The Works of Francis Balfour 1-17
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol I. Separate Memoirs (1885)
XVII. Notes on the Development of the Araneina
(With Plates 30, 31, 32.)
THE following observations do not profess to contain a complete history of the development even of a single species of spider. They are the result of investigations carried on at intervals during rather more than two years, on the ova of Agelena labyrinthica ; and I should not have published them now, if I had any hope of being able to complete them before the appearance of the work I am in the course of publishing on Comparative Embryology. It appeared to me, however, desirable to publish in full such parts of my observations as are completed before the appearance of my treatise, since the account of the development of the Araneina is mainly founded upon them.
My investigations on the germinal layers and organs have been chiefly conducted by means of sections. To prepare the embryos for sections, I employed the valuable method first made known by Bobretzky. I hardened the embryos in bichromate of potash, after placing them for a short time in nearly boiling water. They were stained as a whole with hasmatoxylin after the removal of the membranes, and embedded for cutting in coagulated albumen.
The number of investigators who have studied the development of spiders is inconsiderable. A list of them is given at the end of the paper.
The earliest writer on the subject is'Herold (No. 4) ; he was followed after a very considerable interval of time by Claparede
1 From the Quarterly Jottrn. of Microscopical Science, Vol. XX. 1880.
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 669
(No. 3), whose memoir is illustrated by a series of beautiful plates, and contains a very satisfactory account of the external features of development.
Balbiani (No. i) 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. MetschnikofFs 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 : (i) 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.
Each yolk-segment is formed of a number of large clear somewhat oval yolk-spherules. In hardened specimens the yolkspherules become polygonal, and in ova treated with hot water prior to preservation are not unfrequently broken up. Amongst the yolk-segments are placed a fair number of nucleated bodies of a very characteristic appearance, Each of them is formed of (i) a large, often angular, nucleus, filled with deeply staining bodies (nucleoli ?). (2) Of a layer of protoplasm surrounding the nucleus, prolonged into a protoplasmic reticulum. The exact relation of these nucleated bodies to the yolk-segments is not very easy to make out, but the general tendency of my observations is to shew (i) that each nucleated body belongs to a yolk-sphere, and (2) that it is generally placed not at the centre, but to one side of a yolk-sphere. If the above conclusions are correct each complete yolk-segment is a cell, and each such
670 NOTES ON THE DEVELOPMENT OF THE ARANEINA.
cell consists of a normal nucleus, protoplasm, and yolk-spherules. There is a special layer of protoplasm surrounding the nucleus, while the remainder of the protoplasm consists of a reticulum holding together the yolk-spherules. Yolk-cells of this character are seen in Pis. 31 and 32, figs. 10 21.
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 yolkcolumns 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 yolkspheres 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 Claparede 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 Claparede 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.
After a short time the cumulus elongates itself on one side, and becomes connected by a streak with a white patch, which appears on the surface of the egg, about 90 from the cumulus. This patch gradually enlarges, and soon covers the whole surface of the ovum except the region where the cumulus is placed. It becomes the ventral plate or germinal streak of the embryo, its extremity adjoining the cumulus is the anal extremity, and its opposite extremity the cephalic one. The cumulus itself is placed in a depression on the dorsal surface of the ovum.
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 6/1
Claparede compares the cumulus to the dorsal organ of many Crustacea.
Balbiani (No. i) 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 domes tica 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, (i) 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.
The first change that I find in the embryo (when examined after it has been hardened) 1 is the appearance of a small, whitish spot, which is at first very indistinct. A section through such an ovum (PL 31, fig. 10) shews that the cells of about one half of the ovum have become more columnar than those of the other half, and that there is a point (pr. c.} near one end of the thickened half where the cells are more columnar, and about two layers or so deep. It appears to me probable that this point is the whitish spot visible in the hardened ovum. In a somewhat later stage (PL 30, fig. i) the whitish spot becomes more con 1 I was unfortunately too much engaged, at the time when the eggs were collected, to study them in the fresh condition ; a fact which has added not a little to my difficulties in elucidating the obscure points in the early stages.
