Fig. 2.

Balbiani (No. 1) has gone with some detail into the history of the early stages; and Ludwig (No. 5) has published some very important observations on the development of the blastoderm. Finally, Barrois (No. 2) has quite recently taken up the study of the group, and has added some valuable observations on the development of the germinal layers.

In addition to these papers on the true spiders, important investigations have been published by Metschnikoff on other groups of the Arachnida, notably the scorpion. Metschnikoff's observations on the formation of the germinal layers and organs accord in most points with my own.

The development of the Araneina may be divided into four periods: (1) the segmentation; (2) the period from the close of the segmentation up to the period when the segments commence to be formed; (3) the period from the commencing formation of the segments to the development of the full number of limbs; (4) the subsequent stages up to the attainment of the adult form.

In my earliest stage the segmentation was already completed, and the embryo was formed of a single layer of large flattened cells enveloping a central mass of polygonal yolk-segments.

The nuclei of the yolk-cells are probably derived by division from the nuclei of the segmentation rosettes (vide Ludwig, No. 5), and it is probable that they take their origin at the time when the superficial layer of protoplasm separates from the yolk-columns below to form the blastoderm.

The protoplasm of the yolk-cells undergoes rapid division, as is shewn by the fact that there are often two nucleated bodies close together, and sometimes two nuclei in a single mass of protoplasm (fig. 10). It is probable that in some cases the yolk-spheres divide at the same time as the protoplasm belonging to them; the division of the nucleated bodies is, however, in the main destined to give rise to fresh cells which enter the blastoderm.

I have not elucidated to my complete satisfaction the next stage or two in the development of the embryo; and have not succeeded in completely reconciling the results of my own observations with those of Claparède and Balbiani. In order to shew exactly where my difficulties lie it is necessary briefly to state the results arrived at by the above authors.

According to Claparède the first differentiation in Pholcus consists in the accumulation of the cells over a small area to form a protuberance, which he calls the primitive cumulus. Owing to its smaller specific gravity the part of the ovum with the cumulus always turns upwards, like the blastodermic pole of a fowl's egg.

Balbiani (No. 1) describes the primitive cumulus in Tegenaria domestica, Epeira diadema, and Agelena labyrinthica, as originating as a protuberance at the centre of the ventral surface, surrounded by a specialised portion of the blastoderm (p. 57), which I will call the ventral plate. In Tegenaria domestica he finds that it encloses the so-called yolk-nucleus, p. 62. By an unequal growth of the ventral plate the primitive cumulus comes to be placed at the cephalic pole of the ventral plate. The cumulus now becomes less prominent, and in a few cases disappears. In the next stage the central part of the ventral plate becomes very prominent and forms the procephalic lobe, close to the anterior border of which is usually placed the primitive cumulus (p. 67). The space between the cumulus and the procephalic lobe grows larger, so that the latter gradually travels towards the dorsal surface and finally vanishes. Behind the procephalic lobe the first traces of the segments make their appearance, as three transverse bands, before a distinct anal lobe becomes apparent.

The points which require to be cleared up are, (1) what is the nature of the primitive cumulus? (2) where is it situated in relation to the embryo? Before attempting to answer these questions I will shortly describe the development, so far as I have made it out, for the stages during which the cumulus is visible.

In the following stage which I have obtained (which cannot be very much older than the previous stage, because my specimens of it come from the same batch of eggs), a distinct and fairly circumscribed thickening forming the ventral surface of the embryo has become established. Though its component parts are somewhat indistinct, it appears to consist of a procephalic lobe, a less prominent caudal lobe, and an intermediate portion divided into about three segments; but its constituents cannot be clearly identified with the structures visible in the previous stage. I am inclined, however, to identify the anterior thickened area of the previous stage with the procephalic lobe, and a slight protuberance of the caudal portion (visible from the surface) with the primitive cumulus. I have, however, failed to meet with any trace of the cumulus in my sections.

To this stage, which forms the first of the second period of the larval history, I shall return, but it is necessary now to go back to the observations of Claparède and Balbiani.

There can, in the first place, be but little doubt that what I have called the primitive cumulus in my description is the structure so named by Claparède and Balbiani.

