulus. Whether they are really invagin*ted, i. e. whether the edge of each lamella in the Limulus appendage corresponds to the bottom of the fold between the lamellæ in the Scorpion’s gill-book, or whether the whole appendage has become sunkin a hollow in the abdominal surface without being invagin*ted, it is difficult to say. Undoubtedly, the surface now exposed to the air has always been the external surface, but that would be the case with either of the above modes of derivation. Although the second alternative has the advantage that it is easy to see how the change could take place gradually, I am inclined to think the first is probably the true way in which they have arisen. One argument in its favour is that if the second alternative were correct one would expect the gill-book to commence as a distinct outgrowth, which would become sunk in a pit. Now, there is no such outgrowth in the formation of the gill-book. The first thing to appear on the thickened portion of the epiblast, from which the gill-book is formed, is a pit (Pl. XVII, fig. 41). The lamellæ do not begin to form till a later stage. Again, the abdominal appendages of Limulus are directed towards the tail as one would expect abdominal appendages to be. Now, if the appendage had sunk in without invagin*tion, one would expect it to be still directed towards the tail unless there were some very good reason for its having changed its direction. If, on the contrary, it had become invagin*ted it would naturally be directed in the opposite direction towards the head, and this is what we find in the Scorpion. The inpushing is from the beginning towards the head, and the aperture opens towards the tail (Pl. XVII, fig. 47). I think it is quite conceivable that the changed conditions of development, due to terrestrial life, and the consequent pressure on the embryo, may have produced this change. A detailed account of the development of these appendages in Limulus may throw more light on the matter, but, unfortunately, though many authors have attacked the problem, a complete and satisfactory account of the development of Limulus is not yet in existence.

Illustrating Mr. Malcolm Laurie’s paper on “The Embryology of a Scorpion (Euscorpius italicus).”

Abbreviations

a. c. Air-cavity in gill-book. ac′. Air-spaces between the lamellæ of gillbook. ab. ap. Abdominal appendage, am. Amnion, am. c. Amniotic cavity. ap. Appendage, bl. Blastoderm. bl. s. Blood-space. bl. c. Blood-corpuscle. cau. Caudal segment, ce. Cerebral ganglion, ce. in. Cerebral invagin*tion. ceph. Cephalic segment, cœ. Cœlom. cox. Coxal gland, cox. d. Duct of coxal gland, ep. Epiblast, ep′. Extension of epiblast beyond ventral plate. fol. Follicle, fol′. Outer non-cellular layer of follicle, g. I. Ganglion of cheliceral somite, ger. Germinal epithelium or inner layer of ovarian tube. ger′. Yolk-forming cells derived from germinal epithelium, ge. t. Genital tube. ht. Heart. hy. Hypoblast, hy′. Extension of hypoblast beyond ventral plate, hy. m. Mass of hypoblast in caudal segment, ini. Intestine. l. Gastric gland, mes. Mesoblast, mi. Prolongation of ovarian tube to egg. mlph. Malphigian tubes, n. g. Neural groove, n. n′. Nucleus, nucleolus. n. c. Nerve.cord. n. gl. Nerve-ganglion. n. th. Neural thickening, oc. Central eye. oc′. Lateral eye. o. l. Outer layer of ovarian tube. ov. Ovum. p. gl. Poison-gland, pr. hy. Primitive hypoblast (hypomesoblast). proct. Proctodæum. pr. t. Primitive thickening, rtn. Retina of central eye. rtn′. Third layer of central eye, post-retinal membrane, s. m. Serous membrane. s. m′. “ Peripheral cells.” som. mes. Somatic mesoblast, spl. mes. Splanchnic mesoblast, st. Stomodæum. sie. p. Sternocoxal process, sig. Stigmata, te. Telson, tr. mes. Trabecular mesoblast occupying coelom. vil. Vitreous layer of central eye. y. c. Cells in yolk. yk. Yolk. The somites are numbered I, II, III, &c.

PLATE XIII

FIG. 1.—Transverse section of ovarian tube, showing the two layers; one cell of the inner layer enlarging to form an ovum. ⁠.