6/2 NOTES ON THE DEVELOPMENT OF THE ARANEINA.
spicuous (/.), and appears as a distinct prominence, which is, without doubt, the primitive cumulus, and from it there proceeds on one side a whitish streak. The prominence, as noticed by Claparede and Balbiani, is situated on the flatter side of the ovum. Sections at this stage shew the same features as the previous stage, except that (i) the cells throughout are smaller, (2) those of the thickened hemisphere of the ovum more columnar, and (3) the cumulus is formed of several rows of cells, though not divided into distinct layers. In the next stage the appearances from the surface are rather more obscure, and in some of my best specimens a coagulum, derived from the fluid surrounding the ovum, covers the most important part of the blastoderm. In PI; 30, fig. 2, I have attempted to represent, as truly as I could, the appearances presented by the ovum. There is a well-marked whitish side of the ovum, near one end of which is a prominence (pc-}> which must, no doubt, be identified with the cumulus of the earlier stages. Towards the opposite end, or perhaps rather nearer the centre of the white side of the ovum, is an imperfectly marked triangular white area. There can be no doubt that the line connecting the cumulus with the triangular area is the future long axis of the embryo, and the white area is, without doubt, the procephalic lobe of Balbiani.
A section of the ovum at this stage is represented in PI. 31, fig. ii. It is not quite certain in what direction the section is taken, but I think it probable it is somewhat oblique to the long axis. However this may be, the section shews that the whitish hemisphere of the blastoderm is formed of columnar cells, for the most part two or so layers deep, but that there is, not very far from the middle line, a wedge-shaped internal thickening of the blastoderm where the cells are several rows deep. With what part visible in surface view this thickened portion corresponds is not clear. To my mind it most probably corresponds to the larger white patch, in which case I have not got a section through the terminal prominence. In the other sections of the same embryo the wedge-shaped thickening was not so marked, but it, nevertheless, extended through all the sections. It appears to me probable that it constitutes a longitudinal thickened ridge of the blastoderm. In any case, it is clear that the white hemisphere of the blastoderm is a thickened portion of the
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 673
blastoderm, and that the thickening is in part due to the cells being more columnar, and, in part, to their being more than one row deep, though they have not become divided into two distinct germinal layers. It is further clear that the increase in the number of cells in the thickened part of the blastoderm is, in the main, a result of the multiplication of the original single row of cells, while a careful examination of my sections proves that it is also partly due to cells, derived from the yolk, having been added to the blastoderm.
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 Claparede 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 Claparede and Balbiani.
It is clear that Balbiani and Claparede 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.
Though Claparede and Balbiani differ somewhat as to the position of the organ, they both make it last longer than I do : I feel certainly inclined to doubt whether Claparede is right in considering a body he figures after six segments are present, to
674 NOTES ON THE DEVELOPMENT OF THE ARANEINA.
be the same as the dorsal organ of the embryo before the formation of any segments, especially as all the stages between the two appear to have escaped him. In Agelena there is undoubtedly no organ in the position he gives when six segments are found.
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 Claparede 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 chelicerae and pedipalpi are formed later than those of the first three ambulatory appendages.
Balbiani believes that the segment of the chelicerae is formed later than that of the six succeeding segments. He further concludes, from the fact that this segment is cut off from the procephalic portion in front, that it is really part of the procephalic lobe. I cannot accept the validity of this argument ; though I am glad to find myself in, at any rate, partial harmony with the distinguished French embryologist as to the facts. Balbiani denies for this stage the existence of a caudal lobe. There is certainly, as is very well shewn in my longitudinal
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 675
sections, a thickening of the blastoderm in the caudal region, though it is not so prominent in surface views as the procephalic lobe.
A transverse section through an embryo at this stage (PI. 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 (Mem. Acad. Petersbonrg, 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.