It is clear that Balbiani and Claparède have both failed to appreciate the importance of the organ, which my observations shew to be the part of the ventral thickening of the blastoderm where two rows of cells are first established, and therefore the point where the first traces of the future mesoblast becomes visible.

Balbiani's observations accord fairly with my own up to the stage represented in fig. 2. Beyond this stage my own observations are not satisfactory, but I must state that I feel doubtful whether Balbiani is correct in his description of the gradual separation of the procephalic lobe and the cumulus, and the passage of the latter to the dorsal surface, and think it possible that he may have made a mistake as to which side of the procephalic lobe, in relation to the parts of the embryo, the cumulus is placed.

Although there appear to be grounds for doubting whether either Balbiani and Claparède are correct in the position they assign to the cumulus, my observations scarcely warrant me in being very definite in my statements on this head, but, as already mentioned, I am inclined to place the organ near the posterior end (and therefore, as will be afterwards shewn, in a somewhat dorsal situation) of the ventral embryonic thickening.

In my earliest stage of the third period there is present, as has already been stated, a procephalic lobe, and an indistinct and not very prominent caudal portion, and about three segments between the two. The definition of the parts of the blastoderm at this stage is still very imperfect, but from subsequent stages it appears to me probable that the first of the three segments is that of the first pair of ambulatory limbs, and that the segments of the cheliceræ and pedipalpi are formed later than those of the first three ambulatory appendages.

A transverse section through an embryo at this stage (Pl. 31, fig. 12) shews that there is a ventral plate of somewhat columnar cells more than one row deep, and a dorsal portion of the blastoderm formed of a single row of flattened cells. Every section at this stage shews that the inner layer of cells of the ventral plate is receiving accessions of cells from the yolk, which has not to any appreciable extent altered its constitution. A large cell, passing from the yolk to the blastoderm, is shewn in fig. 12 at y.c.

The cells of the ventral plate are now divided into two distinct layers. The outer of these is the epiblast, the inner the mesoblast. The cells of both layers are quite continuous across the median line, and exhibit no trace of a bilateral arrangement.

This stage is an interesting one on account of the striking similarity which (apart from the amnion) exists between a section through the blastoderm of a spider and that of an insect immediately after the formation of the mesoblast. The reader should compare Kowalevsky's (Mém. Acad. Pétersbourg, Vol. XVI. 1871) fig. 26, Pl. IX. with my fig. 12. The existence of a continuous ventral plate of mesoblast has been noticed by Barrois (p. 532), who states that the two mesoblastic bands originate from the longitudinal division of a primitive single band.

Transverse sections shew that considerable changes have been effected in the constitution of the blastoderm. In the previous stage, the ventral plate was formed of an uniform external layer of epiblast, and a continuous internal layer of mesoblast. The mesoblast has now become divided along the whole length of the embryo, except, perhaps, the procephalic lobes, into two lateral bands which are not continuous across the middle line (Pl. 31, fig. 13, me). It has, moreover, become a much more definite layer, closely attached to the epiblast. Between each mesoblastic band and the adjoining yolk there are placed a few scattered cells, which in a somewhat later stage become the splanchnic mesoblast. These cells are derived from the yolk-cells; and almost every section contains examples of such cells in the act of joining the mesoblast.

The epiblast of the ventral plate has not, to any great extent, altered in constitution. It is, perhaps, a shade thinner in the median line than it is laterally. The division of the mesoblast plate into two bands, together, perhaps, with the slight reduction of the epiblast in the median ventral line, gives rise at this stage to an imperfectly marked median groove.

The dorsal epiblast is still formed of a single layer of flat cells. In the neighbourhood of this layer the yolk nuclei are especially concentrated. The yolk itself remains as before.

The epiblast is distinctly thinner in the median line than at the two sides.

The next stage figured (Pl. 30, figs. 5 and 6) is an important one, as it is characterized by the establishment of the full number of appendages. The whole length of the ventral plate has greatly increased, so that it embraces nearly the circumference of the ovum, and there is left uncovered but a very small arc between the two extremities of the plate (Pl. 30, fig. 6; Pl. 31, fig. 15). This arc is the future dorsal portion of the embryo, which lags in its development immensely behind the ventral portion.

