The Works of Francis Balfour 1-24
Embryology - 19 Apr 2024    Expand to Translate
|
|---|
| Google Translate - select your language from the list shown below (this will open a new external page) |
|
العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt These external translations are automated and may not be accurate. (More? About Translations) |
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.
| Online Editor |
|---|
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.
|
| Historic Disclaimer - information about historic embryology pages |
|---|
|
Draft Version - Notice removed when completed.
Vol I. Separate Memoirs (1885)
XXIV. A renewed study of the germinal layers of the Chick
BY F. M. BALFOUR AND F. DEIGHTON'.
(With Plates 43, 44, 45-)
THE formation of the germinal layers in the chick has been so often and so fully dealt with in recent years, that we consider some explanation to be required of the reasons which have induced us to add to the long list of memoirs on this subject. Our reasons are twofold. In the first place the principal results we have to record have already been briefly put forward in a Treatise on Comparative Embryology by one of us ; and it seemed desirable that the data on which the conclusions there stated rest should be recorded with greater detail than was possible in such a treatise. In the second place, our observations differ from those of most other investigators, in that they were primarily made with the object of testing a theory as to the nature of the primitive streak. As such they form a contribution to comparative embryology ; since our object has been to investigate how far the phenomena of the formation of the germinal layers in the chick admit of being compared with those of lower and less modified vertebrate types.
We do not propose to weary the reader by giving a new version of the often told history of the views of various writers on the germinal layers in the chick, but our references to other investigators will be in the main confined to a comparison of our results with those of two embryologists, who have published their memoirs since our observations were made. One of them is L. Gerlach, who published a short memoir 2 in April last, and
1 From the Quarterly Journal of Microscopical Science, Vol. xxn. N. S. 1882..
2 " Ueb. d. entodennale Entstehungsweise d. Chorda dorsal is," Biol. Ccntralblatt, Vol. I. Nos. i and i.
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 855
the other is C. Roller, who has published his memoir l still more recently. Both of them cover part of the ground of our investigations, and their results are in many, though not in all points, in harmony with our own. Both of them, moreover, lay stress on certain features in the development which have escaped our attention. We desired to work over these points again, but various circumstances have prevented our doing so, and we have accordingly thought it best to publish our observations as they stand, in spite of their incompleteness, merely indicating where the most important gaps occur.
Our observations commence at a stage a few hours after hatching, but before the appearance of the primitive streak.
The area pellucida is at this stage nearly spherical. In it there is a large oval opaque patch, which is continued to the hinder border of the area. This opaque patch has received the name of the embryonic shield a somewhat inappropriate name, since the structure in question has no very definite connection with the formation of the embryo.
Roller describes, at this stage, in addition to the so-called embryonic shield, a sickle-shaped opaque appearance at the hinder border of the area pellucida.
We have not made any fresh investigations for the purpose of testing Roller's statements on this subject.
Embryologists are in the main agreed as to the structure of the blastoderm at this stage. There is (PL 43, Ser. A, I and 2) the epiblast above, forming a continuous layer, extending over the whole of the area opaca and area pellucida. In the former its cells are arranged as a single row, and are cubical or slightly flattened. In the latter the cells are more columnar, and form, in the centre especially, more or less clearly, a double row ; many of them, however, extend through the whole thickness of the layer.
We have obtained evidence at this stage which tends to shew that at its outer border the epiblast grows not merely by the division of its own cells, but also by the addition of cells derived from the yolk below. The epiblast has been observed to extend itself over the yolk by a similar process in many invertebrate forms.
1 " Untersuch. lib. d. Blatterbildung im Hiihnerkeim," Archiv f. mikr. Atiat. Vol. xx. 1 88 1.
856 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
Below the epiblast there is placed, in the peripheral part of the area opaca, simply white yolk ; while in a ring immediately outside and concentric with the area pellucida, there is a closelypacked layer of cells, known as the germinal wall. The constituent cells of this wall are in part relatively small, of a spherical shape, with a distinct nucleus, and a granular and not very abundant protoplasm ; and in part large and spherical, filled up with highly refracting yolk particles of variable size, which usually render the nucleus (which is probably present) invisible (A, I and 2). This mass of cell rests, on its outer side, on a layer of white yolk.
The sickle-shaped structure, visible in surface veins, is stated by Koller to be due to a special thickening of the germinal wall. We have not found this to be a very distinctly marked structure in our sections.
In the region of the area pellucida there is placed below the epiblast a more or less irregular layer of cells. This layer is continuous, peripherally, with the germinal wall ; and is composed of cells, which are distinguished both by their flattened or oval shape and more granular protoplasm from the epiblastcells above, to which, moreover, they are by no means closely attached. Amongst these cells a few larger cells are usually present, similar to those we have already described as forming an important constituent of the germinal wall.
We have figured two sections of a blastoderm of this age (Ser. A, i and 2) mainly to shew the arrangement of these cells. A large portion of them, considerably more flattened than the remainder, form a continuous membrane over the whole of the area pellucida, except usually for a small area in front, where the membrane is more or less interrupted. This layer is the hypoblast (Jiy^). The remaining cells are interposed between this layer and the epiblast. In front of the embryonic shield there are either comparatively few or none of these cells present (Ser. A, i), but in the region of the embryonic shield they are very numerous (Ser. A, 2), and are, without doubt, the main cause of the opacity of this part of the area pellucida. These cells may be regarded as not yet completely differentiated segmentation spheres.
In many blastoderms, not easily distinguishable in surface
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 857
views from those which have the characters just described, the hypoblastic sheet is often much less completely differentiated, and we have met with other blastoderms, again, in which the hypoblastic sheet was completely established, except at the hinder part of the embryonic shield ; where, in place of it and of the cells between it and the epiblast, there was only to be found a thickish layer of rounded cells, continuous behind with the germinal wall.
In the next stage, of which we have examined surface views and sections, there is already a well-formed primitive streak.
The area pellucida is still nearly spherical, the embryonic shield has either disappeared or become much less obvious, but there is present a dark linear streak, extending from the posterior border of the area pellucida towards the centre, its total length being about one third, or even less, of the diameter of the area. This streak is the primitive streak. It enlarges considerably behind, where it joins the germinal wall. By Koller and Gerlach it is described as joining the sickle-shaped structure already spoken of. We have in some instances found the posterior end of the primitive streak extending laterally in the form of two wings (PL 45, fig. L). These extensions are, no doubt, the sickle ; but the figures given by Koller appear to us somewhat diagrammatic. One or two of the figures of early primitive streaks in the sparrow, given by Kupffer and Benecke 1 , correspond more closely with what we have found, except that in these figures the primitive streak does not reach the end of the area pellucida, which it certainly usually does at this early stage in the chick.
Sections through the area pellucida (PL 43, Ser. B and c) give the following results as to the structure of its constituent parts.
The epiblast cells have undergone division to a considerable extent, and in the middle part, especially, are decidedly more columnar than at an earlier stage, and distinctly divided into two rows, the nuclei of which form two more or less distinct layers.
In the region in front of the primitive streak the cells of the. lower part of the blastoderm have arranged themselves as- a 1 " Photogramme d. Ontogcnie d. Vogel." Nova Acta. K. Leop. Carol, Dattschen Akad. d. Naturfor, Bd. x. 41, 1879.
B. 55
858 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
definite layer, the cells of which are not so flat as is the case with the hypoblast cells of the posterior part of the blastoderm, and in the older specimens of this stage they are very decidedly more columnar than in the younger specimens.
The primitive streak is however the most interesting structure in the area pellucida at this stage.