FIG. 2.—Transverse section of ovum and ovarian tube. The egg has now pushed its way through the outer layer, and appears as a small protuberance on the ovarian tube. The follicle is beginning to form from the cells of the inner layer, which have accompanied the ovum, ⁠.

FIG. 3.—Longitudinal section of ovum of ·1 mm. diameter, showing the two-layered follicle and the yolk-forming cells (ger′.). ⁠.

FIG. 4.—Longitudinal section of egg of ·4 mm. in length, showing yolkspheres, indefinite nucleus, and strongly marked nucleolus. The egg is surrounded by a vitelline membrane. The rest as in Fig. 3. ⁠.

FIG. 5.—Section through the base of a ripe egg. ⁠.

FIG. 6.—Yolk-spheres from ripe egg, showing the darkly stained spherical and prismatic bodies and the clear spaces. ⁠.

FIG. 7.—Section through a corpus luteum and part of ovarian tube. ⁠.

PLATE XIV

FIG. 8.—Surface view of one-layered blastoderm. ⁠.

FIG. 9.—Section through one-layered blastoderm, same stage as Fig. 8. ⁠.

FIG. 10.—Section through blastoderm later than Fig. 9, showing the cells multiplying to form a mass at one pole of the egg. ⁠.

FIG. 11.—Section through more advanced blastoderm. ⁠.

FIG. 12.—Transverse section through blastoderm at time of formation of primitive hypoblast and serous membrane. The yolk and yolk-cells are drawn in detail in this figure to show the breaking down of the former. ⁠.

FIG. 13.—Surface view of blastoderm now becoming oval. ⁠.

FIG. 14.—Transverse section through posterior end of embryo figured in Fig. 13, showing serous membrane, primitive thickening, primitive hypoblast, and “peripheral cells.” ⁠.

FIG. 15.—Transverse section through anterior part of embryo about the same stage. ⁠.

FIG. 16.—Longitudinal section through an embryo a little younger than Fig. 17, showing two somites with a third forming. The mesoblast is forming from the primitive hypoblast, the amnion is growing up from the edges of the hypoblast, and the primitive thickening is well seen in the caudal segment. ⁠.

FIG. 17.—Surface view of embryo, with three somites fully formed. ⁠.

FIG. 18.—Transverse section through posterior end of Fig. 17, showing hypoblastic mass, mesoblast, &c. ⁠.

FIG. 19.—Diagrammatic representation of the relative extension of the various layers in an embryo of the stage of Fig. 17.

PLATE XV

FIG. 20.—Surface view of an embryo with seven somites, drawn as if flattened out. a—b and c—d are the planes of the sections figured in Figs. 21 and 23. ⁠.

FIG. 21.—Transverse section through one of the posterior somites of an embryo with seven somites (a—b in Fig. 20), showing the three layers, epiblast thinning in centre, and mesoblast thin; amnion, serous membrane, and cœlomic spaces. ⁠.

FIG. 22.—Transverse section through one of the anterior somites of Fig. 20. ⁠.

FIG. 23.—Transverse section through tail-segment (c-d) of Fig. 20, showing the undivided mesoblast and the hypoblastic mass. ⁠.

FIG. 24.—Surface view of embryo of nine somites, drawn as if extended. ⁠.

FIG. 25.—Transverse section through head-segment of Fig. 24, showing epiblast thickening to form cerebral nervous system and spreading (ap′.), with the amnion beyond the ventral plate, neural groove, thin mesoblast, with small cœlomic space and hypoblast. ⁠.

FIG. 26.—Transverse section through one of the anterior somites of Fig. 24, showing the epiblast very solid where the appendage will develop (ap.) and form the neural thickening (n. th.) at each side of the neural groove. Mesoblast thick, and cœlom not very evident, ⁠.

FIG. 27.—Diagrammatic representation of the relative extension of the various layers in an embryo of the stage of Fig. 24.

FIG. 28.—Surface view of embryo at Stage I (ten somites) extended in a plane, showing appendages, cheliceral ganglion, stomodæum, &c. ⁠.

PLATE XVI

FIG. 29.—Longitudinal section of Stage I in the middle line, showing dorsal flexure of the embryo, commencement of tail outgrowth, stomodæum, &c. ⁠.

PIG. 30.—Longitudinal section to one side of the middle line, showing the appendages and cœlomic spaces. ⁠.