By the close of this stage the full number of post-cephalic segments has become established. They are best seen in the longitudinal section (Pl. 31, fig. 15). There are six anterior appendage-bearing segments, followed by four with rudimentary appendages (not seen in this figure), and six without appendages behind. There are, therefore, sixteen in all. This number accords with the result arrived at by Barrois, but is higher by two than that given by Claparède.

The germinal layers (vide Pl. 31, fig. 14) have by this stage undergone a further development. The mesoblastic somites are more fully developed. The general relations of these somites is shewn in longitudinal section in Pl. 31, fig. 15, and in transverse section in Pl. 31, fig. 14. In the tail, where they are simplest (shewn on the upper side in fig. 14), each mesoblastic somite is formed of a somatic layer of more or less cubical cells attached to the epiblast, and a splanchnic layer of flattened cells. Between the two is placed a completely circumscribed cavity, which constitutes part of the embryonic body-cavity. Between the yolk and the splanchnic layer are placed a few scattered cells, which form the latest derivatives of the yolk-cells, and are to be reckoned as part of the splanchnic mesoblast. The mesoblastic somites do not extend outwards beyond the edge of the ventral plate, and the corresponding mesoblastic somites of the two sides do not nearly meet in the middle line. In the limb-bearing somites the mesoblast has the same general characters as in the posterior somites, but the somatic layer is prolonged as a hollow papilliform process into the limb, so that each limb has an axial cavity continuous with the section of the body-cavity of its somite. The description given by Metschnikoff of the formation of the mesoblastic somites in the scorpion, and their continuation into the limbs, closely corresponds with the history of these parts in spiders. In the region of each procephalic lobe the mesoblast is present as a continuous layer underneath the epiblast, but in the earlier part of the stage, at any rate, is not formed of two distinct layers with a cavity between them.

In the procephalic lobes the epiblast is much thickened, and is formed of several rows of cells. The greater part of it is destined to give rise to the supra-œsophageal ganglia.

During the various changes which have been described the blastoderm cells have been continually dividing, and, together with their nuclei, have become considerably smaller than at first. The yolk cells have in the meantime remained much as before, and are, therefore, considerably larger than the nuclei of the blastoderm cells. They are more numerous than in the earlier stages, but are still surrounded by a protoplasmic body, which is continued into a protoplasmic reticulum. The yolk is still divided up into polygonal segments, but from sections it would appear that the nuclei are more numerous than the segments, though I have failed to arrive at quite definite conclusions on this point.

The next stage figured forms the last of the third period (Pl. 30, figs. 7 and 7a). The ventral plate is still rolled round the egg (fig. 7), and the end of the tail and the procephalic lobes nearly meet dorsally, so that there is but a very slight development of the dorsal region. There are the same number of segments as before, and the chief differences in appearance between the present and the previous stage depend upon the fact (1) that the median ventral integument between the nerve ganglia has become wider, and at the same time thinner; (2) that the limbs have become much more developed; (3) that the stomodæum is definitely established; (4) that the procephalic lobes have undergone considerable development.

Of these features, the three last require a fuller description. The limbs of the two sides are directed towards each other, and nearly meet in the ventral line. The cheliceræ are two-jointed, and terminate in what appear like rudimentary chelæ, a fact which perhaps indicates that the spiders are descended from ancestors with chelate cheliceræ. The four embryonic post-ambulatory appendages are now at the height of their development.

The stomodæum (Pl. 30, fig. 7, and Pl. 31, fig. 17, st) is a deepish pit between the two procephalic lobes, and distinctly in front of the segment of the cheliceræ. It is bordered in front by a large, well-marked, bilobed upper lip, and behind by a smaller lower lip. The large upper lip is a temporary structure, to be compared, perhaps, with the gigantic upper lip of the embryo of Chelifer (cf. Metschnikoff). On each side of and behind the mouth two whitish masses are visible, which are the epiblastic thickenings which constitute the ganglia of the cheliceræ (Pl. 30, fig. 7, ch.g).

The procephalic lobes (pr.l) now form two distinct masses, and each of them is marked by a semicircular groove, dividing them into a narrower anterior and a broader posterior division.

The existence of a section of the body-cavity in the præoral region is a fact of some interest, especially when taken in connection with the discovery, by Kleinenberg, of a similar structure in the head of Lumbricus. The procephalic lobe represents the præoral lobe of Chætopod larvæ, but the prolongation of the body-cavity into it does not, in my opinion, necessarily imply that it is equivalent to a post-oral segment.