In a slightly later stage (PI. 30, fig. 3 a and 3 b] six distinct segments are interpolated between the procephalic and the caudal lobes. The two foremost, ch and pd (especially the first), of these are far less distinct than the remainder, and the first segment is very indistinctly separated from the procephalic lobe. From the indistinctness of the first two somites, I conclude that they are later formations than the four succeeding ones. The caudal and procephalic lobes are very similar in appearance, but the procephalic lobe is slightly the wider of the two. There is a slight protuberance on the caudal lobe, which is possibly the remnant of the cumulus. The superficial appearance of segmentation is produced by a series of transverse valleys, separating raised intermediate portions which form the segments.
676 NOTES ON THE DEVELOPMENT OF THE ARANEINA.
The ventral thickening of the embryo now occupies rather more than half the circumference of the ovum.
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 segments continue to increase regularly, each fresh segment being added in the usual way between the last formed segment and the unsegmented caudal lobe. At the stage when about nine or ten segments have become established, the first rudiments of appendages become visible. At this period (PL 30, fig. 4) there is a distinct median ventral groove, extending through the whole length of the embryo, which becomes, however, considerably shallower behind. The procephalic region is distinctly bilobed. The first segment (that of the cheliceras) is better marked off from it than in the previous stage, but is without a trace of an appendage, and exhibits therefore, in respect to the development of its appendages, the same retardation that characterised its first appearance. The next five segments, viz. those of the pedipalpi and four ambulatory appendages, present
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 6/7
a very well-marked swelling at each extremity. These swellings are the earliest traces of the appendages. Of the three succeeding segments, only the first is well differentiated. The caudal lobe, though less broad than the procephalic lobe, is still a widish structure. The most important internal changes concern the mesoblast, which is now imperfectly though distinctly divided into somites, corresponding with segments visible externally. Each mesoblastic somite is formed of a distinct somatic layer closely attached to the epiblast, and a thinner and less well-marked splanchnic layer. In the appendagebearing segments the somatic layer is continued up into the appendages.
The epiblast is distinctly thinner in the median line than at the two sides.
The next stage figured (PI. 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 (PI. 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.
There is a very distinctly bilobed procephalic region (pr. 1} well separated from the segment with the chelicerse (ch}. It is marked by a shallow groove opening behind into a circular depression (sf.) the earliest rudiment of the stomodaeum. The six segments behind the procephalic lobes are the six largest, and each of them bears two prominent appendages. They constitute the six appendage-bearing segments of the adult. The four future ambulatory appendages are equal in size : they are slightly larger than the pedipalpi, and these again than the chelicerse. Behind the six somites with prominent appendages there are four well-marked somites, each with a small protuberance. These four protuberances are provisional appendages. They have been found in many other genera of Araneina (Claparede, Barrois). The segments behind these are rudimentary and difficult to count, but there are, at any rate, five, and at a slightly later stage probably six, including the anal lobe. These fresh segments have been formed by the continued segmentation of
678 NOTES ON THE DEVELOPMENT OF THE ARANEINA.
the anal lobe, which has greatly altered its shape in the process. The ventral groove of the earlier stage is still continued along the whole length of the ventral plate.
By the close of this stage the full number of post-cephalic segments has become established. They are best seen in the longitudinal section (PI. 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 Claparede.
The germinal layers (vide PI. 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 PI. 31, fig. 15, and in transverse section in PI. 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 limbbearing 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 bodycavity 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.
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 679
The epiblast at this stage has also undergone important changes. Along the median ventral groove it has become very thin. On each side of this groove it exhibits in each appendage-bearing somite a well-marked thickening, which gives in surface views the appearance of a slightly raised area (PI. 30, fig. 5), between each appendage and the median line. These thickenings are the first rudiments of the ventral nerve ganglia. The ventral nerve cord at this stage is formed of two ridge-like thickenings of the epiblast, widely separated in the median line, each of which is constituted of a series of raised divisions the ganglia- united by shorter, less prominent divisions (fig. 14, vg}. The nerve cords are formed from before backwards, and are not at this stage found in the hinder segments. There is a distinct ganglionic thickening for the chelicera quite independent of tJie procephalic lobes.