The feature which most obviously strikes the observer in transverse sections through it is the fact, proved by Kolliker, that it is mainly due to a proliferation of the epiblast cells along an axial streak, which, roughly speaking, corresponds with the dark line visible in surface views. In the youngest specimens and at the front end of the primitive streak, the proliferated cells do not extend laterally beyond the region of their origin, but in the older specimens they have a considerable lateral extension.
The hypoblast can, in most instances, be traced as a distinct layer underneath the primitive streak, although it is usually less easy to follow it in that region than elsewhere, and in some cases it can hardly be distinctly separated from the superjacent cells.
The cells, undoubtedly formed by a proliferation of the epiblast, form a compact mass extending downwards towards the hypoblast ; but between this mass and the hypoblast there are almost always present along the whole length of the primitive streak a number of cells, more or less loosely arranged, and decidedly more granular than the proliferated cells. Amongst these loosely arranged cells there are to be found a certain number of large spherical cells rilled with yolk granules. Sometimes these cells are entirely confined to the region of the primitive streak, at other times they are continuous laterally with cells irregularly scattered between the hypoblast and epiblast (Ser.C,2), which are clearly the remnants of the undifferentiated cells of the embryonic shield. The junction between these cells and the cells of the primitive streak derived from the epiblast is often obscure, the two sets of cells becoming partially intermingled. The facility with which the cells we have just spoken of can be recognized varies moreover greatly in different instances. In some cases they are very obvious (Ser. C), while in other cases they can only be distinguished by a careful examination of good sections.
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 859
The cells of the primitive streak between the epiblast and the hypoblast are without doubt mesoblastic, and constitute the first portion of the mesoblast which is established. The section of these cells attached to the epiblast, in our opinion, clearly originates from the epiblast ; while the looser cells adjoining the hypoblast must, it appears to us, be admitted to have their origin in the indifferent cells of the embryonic shield, placed between the epiblast and the hypoblast, and also very probably in a distinct proliferation from the hypoblast below the primitive streak.
Posteriorly the breadth of the streak of epiblast which buds off the cells of the primitive streak widens considerably, and in the case of the blastoderm with the earliest primitive streaks extends into the region of the area opaca. The widening of the primitive streak behind is shewn in Ser. B, 3 ; Sen c, 2 ; and Ser. E, 4. Where very marked it gives rise to the sickle-shaped appearance upon which so much stress has been laid by. Roller and Gerlach. In the case of one of the youngest of our blastoderms of this stage in which we found in surface views (PI. 45, fig. L) a very well-marked sickle-shaped appearance at the hind end of the primitive streak, the appearance was caused, as is clearly brought out by our sections, by a thickening of the hypoblast of the germinal wall.
There is a short gap in our observations between the stage with a young primitive streak and the first described stage in which no such structure is present. This gap has been filled up both by Gerlach and Koller.
Gerlach states that during this period a small portion of the epiblast, within the region of the area opaca, but close to the posterior border of the area pellucida, becomes thickened by a proliferation of its cells. This portion gradually grows outwards laterally, forming in this way a sickle-shaped structure. From the middle of this sickle a process next grows forward into the area pellucida. This process is the primitive streak, and it is formed, like the sickle, of proliferating epiblast cells.
Koller 1 described the sickle and the growth forwards from it of the primitive streak in surface views somewhat before Gerlach;
1 " Beitr. z. Kenntniss d. Hiihnerkeims im Beginne cl. Bebriitung," Site. d. k. Akad. IViss. iv. Abth. 1879.
552
860 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
and in his later memoir has entered with considerable detail into the part played by the various layers in the formation of this structure.
He believes, as already mentioned, that the sickle-shaped structure, which appears according to him at an earlier stage than is admitted by Gerlach, is in the first instance due to a thickening of the hypoblast. At a later stage he finds that the epiblast in the centre of the sickle becomes thickened, and that a groove makes its appearance in this thickening which he calls the "Sichel-rinne." This groove is identical with that first described by Kupffer and Benecke 1 in the sparrow and fowl. We have never, however, found very clear indications of it in our sections.
In the next stage, Roller states that, in the region immediately in front of the "Sichel-rinne," a prominence appears which he calls the Sichelknopf, and from this a process grows forwards which constitutes the primitive streak. This structure is in main derived from a proliferation of epiblast cells, but Koller admits that some of the cells just above the hypoblast in the region of the Sichelknopf are probably derived from the hypoblast. Since these cells form part of the mesoblast it is obvious that Roller's views on the origin of the mesoblast of the primitive streak closely approach those which we have put forward.
The primitive streak starting, as we have seen, at the hinder border of the area pellucida, soon elongates till it eventually occupies at least two-thirds of the length of the area. As Roller (loc. cit.} has stated, this can only be supposed to happen in one of two ways, viz. either by a progression forward of the region of epiblast budding off mesoblast, or by an interstitial growth of the area of budding epiblast. Roller adopts the second of these alternatives, but we cannot follow him in doing so. The simplest method of testing the point is by measuring the distance between the front end of the primitive streak and the front border of the area pellucida at different stages of growth of the primitive streak. If this distance diminishes with the elongation of the primitive streak then clearly the second of the two alternatives is out of the question.
1 Die erstc Entwick. an Eier d. Reptilien, Konigsberg, 1878.
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 86l
We have made measurements to test this point, and find that the diminution of the space between the front end of the primitive streak and the anterior border of the area pellucida is very marked up to the period in which the medullary plate first becomes established. We can further point in support of our view to the fact that the extent of the growth lateralwards of the mesoblast from the sides of the primitive streak is always less in front than behind; which would seem to indicate that the front part of the streak is the part formed latest. Our view as to the elongation of the primitive streak appears to be that adopted by Gerlach.
Our next stage includes roughly the period commencing slightly before the first formation of a groove along the primitive streak, known as the primitive groove, and terminating immediately before the first trace of the notochord makes its appearance. After the close of the last stage the primitive streak gradually elongates, till it occupies fully two-thirds of the diameter of the area pellucida. The latter structure also soon changes its form from a circular to an oval, and finally becomes pyriform with the narrow end behind, while the primitive streak occupying two-thirds of its long axis becomes in most instances marked by a light linear band along the centre, which constitutes the primitive groove.
In surface views the primitive streak often appears to stop short of the hinder border of the area pellucida.
During the period in which the external changes, which we have thus briefly described, take place in the area pellucida, great modifications are effected in the characters of the germinal layers. The most important of these concern the region in front of the primitive streak; but they will be better understood if we commence our description with the changes in the primitive streak itself.
In the older embryos belonging to our last stage we pointed out that the mesoblast of the primitive streak was commencing to extend outwards from the median line in the form of two lateral sheets. This growth of the mesoblast is continued rapidly during the present stage, so that during the latter part of it any section through the primitive streak has approximately the characters of Ser. I, 5
862 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
The mesoblast is attached in the median line to the epiblast. Laterally it extends outwards to the edge of the area pellucida, and in older embryos may even form a thickening beyond the edge (fig. G). Beneath the denser part of the mesoblast, and attached to the epiblast, a portion composed of stellate cells may in the majority of instances be recognized, especially in the front part of the primitive streak. We believe these stellate cells to be in the main directly derived from the more granular cells of the previous stage. The hypoblast forms a sheet of flattened cells, which can be distinctly traced for the whole breadth of the area pellucida, though closely attached to the mesoblast above.
In sections we find that the primitive streak extends back to the border of the area pellucida, and even for some distance bayond. The attachment to the epiblast is wider behind; but the thickness of the mesoblast is not usually greater in the median line than it is laterally, and for this reason probably the posterior part of the streak fails to shew up in surface views. The thinning out of the median portion of the mesoblast of the primitive streak is shewn in a longitudinal section of a duck's blastoderm of this stage (fig. D). The same figure also shews that the hypoblastic sheet becomes somewhat thicker behind, and more independent of the parts above.