The epiblast of the procephalic lobes is a thick layer several cells deep, but without any trace of a separation of the ganglionic portion from the epidermis.

The nuclei of the yolk have increased in number, but the yolk, in other respects, retains its earlier characters.

The next period in the development is that in which the body of the embryo gradually acquires the adult form. The most important event which takes place during this period is the development of the dorsal region of the embryo, which, up to its commencement, is practically non-existent. As a consequence of the development of the dorsal region, the embryo, which has hitherto had what may be called a dorsal flexure, gradually unrolls itself, and acquires a ventral flexure. This change in the flexure of the embryo is in appearance a rather complicated phenomenon, and has been somewhat differently described by the two naturalists who have studied it in recent times.

Barrois takes quite a different view to that of Claparède, which will perhaps be best understood if I quote a translation of his own words. He says: “At the period of the last stage of the embryonic band (the stage represented in Pl. 31, fig. 7, in the present paper) this latter completely encircles the egg, and its posterior extremity nearly approaches the cephalic region. Finally, the germinal bands, where they unite at the anal lobe (placed above on the dorsal surface), form between them a very acute angle. During the following stages one observes the anal segment separate further and further from the cephalic region, and approach nearer and nearer to the ventral region. This displacement of the anal segment determines, in its turn, a modification in the divergence of the anal bands; the angle which they form at their junction tends to become more obtuse. The same processes continue regularly till the anal segment comes to occupy the opposite extremity to the cephalic region, a period at which the two germinal bands are placed in the same plane and the two sides of the obtuse angle end by meeting in a straight line. If we suppose a continuation of the same phenomenon it is clear that the anal segment will come to occupy a position on the ventral surface, and the germinal bands to approach, but in the inverse way, so as to form an angle opposite to that which they formed at first. This condition ends the process by which the posterior extremity of the embryonic band, at first directed towards the dorsal side, comes to bend in towards the ventral region.”

At the earliest stage of this period of which I have examples, the dorsal region has somewhat increased, though not very much. The limbs have grown very considerably and now cross in the middle line.

The ventral ganglia, though not the supra-œsophageal, have become separated from the epiblast.

The yolk nuclei, each surrounded by protoplasm as before, are much more numerous.

In other respects there are no great changes in the internal features.

The caudal protuberance is more prominent than it was, and somewhat bilobed; it is continued on each side into one of the bands, into which the ventral plate is divided. These bands, as is best seen in side view (fig. 8b), have a ventral curvature, or, perhaps more correctly, are formed of two parts, which meet at a large angle open towards the ventral surface. The posterior of these parts bears the four still very conspicuous provisional appendages, and the anterior the six pairs of thoracic appendages. The four ambulatory appendages are now seven-jointed, as in the adult, but though longer than in the previous stage they do not any longer cross or even meet in the middle line, but are, on the contrary, separated by a very considerable interval. This is due to the great distension by the yolk of the ventral part of the body, in the interval between the two parts of the original ventral plate. The amount of this yolk may be gathered from the section (Pl. 32, fig. 18). The pedipalpi carry a blade on their basal joint. The cheliceræ no longer appear to spring from an independent postoral segment.

There is a conspicuous lower lip, but the upper is less prominent than before. Sections at this stage shew that the internal changes have been nearly as considerable as the external.

The dorsal region is now formed of a (1) flattened layer of epiblast cells, and a (2) fairly thick layer of large and rather characteristic cells which any one who has studied sections of spider's embryos will recognize as derivatives of the yolk. These cells are not, therefore, derived from prolongations of the somatic and splanchnic layers of the already formed somites, but are new formations derived from the yolk. They commenced to be formed at a much earlier period, and some of them are shewn in the longitudinal section (Pl. 31, fig. 15). In the next stage these cells become differentiated into the somatic and splanchnic mesoblast layers of the dorsal region of the embryo.

The rudiment of the heart is in contact with the epiblast above, and there is no greater evidence of its being derived from the splanchnic than from the somatic mesoblast; it is, in fact, formed before the dorsal mesoblast has become differentiated into two layers.