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-cesophageal 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.
As development proceeds the appendages grow longer, and gradually bend inwards. They become very soon divided by a series of ring-like constrictions which constitute the first indications of the future joints (PI. 30, fig. 6). The full number of joints are not at once reached, but in the ambulatory appendages five only appear at first to be formed. There are: four joints in the pedipalpi, while the chelicerae do not exhibit any signs of becoming jointed till somewhat later. The primitive presence of only five joints in the ambulatory appendages
680 NOTES ON THE DEVELOPMENT OF THE ARANEINA.
is interesting, as this number is permanent in Insects and in Peripatus.
The next stage figured forms the last of the third period (PI. 30, figs. 7 and 70). 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 (i) 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 stomodaeum 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 chelicerae are two-jointed, and terminate in what appear like rudimentary chelae, a fact which perhaps indicates that the spiders are descended from ancestors with chelate chelicerae. The four embryonic, postambulatory appendages are now at the height of their development.
The stomodaeum (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 chelicerae. 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 chelicerae (PL 30,
fig- 7. &. g\
The procephalic lobes (pr. 1} 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.
In the region of the trunk the general arrangement of the germinal layers has not altered to any great extent. The ventral ganglionic thickenings are now developed in all the segments in the abdominal as well as in the thoracic region. The individ
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 68 1
ual thickenings themselves, though much more conspicuous than in the previous stage' (PL 31, fig. 16, v. c], are still integral parts of the epiblast. They are more widely separated than before in the middle line. The mesoblastic somites retain their earlier constitution (PI. 31, fig. 16). Beneath the procephalic lobes the mesoblast has, in most respects, a constitution similar to that of a mesoblastic somite in the trunk. It is formed of two bodies, one on each side, each composed of a splanchnic and somatic layer (PI. 31, fig. 17, sp. and so), enclosing between them a section of the body-cavity. But the cephalic somites, unlike those of the trunk, are united by a median bridge of mesoblast, in which no division into two layers can be detected. This bridge assists in forming a thick investment of mesoblast round the stomodaeum (sf).
The existence of a section of the body-cavity in the praeoral 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 praeoral lobe of Chaetopod larvae, 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.
For Claparede the prime cause of the change of flexure is
682 NOTES ON THE DEVELOPMENT OF THE ARANEINA.
the translation dorsalwards of the limbs. He compares the dorsal region of the embryo to the arc of a circle, the two ends of which are united by a cord formed by the line of insertion of the limbs. He points out that if you bring the middle of the cord, so stretched between the two ends of the arc, nearer to the summit of the arc, you necessarily cause the two ends of the arc to approach each other, or, in other words, if the insertion of the limbs is drawn up dorsally, the head and tail must approach each other ventrally.
Barrois takes quite a different view to that of Claparede, 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 PI. 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."
Neither of the above explanations is to my mind perfectly satisfactory. The whole phenomenon appears to me to be very simple, and to be caused by the elongation of the dorsal region, i.e. the region on the dorsal surface between the anal and pro
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 683
cephalic lobes. Such an elongation necessarily separates the anal and procephalic lobes ; but, since the ventral plate does not become shortened in the process, and the embryo cannot straighten itself on account of the egg-shell, it necessarily becomes flexed, and such flexure can only be what I have already called a ventral flexure. If there were but little food yolk this flexure would cause the whole embryo to be bent in, so as to have the ventral surface concave, but instead of this the flexure is confined at first to the two bands which form the ventral plate. These bands are bent in the natural way (PI. 30, fig. 8, B', but the yolk forms a projection, a kind of yolk-sack as Barrois calls it, distending the thin integument between the two ventral bands. This yolk-sack is shewn in surface view in PI. 30, fig. 8, and in section in PI. 32, fig. 18. At a later period, when the yolk has become largely absorbed in the formation of various organs, the true nature of the ventral flexure becomes apparent, and the abdomen of the young Spider is found to be bent over so as to press against the ventral surface of the thorax (PI. 30, fig. 9). This flexure is shewn in section in PI. 32, fig. 21.