A careful study of the peripheral part of the area pellucida, in the region of the primitive streak, in older embryos of this stage, shews that the hypoblast is here thickened, and that its upjjer part, i.e. that adjoining the mesoblast, is often formed of stellate cells, many of which give the impression of being in the act of passing into the mesoblast above. At a later stage the mesoblast of the vascular area undoubtedly receives accessions of cells from the yolk below; so that we see no grounds for mistrusting the appearances just spoken of, or for doubting that they are to be interpreted in the sense suggested.
We have already stated that during the greater part of the present stage a groove, known as the primitive groove, is to be found along the dorsal median line of the primitive streak.
The extent to which this groove is developed appears to be subject to very great variation. On the average it is, perhaps, slightly deeper than it is represented in Ser. I, 5. In some cases
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 863
it is very much deeper. One of the latter is represented in fig. G. It has here the appearance of a narrow slit, and sections of it give the impression of the mesoblast originating from the lips of a fold; in fact, the whole structure appears like a linear blastopore, from the sides of which the mesoblast is growing out; and this as we conceive actually to be the true interpretation of the structure. Other cases occur in which the primitive groove is wholly deficient, or at the utmost represented by a shallow depression along the median axial line of a short posterior part of the primitive streak.
We may now pass to the consideration of the part of the area pellucida in front of the primitive streak.
We called attention to a change in the character of the hypoblast cells of this region as taking place at the end of the last stage. During the very early part of this stage the change in the character of these cells becomes very pronounced.
What we consider to be our earliest stage in this change we have only so far met with in the duck, and we have figured a longitudinal and median section to shew it (PI. 43, fig. D). The hypoblast (hy) has become a thick layer of somewhat cubical cells several rows deep. These cells, especially in front, are characterized by their numerous yolk spherules, and give the impression that part of the area pellucida has been, so to speak, reclaimed from the area opaca. Posteriorly, at the front end of the primitive streak, the thick layer of Jiypoblast, instead of being continuous with the flattened hypoblast tinder the primitive streak, falls, in the axial line, into the mesoblast of the primitive streak (PL 43, fig. D).
In a slightly later stage, of which we have specimens both of the duck and chick, but have only figured selected sections of a chick series, still further changes have been effected in the constitution of the hypoblast (PI. 44, Ser. H, I and 2).
Near the front border of the area pellucida (i) it has the general characters of the hypoblast of the duck's blastoderm just described. Slightly further back the cells of the hypoblast have become differentiated into stellate cells several rows deep, which can hardly be resolved in the axial line into hypoblast and mesoblast, though one can fancy that in places, especially laterally, they are partially differentiated into two layers. The axial
864 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
sheet of stellate cells is continuous laterally with cubical hypoblast cells.
As the primitive streak is approached an axial prolongation forwards of the rounded and closely-packed mesoblastic elements of the primitive streak is next met with ; and at the front end of the primitive streak, where this prolongation unites with the epiblast, it also becomes continuous with the stellate cells just spoken of. In fact, close to the end of the primitive streak it becomes difficult to say which mesoblast cells are directly derived from the primitive layer of hypoblast in front of the primitive streak, and which from the forward growth of the mesoblast of the primitive streak. There is, in fact, as in the earlier stage, a fusion of the layers at this point.
Sections of a slightly older chick blastoderm are represented in PI. 45, Ser.l, I, 2, 3, 4 and 5.
Nearly the whole of the hypoblast in front of the primitive streak has now undergone a differentiation into stellate cells. In the second section the products of the differentiation of this layer form a distinct mesoblast and hypoblast laterally, while in the median line they can hardly be divided into two distinct layers.
In a section slightly further back the same is true, except that we have here, in the axial line above the stellate cells, rounded elements derived from a forward prolongation of the cells of the primitive streak. In the next section figured, passing through the front end of the primitive streak, the axial cells have become continuous with the axial mesoblast of the primitive streak, while below there is an independent sheet of flattened hypoblast cells.
The general result of our observations on the part of the blastoderm in front of the primitive streak during this stage is to shew that the primitive hypoblast of this region undergoes considerable changes, including a multiplication of its cells; and that these changes result in its becoming differentiated on each side of the middle line, with more or less distinctness, into (i) a hypoblastic sheet below, formed of a single row of flattened cells, and (2) a mesoblast plate above formed of stellate cells, while in the middle line there is a strip of stellate cells in which there is no distinct differentiation into two layers.
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 865
Since the region in which these changes take place is that in which the medullary plate becomes subsequently formed, the lateral parts of the mesoblast plate are clearly the permanent lateral plates of the trunk, from which the mesoblastic somites, &c., become subsequently formed ; so that the main part of the 'mesoblast of the trunk is not directly derived from the primitive streak.
Before leaving this stage we would call attention to the presence, in one of our blastoderms of this stage, of a deep pit at the junction of the primitive streak with the region in front of it (PI. 44, Ser. F, I and 2). Such a pit is unusual, but we think it may be regarded as an exceptionally early commencement of that most variable structure in the chick, the neurenteric canal.
The next and last stage we have to deal with is that during which the first trace of the notochord and of the medullary plate make their appearance.
In surface views this stage is marked by the appearance of a faint dark line, extending forwards, from the front end of the primitive streak, to a fold, which has in the mean time made its appearance near the front end of the area pellucida, and constitutes the head fold.
PI. 45, Ser. K, represents a series of sections through a blastoderm of this stage, which have been selected to illustrate the mode of formation of the notochord.
In a section immediately behind the head fold the median part of the epiblast is thicker than the lateral parts, forming the first indication of a medullary plate (Ser. K, i). Below the median line of the epiblast is a small cord of cells, not divided into two layers, but continuous laterally, both with the hypoblast and mesoblast, which are still more distinctly separated than in the previous stage.
A section or so further back (Ser. K, 2) the axial cord, which we need scarcely say is the rudiment of the notochord, is thicker, and causes a slight projection in the epiblast above. It is, as before, continuous laterally, both with the mesoblast and with the hypoblast. The medullary plate is more distinct, and a shallow but unmistakable medullary groove has made its appearance.
866 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
As we approach the front end of the primitive streak the notochord becomes (Sen K, 3) very much more prominent, though retaining the same relation to the germinal layers as in front.
In the section immediately behind (Ser. K, 4) the convex upper surface of the notochord has become continuous with the epiblast for a very small region. The section, in fact, traverses the front end of the primitive streak.
In the next section the attachment between the epiblast and the cells below becomes considerably wider. It will be noticed that this part of the primitive streak is placed on the floor of the wide medullary groove, and there forms a prominence known as the anterior swelling of the primitive streak.
It will further be noticed that in the two sections passing through the primitive streak, the hypoblast, instead of simply becoming continuous with the axial thickening of the cells, as in front, forms a more or less imperfect layer underneath it. This layer becomes in the sections following still more definite, and forms part of the continuous layer of hypoblast present in the region of the primitive streak.
A comparison of this stage with the previous one shews very clearly that the notochord is formed out of the median plate of cells of the earlier stage, which was not divided into mesoblast and hypoblast, together with the short column of cells which grew forwards from the primitive streak;
The notochord, from its mode of origin, is necessarily contios -behind -with the axial cells of the primitive streak.
The sections immediately behind the last we have represented shew a rudiment of the neurenteric canal of the same form as that first figured by Gasser, viz. a pit perforating the epiblast with a great mass of rounded cells projecting upwards through it.