In the abdomen three or four transverse septa, derived from the splanchnic mesoblast, grow a short way into the yolk. They become more conspicuous during the succeeding stage, and are spoken of in detail in the description of that stage. In the anterior part of the thorax a longitudinal and vertical septum is formed, which grows downwards from the median dorsal line, and divides the yolk in this region into two parts. In this septum there is formed at a later stage a vertical muscle attached to the suctorial part of the stomodæum.

The mesoblastic somites of the earlier stage are but little modified; and there are still prolongations of the body-cavity into the limbs (Pl. 32, fig. 18).

The lateral parts of the ventral nerve cords are now at their maximum of separation (Pl. 32, fig. 18, v.g.). Considerable differentiation has already set in in the constitution of the ganglia themselves, which are composed of an outer mass of ganglion cells enclosing a kernel of nerve fibres, which lie on the inner side and connect the successive ganglia. There are still distinct thoracic and abdominal ganglia for each segment, and there is also a pair of separate ganglion for the cheliceræ, which assists, however, in forming the œsophageal commissures.

The thickenings of the præoral lobe which form the supra-œsophageal ganglia are nearly though not quite separated from the epiblast. The semicircular grooves of the earlier stages are now deeper than before, and are well shewn in sections nearly parallel to the outer anterior surface of the ganglion (Pl. 32, fig. 19). The supra-œsophageal ganglia are still entirely formed of undifferentiated cells, and are without commissural tissue like that present in the ventral ganglia.

The next stage constitutes (Pl. 30, fig. 9) the last which requires to be dealt with so far as the external features are concerned. The yolk has now mainly passed into the abdomen, and the constriction separating the thorax and abdomen has begun to appear. The yolk-sack has become absorbed, so that the two halves of the ventral plate in the thorax are no longer widely divaricated. The limbs have to a large extent acquired their permanent structure, and the rings of which they are formed in the earlier stages are now replaced by definite joints. A delicate cuticle has become formed, which is not figured in my sections. The four rudimentary appendages have disappeared, unless, which seems to me in the highest degree improbable, they remain as the spinning mammillæ, two pairs of which are now present. Behind is the anal lobe, which is much smaller and less conspicuous than in the previous stage. The spinnerets and anal lobe are shewn as five papillæ in Pl. 30, fig. 9. Dorsally the heart is now very conspicuous, and in front of the cheliceræ may be seen the supra-œsophageal ganglia.

The indifferent mesoblast has now to a great extent become converted into the permanent tissues. On the dorsal surface there was present in the last stage a great mass of unformed mesoblast cells. This mass of cells has now become divided into a somatic and splanchnic layer (Pl. 32, fig. 22). It has, moreover, in the abdominal region at any rate, become divided up into somites. At the junction between the successive somites the splanchnic mesoblast on each side of the abdomen dips down into the yolk and forms a septum (Pl. 32, fig. 22, s). The septa so formed, which were first described by Barrois, are not complete. The septa of the two sides do not, in the first place, quite meet along the median dorsal or ventral lines, and in the second place they only penetrate the yolk for a certain distance. Internally they usually end in a thickened border.

In the intervals between these venous spaces the somatic and splanchnic layers of mesoblast are in contact with each other.

I have not been able to work out satisfactorily the later stages of development of the septa, but I have found that they play an important part in the subsequent development of the abdomen. In the first place they send off lateral offshoots, which unite the various septa together, and divide up the cavity of the abdomen into a number of partially separated compartments. There appears, however, to be left a free axial space for the alimentary tract, the mesoblastic walls of which are, I believe, formed from the septa.

At the present stage the splanchnic mesoblast, apart from the septa, is a delicate membrane of flattened cells (fig. 22, sp). The somatic mesoblast is thicker, and is formed of scattered cells (so).

The somatic layer is in part converted, in the posterior region of the abdomen, into a delicate layer of longitudinal muscles, the fibres of which are not continuous for the whole length of the body, but are interrupted at the lines of junction of the successive segments. They are not present in the anterior part of the abdomen. The longitudinal direction of these fibres, and their division with myotomes, is interesting, since both these characters, which are preserved in Scorpions, are lost in the abdomen of the adult Spider.