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-cesophageal, 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.
In my next stage, represented in PI. 30, figs. 8 a, and 8 b, a very considerable advance has become effected. In the first place the dorsal surface has increased in length to rather more than one half the circumference of the ovum. The dorsal region has, however, not only increased in length, but also in definiteness, and a series of transverse markings (figs. 8 a and b}, which are very conspicuous in the case of the four anterior abdominal segments (the segments with rudimentary appendages), have appeared, indicating the limits of segments dorsally. The terga of the somites may, in fact, be said to have become formed.
684 NOTES ON THE DEVELOPMENT OF THE ARANEINA.
The posterior terga (fig. 8 a} are very narrow compared to the anterior.
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. 8 b), 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 chelicerae 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 (i) 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 (PI. 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.
In the dorsal region of the abdomen the heart has already become established. So far as I have been able to make out it is formed from a solid cord of the cells of the dorsal region.
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 685
The peripheral layer of this cord gives rise to the walls of the heart, while the central cells become converted into the corpuscles of the blood.
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 stomodseum.
The mesoblastic somites of the earlier stage are but little modified ; and there are still prolongations of the body cavity into the limbs (PI. 32, fig. 18).
The lateral parts of the ventral nerve cords are now at their maximum of separation (PI. 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 chelicerae, which assists, however, in forming the cesophageal commissures.
The thickenings of the praeoral lobe which form the supracesophageal 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-cesophageal ganglia are still entirely formed of undifferentiated cells, and are without commissural tissue like that present in the ventral ganglia.
The stomodasum has considerably increased in length, and the proctodaeum has become formed as a short, posteriorly
686 NOTES ON THE DEVELOPMENT OF THE ARANEINA.
directed involution of the epiblast. I have seen traces of what I believe to be two outgrowths from it, which form the Malpighian bodies.
The next stage constitutes (PL 3.0, 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 seerns to me in the highest degree improbable, they remain as the spinning mammillae, 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 papillae in PI. 30, fig. 9. Dorsally the heart is now very conspicuous, and in front of the chelicerae may be seen the supra-oesophageal 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 (PI. 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 (PI. 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.
Along the line of insertion of each of these septa there is developed a considerable space between the somatic and splanchnic layers of mesoblast. The parts of the body-cavity so estab
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 687
lished are transversely directed channels passing from the heart outwards. They probably constitute the venous spaces, and perhaps also contain the transverse aortic branches.
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 original mesoblastic somites have undergone quite as important changes as the dorsal mesoblast. In the abdominal region the somatic layer constitutes two powerful bands of longitudinal muscles, inserted anteriorly at the root of the fourth ambulatory appendage, and posteriorly at the spinning mammillae. Between these two bands are placed the nervous bands. The relation of these parts are shewn in the section in PL 32, fig. 20 d, which cuts the abdomen horizontally and longitudinally. The mesoblastic bands are seen at m., and the nervous bands within them at ab. g. In the thoracic region the part of the somatic layer in each limb is converted into muscles, which are continued into dorsal and ventral muscles
688 NOTES ON THE DEVELOPMENT OF THE ARANEINA.
in the thorax (vide fig. 20 c). There are, in addition to these, intrinsic transverse fibres on the ventral side of the thorax. Besides these muscles there are in the thorax, attached to the suctorial extremity of the stomodaeum, three powerful muscles, which I believe to be derived from the somatic mesoblast One of these passes vertically down from the dorsal surface, in the septum the commencement of which was described in the last stage. The two other muscles are lateral, one on each side (PL 31, fig. 20 c.).
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 ///). 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.
In the thoracic region changes of not less importance have taken place. The ganglia are still distinct. The two cords formed of these ganglia are no longer widely separated in median line, but meet, in the usual way, in the ventral line. Transverse commissures have become established (fig. 20 c) between the ganglia of the .two sides. There is as little trace at this, as at the previous stages, of an ingrowth of epiblast, to form a median portion of the central nervous system. Such a median structure has been described by Hatschek for Lepidoptera, and he states that it gives rise to the transverse com
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 689
missures between the ganglia. My observations shew that for the spider, at any rate, nothing of the kind is present.