The observations just recorded practically deal with two much disputed points in the ontogeny of birds, viz. the origin of the mesoblast and the origin of the notochord.
With reference to the first of these our results are briefly as follows :
The first part of the mesoblast to be formed is that which arises in connection with the primitive streak. This part is in
RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 867
the main formed by a proliferation from an axial strip of the epiblast along the line of the primitive streak, but in part also from a simultaneous differentiation of hypoblast cells also along the axial line of the primitive streak. The two parts of the mesoblast so formed become subsequently indistinguishable. The second part of the mesoblast to be formed is that which gives rise to the lateral plates of mesoblast of the head and trunk of the embryo. This part appears as two plates one on each side of the middle line which arise by direct differentiation from the hypoblast in front of the primitive streak. They are continuous behind with the lateral wings of mesoblast which grow out from the primitive streak, and on their inner side are also at first continuous with the cells which form the notochord.
In addition to the parts of mesoblast, formed as just described, the mesoblast of the vascular area is in a large measure developed by a direct formation of cells round the nuclei of the germinal wall.
The mesoblast formed in connection with the primitive streak gives rise in part to the mesoblast of the allantois, and ventral part of the tail of the embryo (?), and in part to the vascular structures found in the area pellucida.
With reference to the formation of the mesoblast of the primitive streak, our conclusions are practically in harmony with those of Koller ; except that Koller is inclined to minimise the share taken by the hypoblast in the formation of the mesoblast of the primitive streak.
Gerlach, with reference to the formation of this part of the mesoblast, adopts the now generally accepted view of Kolliker, according to which the whole of the mesoblast of the primitive streak is derived from the epiblast.
As to the derivation of the lateral plates of mesoblast of the trunk from the hypoblast of the anterior part of the primitive streak, our general result is in complete harmony with Gerlach's results, although in our accounts of the details of the process we differ in some not unimportant particulars.
As to the origin of the notochord, our main result is that this structure is formed as an actual thickening of the primitive hypoblast of the anterior part of the area pellucida. We find
868 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
that it unites posteriorly with a forward growth of the axial tissue of the primitive streak, while it is laterally continuous, at first, both with the mesoblast of the lateral plates and with the hypoblast. At a later period its connection with the mesoblast is severed, while the hypoblast becomes differentiated as a continuous layer below it.
As to the hypoblastic origin of the notochord, we are again in complete accord with Gerlach ; but we differ from him in admitting that the notochord is continuous posteriorly with the axial tissue of the primitive streak, and also at first continuous with the lateral plates of mesoblast
The account we have given of the formation of the mesoblast may appear to the reader somewhat fantastic, and on that account not very credible. We believe, however, that if the view which has been elsewhere urged by one of us, that the primitive streak is the homologue of the blastopore of the lower vertebrates is accepted, the features we have described receive an adequate explanation.
The growth outwards of part of the mesoblast from the axial line of the primitive streak is a repetition of the well-known growth from the lips of the blastopore. It might have been anticipated that all the layers would fuse along the line of the primitive streak, and that the hypoblast as well as part of the mesoblast would grow out from it. There is, however, clearly a precocious formation of the hypoblast ; but the formation of the mesoblast of the primitive streak, partly from the epiblast and partly from the hypoblast, is satisfactorily explained by regarding the whole structure as the blastopore. The two parts of the mesoblast subsequently become indistinguishable, and their difference in origin is, on the above view, to be regarded as simply due to a difference of position, and not as having a deeper significance.
The differentiation of the lateral plates of mesoblast of the trunk directly from the hypoblast is again a fundamental feature of vertebrate embryology, occurring in all types from Amphioxus upwards, the meaning of which has been fully dealt with in the Treatise on Comparative Embryology by one of us. Lastly, the formation of the notochord from the hypoblast is the typical vertebrate mode of formation of this organ, while
EXPLANATION OF PLATES. 869
the fusion of the layers at the front end of the primitive streak is the universal fusion of the layers at the dorsal lip of the blastopore, which is so well known in the lower vertebrate types.
EXPLANATION OF PLATES 4345. N. B. The series of sections are in all cases numbered from before backwards.
LIST OF REFERENCE LETTERS.
a. p. Area pellucida. ep. Epiblast. ch. Notochord. gr. Germinal wall. hy. Hypoblast. m. Mesoblast. o. p. Area opaca. pr. g. Primitive groove. pv s. Primitive streak, yk. Yolk of germinal wall.
PLATE 43.
SERIES A, i and 2. Sections through the blastoderm before the appearance of primitive streak.
I. Section through anterior part of area pellucida in front of embryonic shield. The hypoblast here forms an imperfect layer. The figure represents about half the section, i. Section through same blastoderm, in the region of the embryonic shield. Between the epiblast and hypoblast are a number of undifferentiated cells. The figure represents considerably more than half the section.
SERIES B, i, 2 and 3. Sections through a blastoderm with a very young primitive streak.
i. Section through the anterior part of the area pellucida in front of the primitive streak. 2. Section through about the middle of the primitive streak. 3. Section through the posterior part of the primitive streak.
SERIES C, i and 2. Sections through a blastoderm with a young primitive streak. r. Section through the front end of the primitive streak. 2. Section through the primitive streak, somewhat behind i. Both figures shew very clearly the difference in character between the cells of the epiblastic mesoblast of the primitive streak, and the more granular cells of the mesoblast derived from the hypoblast.
FIG. D. Longitudinal section through the axial line of the primitive streak, and the part of the blastoderm in front of it, of an embryo duck with a well-developed primitive streak.
PLATE 44.
SERIES E, i, 2, 3 and 4. Sections through blastoderm with a primitive streak, towards the end of the first stage.
i. Section through the anterior part of the area pellucida. 2. Section a little way behind i shewing a forward growth of mesoblast from the primitive streak. 3. Section through primitive streak. 4. Section through posterior part of primitive streak, shewing the great widening of primitive streak behind.
8/0 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK.
SERIES F, i and 2. Sections through a blastoderm with primitive groove.
i. Section shewing a deep pit in front of primitive streak, probably an early indication of the neurenteric canal. 2. Section immediately following i.
FIG. G. Section through blastoderm with well-developed primitive streak, shewing an exceptionally deep slit-like primitive groove.
SERIES H, i and 2. Sections through a blastoderm with a fully-developed primitive streak.
i. Section through the anterior part of area pellucida, shewing the cubical granular hypoblast cells in this region. 2. Section slightly behind i, shewing the primitive hypoblast cells differentiated into stellate cells, which can hardly be resolved in the middle line into hypoblast and mesoblast.
PLATE 45.
SERIES I, i, 2, 3, 4 and 5. Sections through blastoderm somewhat older than Series H.
i. Section through area pellucida well in front of primitive streak. 2. Section through area pellucida just in front of primitive streak. 3. Section through the front end of primitive streak. 4. Section slightly behind 3. 5. Section slightly behind 4.
SERIES K, 1,2, 3, 4 and 5. Sections through a blastoderm in which the first traces of notochord and medullary groove have made their appearance. Rather more than half the section is represented in each figure, but the right half is represented in i and 3, and the left in 2 and 4.
i. Section through notochord immediately behind the head-fold. 2. Section shewing medullary groove a little behind i. 3. Section just in front of the primitive streak. 4 and 5. Sections through the front end of the primitive streak.
FlG. L. Surface view of blastoderm with a very young primitive streak.
XXV. The Anatomy and Development of Peripatus Capensis
(With Plates 4653.)
INTRODUCTION.