FIG. 31.—Transverse section through the third somite of Stage I, showing the formation of the appendage, the neural thickening, &c. ⁠.

FIG. 32.—Surface view of embryo at Stage K, showing the cerebral invagin*tions, abdominal appendages, tail, &c. ⁠.

FIG. 33.—Transverse section through the base of a thoracic appendage, showing the sternocoxal process.

FIG. 34, a—h.—Series of sections through base of fifth appendage, showing the coxal gland.

FIG. 35.—Transverse section through one of the abdominal appendages and the tail, showing the appendage, the neural thickening beginning to separate from the epiblast, the gut forming in the tail, &c. ⁠.

FIG. 36.—Transverse section through the cephalic segment of a somewhat earlier embryo, showing the beginning of the cerebral invagin*tion. ⁠.

FIG. 37, A—D.—Sections through the head of an embryo of Stage K, showing the cerebral-optic invagin*tions. ⁠.

PLATE XVII

FIG 38.—Surface view of an embryo of Stage L extended in a plane, showing the cerebral invagin*tion, the central eyes, &c. ⁠.

FIG. 39.—Transverse section through the base of the fifth appendage, showing the coxal gland. ⁠.

FIG. 40.—Section through the pectines. ⁠.

FIG. 41.—Section through an abdominal appendage, showing the inpushing to form the gill-book. ⁠.

FIG. 42, a, b.—Transverse sections through the cerebro-optic invagin*tions. ⁠.

FIG. 43.—Longitudinal section through the cerebro-optic invagin*tions, showing the formation of the brain and the central eye. ⁠.

FIG. 44.—Surface view of an embryo of Stage M, from the ventral surface. ⁠.

FIG. 45.—Surface view of the dorsal side of the head of the same embryo, showing the central and lateral eyes, ⁠.

FIG. 46.—Section through the seventh somite, showing the formation of the genital tube from part of the cœlom. ⁠.

FIG. 47.—Longitudinal secion through a gill-book, showing the commencement of the formation of the lamellæ. ⁠.

FIG. 48.—Longitudinal section through the head end, showing the stomodæum and the cerebro-optic invagin*tion from which the brain is now entirely separate. ⁠.

FIG. 49.—Transverse section through the same region. ⁠.

FIG. 50.—Longitudinal section through the lateral eye, showing its formation by a thickening of the hypodermis. ⁠.

FIG. 50a.—Section through a somewhat older lateral eye in which the inpushing of the hypodermis has disappeared. ⁠.

PLATE XVIII

FIG. 51.—Longitudinal section through the tail end of Stage M, showing the poison-gland, proctodæum, intestine, &c. ⁠.

FIG. 52.—Transverse section through the posterior end of the body, showing the intestine, with the Malpighian tubes, the heart, &c. ⁠.

FIG. 53.—Transverse section a little further foward than Fig. 52, showing the intestine, which has not yet closed into a tube. ⁠.

FIG. 54.—Section through the coxal gland of a newly-hatched scorpion, showing the opening to the exterior, &c. ⁠.

FIG. 55.—Longitudinal section through a gill-book of a newly-hatched scorpion. ⁠.

FIG. 56.—Longitudinal section through the central eye of an embryo a short time before hatching, showing the closure of the cerebro-optic invagin*tion and the three layers of the eye. ⁠.

FIG. 57.—Longitudinal somewhat oblique section through the central eyes of a newly-hatched scorpion. ⁠.

FIG. 58.—A few cells of the same eye more highly magnified, and showing the inpushing in the vitreous layer.

FIGS. 58, a, b, c.—Transverse sections through the same eye at different levels.

FIG. 59.—Section through the lateral eyes of a newly-hatched scorpion. ⁠.

FIG. 60.—Transverse section through the posterior part of the body of a newly-hatched scorpion, showing the fully-formed intestine, the Malpighian tubes, the nerve-cord, and the trabecular tissue filling up the cœlomic space. ⁠.

FIG. 61.—Transverse section through the intestine further forward, where it is not yet properly formed, showing the irregular hypoblast cells and yolkspheres. ⁠.

Copyright © 1890 by the Company of Biologists Ltd.

1890