The heart has now, in most respects, reached its full development. It is formed of an outer muscular layer, within which is a doubly-contoured lining, containing nuclei at intervals, which is probably of the nature of an epithelioid lining (Pl. 32, fig. 22, ht). In its lumen are numerous blood-corpuscles (not represented in my figure). The heart lies in a space bound below by the splanchnic mesoblast, and to the sides by the somatic mesoblast. This space forms a kind of pericardium (fig. 22, pc), but dorsally the heart is in contact with the epiblast. The arterial trunks connected with it are fully established.

The nervous system has undergone very important changes.

In the abdominal region the ganglia of each side have fused together into a continuous cord (fig. 21, ab.g). In fig. 20, in which the abdomen is cut horizontally and longitudinally, there are seen the two abdominal cords (ab.g) united by two transverse commissures; and I believe that there are at this stage three or four transverse commissures at any rate, which remain as indications of the separate ganglia, from the coalescence of which the abdominal cords are formed. The two abdominal cords are parallel and in close contact.

As shewn in the longitudinal section (Pl. 32, fig. 21), the ganglion of the cheliceræ has now united with the supra-œsophageal ganglion. It forms, as is shewn in fig. 20b (ch.g.), a part of the œsophageal commissure, and there is no sub-œsophageal commissure uniting the ganglia of the cheliceræ, but the œsophageal ring is completed below by the ganglia of the pedipalpi (fig. 20c, pd.g.).

The supra-œsophageal ganglia have become completely separated from the epiblast.

I have unfortunately not studied their constitution in the adult, so that I cannot satisfactorily identify the parts which can be made out at this stage.

I distinguish, however, the following regions:

(1) A central region containing the commissural part, and continuous below with the ganglia of the cheliceræ.

(2) A dorsal region formed of two hemispherical lobes.

(3) A ventral anterior region.

The central region contains in its interior the commissural portion, forming a punctiform, rounded mass in each ganglion. A transverse commissure connects the two (vide fig. 20b).

The dorsal hemispherical lobes are derived from the part which, at the earlier stage, contained the semicircular grooves. When the supra-œsophageal ganglia become separated from the epidermis the cells lining these grooves become constricted off with them, and form part of these ganglia. Two cavities are thus formed in this part of the supra-œsophageal ganglia. These cavities become, for the most part, obliterated, but persist at the outer side of the hemispherical lobes (figs. 20a and 21).

The ventral lobe of the brain is a large mass shewn in longitudinal section in fig. 21. It lies immediately in front of and almost in contact with the ganglia of the cheliceræ.

The two hemispherical lobes agree in position with the fungiform body (pilzhutförmige Körpern), which has attracted so much the attention of anatomists, in the supra-œsophageal ganglia of Insects and Crustacea; but till the adult brain of Spiders has been more fully studied it is not possible to state whether the hemispherical lobes become fungiform bodies.

The yolk retains its earlier constitution, being divided into polygonal segments, formed of large yolk vesicles. The nuclei are more numerous than before. In the thorax the yolk is anteriorly divided into two lobes by the vertical septum, which contains the vertical muscle of the suctorial pouch. In the posterior part of the thorax it is undivided.

I have not yet been able clearly to make out the eventual fate of the yolk. At a subsequent stage, when the cavity of the abdomen is cut up into a series of compartments by the growth of the septa, described above, the yolk fills these compartments, and there is undoubtedly a proliferation of yolk cells round the walls of these compartments. It would not be unreasonable to conclude from this that the compartments were destined to form the hepatic cæca, each cæcum being enclosed in a layer of splanchnic mesoblast, and its hypoblastic wall being derived from the yolk cells. I think that this hypothesis is probably correct, but I have met with some facts which made me think it possible that the thickenings at the ends of the septa, visible in Pl. 32, fig. 22, were the commencing hepatic cæca.

I must, in fact, admit that I have hitherto failed to work out satisfactorily the history of the mesenteron and its appendages. The firm cuticle of young spiders is an obstacle both in the way of making sections and of staining, which I have not yet overcome.

General Conclusions.

Without attempting to compare at length the development of the spiders with that of other Arthropoda, I propose to point out a few features in the development of spiders, which appear to shew that the Arachnida are undoubtedly more closely related to the other Tracheata than to the Crustacea.

The whole history of the formation of the mesoblast is very similar to that in insects. The mesoblast in both groups is formed by a thickening of the median line of the ventral plate (germinal streak).