THE late Professor Balfour was engaged just before his death in investigating the structure and embryology of Peripatus capensis, with the view of publishing a complete monograph of the genus. He left numerous drawings intended to serve as illustrations to the monograph, together with a series of notes and descriptions of a large part of the anatomy of Peripatus capensis. Of this manuscript some portions were ready for publication, others were more or less imperfect ; while of the figures many were without references, and others were provided with only a few words of explanation.
It was obviously necessary that Professor Balfour's work embodying as it did much important discovery should be published without delay; and the task of preparing his material for the press was confided to us. We have printed all his notes and descriptions without alteration 2 . Explanations which appeared to be necessary, and additions to the text in cases in which he had prepared figures without writing descriptions, together with full descriptions of all the plates, have been added by us, and are distinguished by enclosure in square brackets 3 .
We have to thank Miss Balfour, Professor Balfour's sister, for the important service which she has rendered by preparing
1 From the Quarterly Jotirnal of Microscopical Science, April, 1883.
2 Excepting in an unimportant matter of change of nomenclature used with regard to the buccal cavity.
3 The account of the external characters, generative organs, and development, has been written by the editors.
872 ANATOMY AND DEVELOPMENT
a large part of the beautiful drawings with which the monograph is illustrated. Many of these had been executed by her under Professor Balfour's personal supervision ; and the knowledge of his work which she then acquired has been of the greatest assistance to us in preparing the MSS. and drawings for publication.
Since his death she has spared no pains in studying the structure of Peripatus, so as to enable us to bring out the first part of the monograph in as complete a state as possible. It is due to her skill' that the first really serviceable and accurate representation of the legs of any species of Peripatus available for scientific purposes are issued with the present memoir 1 .
We have purposely refrained from introducing comments on the general bearing of the new and important results set forth in this memoir, and have confined ourselves to what was strictly necessary for the presentation of Mr Balfour's discoveries in a form in which they could be fully comprehended.
Mr Balfour had at his disposal numerous specimens of Peripatus nova zealandia, collected for him by Professor Jeffrey Parker, of Christchurch, New Zealand ; also specimens from the Cape of Good Hope collected by Mr Lloyd Morgan, and brought to England by Mr Roland Trimen in 1881 ; and others given to him by Mr Wood Mason, together with all the material collected by Mr Moseley during the "Challenger" voyage.
A preliminary account of the discoveries as to the embryology of Peripatus has already been communicated to the Royal Society 2 . It is intended that the present memoir shall be followed by others, comprising a complete account of all the species of the genus Peripatus.
H. M. MOSELEY. A. SEDGWICK.
1 The drawings on PI. 47, figs. 9 and 10 on PI. 48, and the drawings of the embryos (except fig. 37), have been made by Miss Balfour since Professor Balfour's death.
5 Proc. Royal Soc. 1883.
OF PERIPATUS CAPENSTS. 873
PART I. DESCRIPTION OF THE SPECIES.
Peripatus capensis (fig. i).
[The body is elongated, and slightly flattened dorso-ventrally. The dorsal surface is arched, and darkly pigmented ; while the ventral surface is nearly flat, and of a lighter colour.
The mouth is placed at the anterior end of the body, on the ventral surface.
The anus is posterior and terminal.
The generative opening is single and median, and placed in both sexes on the ventral surface, immediately in front of the anus.
There are a pair of ringed antennae projecting from the anterior end of the head, and a pair of simple eyes, placed on the dorsal surface at the roots of the antennae.
The appendages of the body behind the antennae are disposed in twenty pairs.
1. The single pair of jaws placed within the buccal cavity in front of the true mouth opening, and consisting each of a papilla, armed at its termination with two cutting blades.
2. The oral papillae placed on each side of the mouth. At their apices the ducts of the slime glands open.
3. The seventeen pairs of ambulatory appendages, each provided with a pair of chitinous claws at its extremity.
4. The anal papillae placed on each side of the generative opening.
Colour. The following statements on this head are derived from observations of spirit specimens. The colour varies in different individuals. It always consists of a groundwork of green and bluish grey, with a greater or less admixture of brown. The chief variations in the appearance of the animal, so far as colour is concerned, depend on the shade of the green. In some it is dark, as in the specimen figured (fig. i) ; in others it is of a lighter shade.
There is present in most specimens a fairly broad light band on each side of the body, immediately dorsal to the attachment B. 56
8/4 ANATOMY AND DEVELOPMENT
of the legs. This band is more prominent in the lighter coloured vaiieties than in the dark, and is especially conspicuous in large individuals. It is due to a diminution in the green pigment, and an increase in the brown.
There is a dark line running down the middle of the dorsal surface, in the middle of which is a fine whitish line.
The ventral surface is almost entirely free from the green pigment, but possesses a certain amount of light brown. This brown pigment is more conspicuous and of a darker shade on the spinous pads of the foot.
In parts of the body where the pigment is scarce, it is seen to be confined to the papillae. This is especially evident round the mouth, where the sparse green pigment is entirely confined to the papillae.
In some specimens a number of white papillae, or perhaps light brown, are scattered over the dorsal surface ; and sometimes there is a scattering of green papillae all over the ventral surface. These two peculiarities are more especially noticeable in small specimens.
Ridges and Papilla of the Skin. The skin is thrown into a number of transverse ridges, along which the primary wartlike papillae are placed.
The papillae, which are found everywhere, are specially developed on the dorsal surface, less so on the ventral. The papillae round the lips differ from the remaining papillae of the ventral surface in containing a green pigment. Each papilla bears at its extremity a well-marked spine.
The ridges of the skin are not continued across the dorsal middle line, being interrupted by the whitish line already mentioned. Those which lie in the same transverse line as the legs are not continued on to the latter, but stop at the junction of the latter with the body. All the others pass round to the ventral surface and are continued across the middle line ; they do not, however, become continuous with the ridges of the other side, but passing between them gradually thin off and vanish.
The ridges on the legs are directed transversely to their long axes, i.e. are at right angles to the ridges of the rest of the body.
OF PERIPATUS CAPENSIS. 875
The -antennae are ringed and taper slightly till near their termination, where they present a slight enlargement in spirit specimens, which in its turn tapers to its termination.
The rings consist essentially of a number of coalesced primary papillae, and are, therefore, beset by a number of spines like those of the primary papillae (described below). They are more deeply pigmented than the rest of the antenna.
The free end of the antenna is covered by a cap of tissue like that of the rings. It is followed by four or more rings placed close together on the terminal enlargement. There appears to be about thirty rings on the antennae of all adults of this species. But they are difficult to count, and a number of small rings occur between them, which are not included in the thirty.
The antennae are prolongations of the dorso-lateral parts of the anterior end of the body.
The eyes are paired and are situated at the roots of the antennae on the dorso-lateral parts of the head. Each is placed on the side of a protuberance which is continued as the antenna, and presents the appearance of a small circular crystalline ball inserted on the skin in this region.
The rings of papillae on that part of the head from which the antennae arise lose their transverse arrangement. They are arranged concentrically to the antennal rings, and have a straight course forwards between the antennae.
The oral papillae are placed at the side of the head. They are attached ventro-Iaterally on each side of the lips. The duct of the slime gland opens through their free end. They possess two main rings of projecting tissue, which are especially pigmented on the dorsal side ; and their extremities are covered by papillae irregularly arranged.
The buccal cavity, jaws, and lips are described below.
The Ambulatory Appendages. The claw-bearing legs are usually seventeen in number ; but in two cases of small females we have observed that the anal papillae bear claws, and present all the essential features of the ambulatory appendages. In one small female specimen there were twenty pairs of clawbearing appendages, the last being like the claw-bearing anal papillae last mentioned, and the generative opening being placed between them.