As shewn in the longitudinal section (PI. 32, fig. 21), the ganglion of the chelicerae has now united with the supra-cesophageal ganglion. It forms, as is shewn in fig. 20 b (ch. g.}, a part of the oesophageal commissure, and there is no subcesophageal commissure uniting the ganglia of the chelicerae, but the cesophageal ring is completed below by the ganglia of the pedipalpi (fig. 20 c,pd.g.}.
The supra-cesophageal 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 chelicerae.
(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. 20 b}.
The dorsal hemispherical lobes are derived from the part which, at the earlier stage, contained the semicircular grooves. When the supra-cesophageal 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 cesophageal ganglia. These cavities become, for the most part, obliterated, but persist at the outer side of the hemispherical lobes (figs. 20 a 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 chelicerae.
The two hemispherical lobes agree in position with the fungiform body (pilzhutformige Korperti), which has attracted so much the attention of anatomists, in the supra-cesophageal 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 fungi form bodies.
690 NOTES ON THE DEVELOPMENT OF THE ARANEINA.
Hatschek 1 has described a special epiblastic invagination in the supra-cesophageal ganglion of Bombyx, which is probably identical with the semicircular groove of Spiders and Scorpions, but in the figure he gives the groove does not resemble that in the Arachnida. A similar groove is found in Peripatus, and there forms, as I have found, a large part of the supra-cesophageal ganglia. It is figured by Moseley, Phil. Trans., Vol. CLXIV. pi. Ixxv, fig. 9.
The stomodaeum is considerably larger than in the last stage, and is lined by a cuticle; it is a blind tube, the blind end of which is the suctorial pouch of the adult. To this pouch are attached the vertical dorsal, and two lateral muscles spoken of above.
The protodaeum (pr.} has also grown in length, and the two Malpighian vessels which grow out from its blind extremity (fig. 20 e. mp. g^) have become quite distinct. The part now formed is the rectum of the adult. The proctodaeum is surrounded by a great mass of splanchnic mesoblast. The mesenteron has as yet hardly commenced to be developed. There is, however, a short tube close to the proctodaeum (fig. 20 e. mes], which would seem to be the commencement of it. It ends blindly on the side adjoining the rectum, but is open anteriorly towards the yolk, and there can be very little doubt that it owes its origin to cells derived from the yolk. On its outer surface is a layer of mesoblast.
From the condition of the mesenteron at this stage there can be but little doubt that it will be formed, not on the surface, but in the interior of the yolk, I failed to find any trace of an anterior part of the mesenteron adjoining the stomodaeum. In the posterior part of the thorax (vide fig. 20 d], there is undoubtedly no trace of the alimentary tract.
The presence of this rudiment shews that Barrois is mistaken in supposing that the alimentary canal is formed entirely from the stomodaeum and proctodaeum, which are stated by him to grow towards each other, and to meet at the junction of the thorax and abdomen. My own impression is that the stomodaeum and proctoda;um have reached their full extension at the
1 " Ik-itiagc z. Entwick. d. Lepidopteren," JenaischeZeit. t Vol. xi. p. 124.
NOTES ON THE DEVELOPMENT OF THE ARANEINA. 69!
present stage, and that both the stomach in the thorax and the intestine in the abdomen are products of the mesenteron.
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 caeca, each caecum 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 PI. 32, fig. 22, were the commencing hepatic caeca.
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.
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).