56 2
8/6 ANATOMY AND DEVELOPMENT
The ambulatory appendages, with the exception of the fourth and fifth pairs in both sexes, and the last pair (seventeenth) in the male, all resemble each other fairly closely. A typical appendage (figs. 2 and 3) will first be described, and the small variations found in the appendages just mentioned will then be pointed out. Each consists of two main divisions, a larger proximal portion, the leg, and a narrow distal claw-bearing portion, the foot.
The leg has the form of a truncated cone, the broad end of which is attached to the ventro-lateral body-wall, of which it appears to be, and is, a prolongation. It is marked by a number of rings of primary papillae, placed transversely to the long axis of the leg, the dorsal of which contain a green and the ventral a brown pigment. These rings of papillae, at the attachment of the leg, gradually change their direction and merge into the body rings. At the narrow end of the cone there are three ventrally placed pads, in which the brown pigment is dark, and which are covered by a number of spines precisely resembling the spines of the primary papillae. These spinous pads are continued dorsally, each into a ring of papillae.
The papillae of the ventral row next the proximal of these spinous pads are intermediate in character between the primary papillae and the spinous pads. Each of these papillae is larger than a normal papilla, and bears several spines (fig. 2). This character of the papilla of this row is even more marked in some of the anterior legs than in the one figured ; it seems probable that the pads have been formed by the coalescence of several rows of papillae on the ventral surface of the legs. On the outer and inner sides of these pads the spines are absent, and secondary papillae only are present.
In the centre of the basal part of the ventral surface of the foot there are present a group of larger papillae, which are of a slightly paler colour than the others. They are arranged so as to form a groove, directed transversely to the long axis of the body, and separated at its internal extremity by a median papilla from a deep pit which is placed at the point of junction of the body and leg. The whole structure has the appearance, when viewed with the naked eye, of a transverse slit placed at the base of the leg. The segmental organs open by the deep pit placed
OF FERIPATUS CAPENSIS. 877
at the internal end of this structure. The exact arrangement of the papillae round the outer part of the slit does not appear to be constant.
The foot is attached to the distal end of the leg. It is slightly narrower at its attached extremity than at its free end, which bears the two claws. The integument of the foot is covered with secondary papillae, but spines and primary papillae are absent, except at the points now to be described.
On each side of the middle ventral line of the proximal end of the foot is placed an elliptical elevation of the integument covered with spines. Attached to the proximal and lateral end of this is a primary papilla. At the distal end of the ventral side of the foot on each side of the middle line is a group of inconspicuous pale elevations, bearing spines.
On the front side of the distal end of the foot, close to the socket in which the claws are placed, are two primary papillae, one dorsal and the other ventral.
On the posterior side of the foot the dorsal of these only is present. The claws are sickle-shaped, and placed on papillae on the terminal portion of the foot. The part of the foot on which they are placed is especially retractile, and is generally found more or less telescoped into the proximal part (as in the figure).
The fourth and fifth pairs of legs exactly resemble the others, except in the fact that the proximal pad is broken up into three, a small central and two larger lateral. The enlarged segmental organs of these legs open on the small central division.
The last (17) leg of the male (PL 47, fig. 4) is characterized by possessing a well-marked white papilla on the ventral surface. This papilla, which presents a slit-like opening at its apex, is placed on the second row of papillae counting from the innermost pad, and slightly posterior to the axial line of the leg.
The anal papillae, or as they should be called, generative papillae, are placed one on each side of the generative aperture. They are most marked in small and least so in large specimens. That they are rudimentary ambulatory appendages is shewn by the fact that they are sometimes provided with claws, and resemble closely the anterior appendages.]
8/8 ANATOMY AND DEVELOPMENT
PART II. ALIMENTARY CANAL.
The alimentary canal of Peripatus capensis forms, in the extended condition of the animal, a nearly straight tube, slightly longer than the body, the general characters of which are shewn in figs. 6 and 7.
For the purposes of description, it may conveniently be divided into five regions, viz. (i) the buccal cavity with the tongue, jaws, and salivary glands, (2) pharynx, (3) the oesophagus, (4) the stomach, (5) the rectum.
The Buccal Cavity. The buccal cavity has the form of a fairly deep pit, of a longitudinal oval form, placed on the ventral surface of the head, and surrounded by a tumid lip.
[The buccal cavity has been shewn by Moseley to be formed in the embryo by the fusion of a series of processes surrounding the true mouth-opening, and enclosing in their fusion the jaws.]
The 'lip is covered by a soft skin, in which are numerous organs of touch, similar to those in other parts of the skin having their projecting portions enclosed in delicate spines formed by the cuticle. The skin of the lips differs, however, from the remainder of the skin, in the absence of tubercles, and in the great reduction of the thickness of the dermis. It is raised into a series of papilliform ridges, whose general form is shewn in fig. 5 ; of these there is one unpaired and median behind, and a pair, differing somewhat in character from the remainder, in front, and there are, in addition, seven on each side.
The structures within the buccal cavity are shewn as they appear in surface views in figs. 5 and 7, but their real nature is best seen in sections, and is illustrated by PL 49, figs. II and 12, representing the oral cavity in transverse section, and by PL 49, figs. 17 and 1 8, representing it in horizontal longitudinal sections. In the median line of the buccal cavity in front is placed a thick muscular protuberance, which may perhaps conveniently be called the tongue, though attached to the dorsal instead of the ventral wall of the mouth. It has the form of an elongated
OF PERIPATUS CAPENSIS. 879
ridge, which ends rather abruptly behind, becoming continuous with the dorsal wall of the pharynx. Its projecting edge is armed by a series of small teeth, which are thickenings of the chitinous covering, prolonged from the surface of the body over the buccal cavity. Where the ridge becomes flatter behind, the row of teeth divides into two, with a shallow groove between them (PI. 48, fig. 7).
The surface of the tongue is covered by the oral epithelium, in parts of which are organs of special sense, similar to those in the skin; but its interior is wholly formed of powerful muscles. The muscles form two groups, intermingled amongst each other. There are a series of fibres inserted in the free edge of the tongue, which diverge, more or less obliquely, towards the skin at the front of the head anteriorly, and towards the pharynx behind. The latter set of fibres are directly continuous with the radial fibres of the pharynx. The muscular fibres just described are clearly adapted to give a sawing motion to the tongue, whose movements may thus, to a certain extent, be compared to those of the odontophor of a mollusc.
In addition to the above set of muscles, there are also transverse muscles, forming laminae between the fibres just described. They pass from side to side across the tongue, and their action is clearly to narrow it, and so cause it to project outwards from the buccal cavity.
On each side of the tongue are placed the jaws, which are, no doubt, a pair of appendages, modified in the characteristic arthropodan manner, to subserve mastication. Their structure has never been satisfactorily described, and is very complicated. They are essentially short papillae, moved by an elaborate and powerful system of muscles, and armed at their free extremities by a pair of cutting blades or claws. The latter structures are, in all essential points, similar to the claws borne by the feet, and, like these, are formed as thickenings of the cuticle. They have therefore essentially the characters of the claws and jaws of the Arthropoda, and are wholly dissimilar to the setae of Chsetopoda. The claws are sickle-shaped and, as shewn in PL 47, fig. 5, have their convex edge directed nearly straight forwards, and their concave or cutting edge pointed backwards. Their form differs somewhat in the different species, and, as will
88O ANATOMY AND DEVELOPMENT
be shewn in the systematic part of this memoir 1 , forms a good specific character. In Peripatus capensis (PI. 48, fig. 10) the cutting surface of the outer blade is smooth and without teeth, while that of the inner blade (fig. 9), which is the larger of the two, is provided with five or six small teeth, in addition to the main point. A more important difference between the two blades than that in the character of the cutting edge just spoken of, is to be found in their relation to the muscles which move them. The anterior parts of both blades are placed on two epithelial ridges, which are moved by muscles common to both blades (PI. 49, fig. 1 1). Posteriorly, however, the behaviour of the two blades is very different. The epithelial ridge bearing the outer blade is continued back for a short distance behind the blade, but the cuticle covering it becomes very thin, and it forms a simple epithelial ridge placed parallel to the inner blade. The cuticle covering the epithelial ridge of the inner blade is, on the contrary, prolonged behind the blade itself as a thick rod, which, penetrating backwards along a deep pocket of the buccal epithelium, behind the main part of the buccal cavity for the whole length of the pharynx, forms a very powerful lever, on which a great part of the muscles connected with the jaws find their insertion. The relations of the epithelial pocket bearing this lever are somewhat peculiar.
The part of the epithelial ridge bearing the proximal part of this lever is bounded on both its outer and inner aspect by a deep groove. The wall of the outer groove is formed by the epithelial ridge of the outer blade, and that of the inner by a special epithelial ridge at the side of the tongue. Close to the hinder border of the buccal cavity (as shewn in PL 49, fig. 12, on the right hand side), the outer walls of these two grooves meet over the lever, so as completely to enclose it in an epithelial tube, and almost immediately behind this point the epithelial tube is detached from the oral epithelium, and appears in section as a tube with a chitinous rod in its interior, lying freely in the body-cavity (shewn in PI. 49, figs. 13 16 le). This apparent tube is the section of the deep pit already spoken of. It may
1 Some material for this memoir was left by Prof. Balfour, which will be published separately.
OF PERIPATUS CAPENSIS. 88 1
be traced back even beyond the end of the pharynx, and serves along its whole length for the attachment of muscles.
The greater part of the buccal cavity is filled with the tongue and jaws just described. It opens dorsally and behind by the mouth into the pharynx, there being no sharp line of demarcation between the buccal cavity and the pharynx. Behind the opening into the pharynx there is a continuation of the buccal cavity shewn in transverse section in fig. 13, and in longitudinal and horizontal section in fig. 17, into which there opens the common junction of the two salivary glands. This diverticulum is wide at first and opens by a somewhat constricted mouth into the pharynx above (PL 49, fig. 13, also shewn in longitudinal and horizontal section in fig. 17). Behind it narrows, passing insensibly into what may most conveniently be regarded as a common duct for the two salivary glands (PL 49, fig. 17).
The Salivary Glands, These two bodies were originally described by Grube, by whom their nature was not made out, and subsequently by Moseley, who regarded them as fat bodies. They are placed in the lateral compartments of the body-cavity immediately dorsal to the ventral nerve cords, and extend for a very variable distance, sometimes not more than half the length of the body, and in other instances extending for nearly its whole length. Their average length is perhaps about twothirds that of the body. Their middle portion is thickest, and they thin off very much behind and to a slight extent in front. Immediately behind the mouth and in front of the first pair of legs, they bend inwards and downwards, and fall (fig. 7) one on each side into the hind end of the narrow section of the oral diverticulum just spoken of as the common duct for the two salivary glands. The glandular part of these organs is that extending back from the point where they bend inwards. This part (fig. 1 6) is formed of very elongated cells supported by a delicate membrana propria. The section of this part is somewhat triangular, and the cells are so long as to leave a comparatively small lumen. The nuclei of the cells are placed close to the supporting membrane, and the remainder of the cells arc filled with very closely packed secretory globules, which have a high index of refraction. It was the presence of these globules which probably led Moseley to regard the salivary glands as fat
882 ANATOMY AND DEVELOPMENT
bodies. The part of each gland which bends inwards must be regarded as the duct.
The cells lining the ducts are considerably less columnar than those of the gland proper. Their nuclei (fig. 14) are situated at the free extremities instead of at the base of the cells, and they are without secretory globules. The cells lining the ducts of the salivary glands pass, without any sharp line of demarcation, into those of the oral epithelium, which are flatter and have their nuclei placed in the middle.
The Pharynx. The Pharynx is a highly muscular tube (fig. 7) with a triangular lumen (figs. 14, 15), .which extends from the mouth to about half way between the first and second pair of legs. It is lined by a flattish epithelium bounded by a cuticle continuous with that of the mouth. On the dorsal side is a ridge projecting into the lumen of the pharynx. This ridge may be traced forwards (PI. 49, figs. II 14) into the tongue, and the two grooves at the side of this ridge, forming the two upper angles of the triangular lumen, may be followed into those at the sides of the tongue. The muscles of the pharynx are very highly developed, consisting of an intrinsic and an extrinsic set. The former consists, as is best seen in longitudinal sections, of (PI. 51, fig. 23) radial fibres, arranged in somewhat wedgeshaped laminae, between which are rings of circular fibres. The latter are thicker externally than internally, and so also appear wedge-shaped in longitudinal sections. Very characteristic of the pharynx are the two sympathetic nerves placed close to the two dorsal angles of the triangular lumen (fig. 14, sy).
The pharynx of Peripatus is interesting in that it is unlike, so far as I know, the pharynx of any true Arthropod, in all of which the region corresponding with the pharynx of Peripatus is provided with relatively very thin walls.
The pharynx of Peripatus has, on the other hand, a very close and obvious resemblance to that of many of the Chaetopoda, a resemblance which is greatly increased by the characteristic course of the sympathetic nerves.
The form of the lumen, as already pointed out by Grube, resembles that of the Nematoda.
Ttte (Esophagus. Behind the pharynx there follows a narrow oesophagus (fig. 7, o e] shewn in section in fig. 16. It has some
OF PERIPATUS CAPENSIS. 883
what folded and fairly thick walls, and lies freely in the central division of the body-cavity without any mesenteric support. Its walls are formed of five layers, viz. from without inwards.
(1) A peritoneal investment.
(2) A layer of longitudinal fibres.
(3) A layer of circular fibres, amongst which are numerous nuclei.
(4) A connective-tissue layer supporting (5) a layer of fairly columnar hyaline epithelium, bounded on its inner aspect by a cuticle continued from that of the pharynx. In front it passes insensibly into the pharynx, and beyond the region where the dorsal walls of the pharynx have clearly commenced, the ventral walls still retain the characters of the cesophageal walls. The oesophagus is vertically oval in front, but more nearly circular behind. Characteristic of the cesophagus is the junction of the two sympathetic nerves on its dorsal wall (fig. 16). These nerves cannot be traced far beyond their point of junction.
The Stomach. The next section of -the alimentary tract is the stomach or rnesenteron (fig. 6). It is by far the largest part of the alimentary tract, commencing at about the second pair of legs and extending nearly to the hind end of the body. It tapers both in front and behind, and is narrowest in the middle, and is marked off sharply both from the cesophagus in front and the rectum behind, and is distinguished from both of these by its somewhat pinker hue. In the retracted condition of the animal it is, as pointed out by Moseley, folded in a single short dorsal loop, at about the junction of its first with its second third, and also, according to my observations, at its junction with the rectum ; but in the extended condition it is nearly straight, though usually the posterior fold at the junction of the rectum is not completely removed. Its walls are always marked by plications which, as both Moseley and Grube have stated, do not in any way correspond with the segmentation of the body. In its interior I have frequently found the chitinous remains of the skins of insects, so that we are not justified in considering that the diet is purely vegetable. It lies free, and is, like the remainder of the alimentary tract, without a mesentery. The structure of the walls of the stomach has not hitherto been very satisfactorily described.