692 NOTES ON THE DEVELOPMENT OF THE ARANEINA.
In insects there is usually formed a median groove, the walls of which become converted into a plate of mesoblast. In spiders there is no such groove, but a median keel- like thickening of the ventral plate (PI. 31, fig. 11), is very probably an homologous structure. The unpaired plate of mesoblast formed in both insects and Arachnida is exactly similar, and becomes divided, in both groups, into two bands, one on each side of the middle line. Such differences as there are between Insects and Arachnida sink into insignificance compared with the immense differences in the origin of the mesoblast between either group, and that in the Isopoda, or, still more, the Malacostraca and most Crustacea. In most Crustacea we find that the mesoblast is budded off from the walls of an invagination, which gives rise to the mesenteron.
In both spiders and Myriopoda, and probably insects, the mesoblast is subsequently divided into somites, the lumen of which is continued into the limbs. In Crustacea mesoblastic somites have not usually been found, though they appear occasionally to occur, e.g. Mysis, but they are in no case similar to those in the Tracheata.
In the formation of the alimentary tract, again, the differences between the Crustacea and Tracheata are equally marked, and the Arachnida agree with the Tracheata. There is generally in Crustacea an invagination, which gives rise to the mesenteron. In Tracheata this never occurs. The proctodaeum is usually formed in Crustacea before or, at any rate, not later than the stomodaeum 1 . The reverse is true for the Tracheata. In Crustacea the proctodaeum and stomodaeum, especially the former, are very long, and usually give rise to the greater part of the alimentary tract, while the mesenteron is usually short.
In the Tracheata the mesenteron is always considerable, and the proctodaeum is always short. The derivation of the Malpighian bodies from the proctodaeum is common to most Tracheata. Such organs are not found in the Crustacea.
With reference to other points in my investigations, the evidence which I have got that the chelicerae are true postoral appendages supplied in the embryo from a distinct postoral
1 If Grobben's account of the development of Moina is correct this statement must be considered not to be universally true.
NOTES ON THE DEVELOPMENT OF THE ARANETNA. 693
ganglion, confirms the conclusions of most previous investigators, and shews that these appendages are equivalent to the mandibles, or possibly the first pair of maxillae of other Tracheata. The invagination, which I have found, of part of a groove of epiblast in the formation of the supra-cesophageal ganglia is of interest, owing to the wide extension of a similar occurrence amongst the Tracheata.
The wide divarication of the ventral nerve cords in the embryo renders it easy to prove that there is no median invagination of epiblast between them, and supports Kleinenberg's observations on Lumbricus as to the absence of this invagination. I have further satisfied myself as to the absence of such an invagination in Peripatus. It is probable that Hatschek and other observers who have followed him are mistaken in affirming .the existence of such an invagination in either the Chaetopoda or the Arthropoda.
The observations recorded in this paper on the yolk cells and their derivations are, on the whole, in close harmony with the observations of Dohrn, Bobretzky, and Graber, on Insects. They shew, however, that the first formed mesoblastic plate does not give rise to the whole of the mesoblast, but that during the whole of embryonic life the mesoblast continues to receive accessions of cells derived from the cells of the yolk.
1. Balbiani, " Mdmoire sur le DeVeloppement des Araneides," Ann, Set. Nat., series v, Vol. xvn. 1873.
2. J. Barrois, " Recherches s. 1. DeVeloppement des Araigne"es," Journal de I'Anat. et de la PhysioL, 1878.
3. E. Claparede, Recherches s, VEvolution des Araigne"es, Utrecht, 1860.
4. Herold, De Generatione Araniorum in Ovo, Marburg, 1824.
5. H. Ludwig, "Ueb. d. Bildung des Blastoderm bei d. Spinnen," Zeit.f. iviss. Zool., Vol. xxvi. 1876.
694 NOTES ON THE DEVELOPMENT OF THE AKANETNA.
EXPLANATION OF PLATES 30, 31, AND 32.
COMPLETE LIST OF REFERENCE LETTERS.
ch. Chelicerse. ch. g. Ganglion of chelicera?. c. 1. Caudal lobe. p. c. Primitive cumulus, pd. Pedipalpi. pr. I. Prreoral lobe. . pp 1 . // 2 . etc. Provisional appendages, sp. Spinnerets, st. Stomodreum.
I IV. Ambulatory appendages, i 16. Postoral segments.
Fig. i. Ovum, with primitive cumulus and streak proceeding from it.
Fig. 2. Somewhat later stage, in which the primitive cumulus is still visible. Near the opposite end of the blastoderm is a white area, which is probably therudiment of the procephalic lobe.
Fig. 3 and 3$. View of an embryo from the ventral surface and from the side when six segments have become established.
Fig. 4. View of an embryo, ideally unrolled, when the first rudiments of the appendages become visible.
Fig. 5. Embryo ideally unrolled at the stage when all the appendages have become established.
Fig. 6. Somewhat older stage, when the limbs begin to be jointed. Viewed from the side.
Fig. 7. Later stage, viewed from the side.
Fig. "ja. Same embryo as fig, 7, ideally unrolled.
Figs. 8 and 8/'. View from the ventral surface and from the side of an embryo, after the ventral flexure has considerably advanced.
Fig. 9. Somewhat older embryo, viewed from the ventral surface.
PLATES 31 AND 32.
COMPLETE LIST OF REFERENCE LETTERS.
ao. Aorta, ab. g. Abdominal nerve cord. ch. Cheliceraj. ch. g. Ganglion of chelicerae. ep. Epiblast. hs. Hemispherical lobe of supra-cesophageal ganglion. ///.Heart. //. Lower lip. m. Muscles, me. Mesoblast. mes. Mesenteron. mp.g. Malpighian tube. ms. Mesoblastic somite, cc. (Esophagus. /. c. Pericardium. pd. Pedipalpi. pd. g. Ganglion of pedipalpi. pr. Proctodxum (rectum), pr. c. Primitive cumulus, s. Septum in abdomen. st>. Somatopleure. sp. Splanchnopleure.
EXPLANATION OF PLATES 30, 31, 32. 695
st. Stomodseum. sit. Suctorial apparatus. sn. g. Supra-<esophageal ganglion. th. g. Thoracic ganglion, v. g. Ventral nerve cord, y, c. Cells derived from yolk. yk. Yolk. y. n. Nuclei of yolk cells.
I g IV g. Ganglia of ambulatory limbs, i 16. Postoral segments.
Fig. 10. Section through an ovum, slightly younger than fig. i. Shewing the primitive cumulus and the columnar character of the cells of one half of the blastoderm.
Fig. n. Section through an embryo of the same age as fig. 2. Shewing the median thickening of the blastoderm.
Fig. 12. Transverse section through the ventral plate of a somewhat older embryo. Shewing the division of the ventral plate into epiblast and mesoblast.
Fig. 13. Section through the ventral plate of an embryo of the same age as fig. 3, shewing the division of the mesoblast of the ventral plate into two mesoblastic bands.
Fig. 14. Transverse section through an embryo of the same age as fig. 5, passing through an abdominal segment above and a thoracic segment below.
Fig. 15. Longitudinal section slightly to one side of the middle line through an embryo of the same age.
Fig. 1 6. Transverse section through the ventral plate in the thoracic region of an embryo of the same age as fig. 7.
Fig. 17. Transverse section through the procephalic lobes of an embryo of the same age. gr. Section of hemicircular groove in procephalic lobe.
Fig. 1 8. Transverse section through the thoracic region of an embryo of the same age as fig. 8.
Fig. 19. Section through the procephalic lobes of an embryo of the same age.
Fig. 20 a, b, c, d, e. Five sections through an embryo of the same age as fig. 9. a and b are sections through the procephalic lobes, c through the front part of the thorax, d cuts transversely the posterior parts of the thorax, and longitudinally and horizontally the ventral surface of the abdomen, e cuts the posterior part of the abdomen longitudinally and horizontally, and shews the commencement of the mesenteron.
Fig. 21. Longitudinal and vertical section of an embryo of the same age. The section passes somewhat to one side of the middle line, and shews the structure of the nervous system.
Fig. 22. Transverse section through the dorsal part of the abdomen of an embryo of the same stage as fig. 9.