884 ANATOMY AND DEVELOPMENT
The connective tissue and muscular coats are extremely thin. There is present everywhere a peritoneal covering, and in front a fairly well-marked though very thin layer of muscles formed of an external circular and an internal longitudinal layer. In the middle and posterior parts, however, I was unable to recognize these two layers in section ; although in surface view Grube found an inner layer of circular fibres and an outer layer formed of bands of longitudinal fibres, which he regards as muscular.
The layer supporting the epithelium is reduced to a basement membrane. The epithelial part of the wall of the stomach is by far the thickest (fig. 20), and is mainly composed of enormously elongated, fibre-like cells, which in the middle part of the stomach, where they are longest, are nearly half a millimetre in length, and only about '006 mm. in breadth. Their nuclei, as seen in fig. 20, are very elongated, and are placed about a quarter of the length from the base.
The cells are mainly filled with an immense number of highly refracting spherules, probably secretory globules, but held by Grube, from the fact of their dissolving in ether, to be fat. The epithelial cells are raised into numerous blunt processes projecting into the lumen of the stomach.
In addition to the cells just described there are present in the anterior part of the stomach a fair sprinkling of mucous cells. There are also everywhere present around the bases of the columnar cells short cells with spherical nuclei, which are somewhat irregularly scattered in the middle and posterior parts of the stomach, but form in the front part a definite layer. I have not been able to isolate these cells, and can give no account of their function.
The rectum extends from the end of the stomach to the anus. The region of junction between the stomach and the rectum is somewhat folded. The usual arrangement of the parts is shewn in fig. 6, where the hind end of the stomach is seen to be bent upon itself in a U-shaped fashion, and the rectum extending forwards under this bent portion and joining the front end of the dorsal limb of the U. The structure of the walls of the rectum is entirely different to that of the stomach, and the transition between the two is perfectly sudden.
OF PERIPATUS CAPENSIS. 885
Within the peritoneal investment comes a well-developed muscular layer with a somewhat unusual arrangement of its layers, there being an external circular layer and an internal layer formed of isolated longitudinal bands. The epithelium is fairly columnar, formed of granular cells with large nuclei, and is lined by a prolongation of the external cuticle. It is raised into numerous longitudinal folds, which are visible from the surface, and give a very characteristic appearance to this part of the alimentary tract. The muscular layers do not penetrate into the epithelial folds, which are supported by a connective tissue layer.
NERVOUS SYSTEM.
The central nervous system consists of a pair of supra-cesophageal ganglia united in the middle line, and of a pair of widely divaricated ventral cords, continuous in front with the supra-cesophageal ganglia.
It will be convenient in the first instance to deal with the general anatomy of the nervous system and then with the histology.
Ventral Cords. The ventral cords at first sight appear to be without ganglionic thickenings, but on more careful examination they are found to be enlarged at each pair of legs (PI. 48, fig. 8). These enlargements may be regarded as imperfect ganglia. There are, therefore, seventeen such pairs of ganglia corresponding to the seventeen pairs of legs. There is in addition a ganglionic enlargement at the commencement of the cesophageal commissures, where the nerves to the oral papillae are given off (PL 51, fig. 22 or. g.\ and the region of junction between the cesophageal commissures with the supra-cesophageal ganglia, where another pair of nerves are given off to the jaws (PI. 51, fig. 22/0), may be regarded as the anterior ganglion of the ventral cords. There are, therefore, according to the above reckoning, nineteen pairs of ganglia connected with the ventral cords.
The ventral cords are placed each in the lateral compartments of the body-cavity, immediately within the longitudinal layer of muscles.
886 ANATOMY AND DEVELOPMENT
They are connected with each other, rather like the pedal nerves of Chiton and the lower Prosobranchiata, by a number of commissures. These commissures exhibit a fairly regular arrangement from the region included between the first and the last pair of true feet. There are nine or ten of them between each pair of feet (PI. 52, fig. 26). They pass along the ventral wall of the body, perforating the ventral mass of longitudinal muscles. On their way they give off nerves which innervate the skin.
In Peripatus nova zealandicz, and probably also in P. capensis, two of these nerves, coming off from each pair of ganglia, are distinguished from the remainder by the fact that they are provided with numerous nerve-cells, instead of being composed of nerve-fibres only, like the remaining commissures (PL 52, fig. 26 g co). In correlation with the nerves given off from them to the skin the commissures are smaller in the middle than at the two ends.
Posteriorly the two nerve-cords nearly meet immediately in front of the generative aperture, and between this aperture and the last pair of feet there are about six commissures passing between them (PL 48, fig. 8). Behind the generative aperture the two cords bend upwards, and, as is shewn in fig. 8, fall into each other dorsally to the rectum. The section of the two cords placed dorsally to the rectum is solely formed of nerve-fibres; the nerve-cells, present elsewhere, being here absent.
In front of the ganglion of the first foot the commissures have a more dorsal situation than in the remainder of the body. The median longitudinal ventral muscle here gradually thins out and comes to an end, while the commissures pass immediately below the wall of the pharynx (PL 49, figs. 14, 15). The ventral cords themselves at first approach very close to each other in this region, separating again, however, to envelope between them the pharynx (PL 51, fig. 22).
There are eleven commissures in front of the first pair of legs (PL 51, fig. 22). The three foremost of these are very close together, the middle one arising in a more ventral position than the other two, and joining in the median ventral line a peculiar mass of cells placed in contact with the oral epithelium (fig. 14). It is probably an organ of special sense.
OF PERIPATUS CAPENSIS. 887
The ventral cords give off a series of nerves from their outer borders, which present throughout the trunk a fairly regular arrangement. From each ganglion two large nerves (figs. 8, 22, 26) are given off, which, diverging somewhat from each other, pass into the feet, and, giving off branches on their way, may be traced for a considerable distance within the feet along their anterior and posterior borders.
In front of each of the pair of pedal nerves a fairly large nerve may be seen passing outwards towards the side of the body (fig. 22). In addition to this nerve there are a number of smaller nerves passing off from the main trunk, which do not appear to be quite constant in number, but which are usually about seven or eight. Similar nerves to those behind are given off from the region in front of the first pair of legs, while at the point where the two ventral cords pass into the oesophageal commissures two large nerves (fig. 22), similar to the pairs of pedal nerves, take their origin. These nerves may be traced forwards into the oral papillae, and are therefore to be regarded as the nerves of these appendages. On the ventral side of the cords, where they approach most closely, between the oral papillae and the first pair of legs, a number of small nerves are given off to the skin, whose distribution appears to be to the same region of the skin as that of the branches from the commissures behind the first pair of legs.
From the cesophageal commissures, close to their junction with the supra-cesophageal ganglia, a nerve arises on each side which passes to the jaws, and a little in front of this, apparently from the supra-cesophageal ganglion itself, a second nerve to the jaws also takes its origin (PI. 51, fig. 22 j n}.. These two nerves I take to be homologous with a pair of pedal nerves.
Between the nerves to the jaws and those to the oral papillae a number of small nerves take their origin. Three of these on each side pass in a dorsal direction and one or two in a ventral one.
The Supra-cesophageal Ganglia. The supra-cesophageal ganglia (figs. 8 and 22) are large, somewhat oval masses, broader in front than behind, completely fused in the middle, but free at their extremities. Each of them is prolonged anteriorly into an antennary nerve, and is continuous behind with one of the
888 ANATOMY AND DEVELOPMENT
cesophageal commissures. On the ventral surface of each, rather behind the level of the eye, is placed a very peculiar protuberance (fig. 22 d], of which I shall say more in dealing with the histology of the nervous system.
A number of nerves arise from the supra-cesophageal ganglia, mainly from their dorsal surface.
In front are the immense antennary nerves extending along the whole length of each antenna, and giving off numerous lateral twigs to the sense organs. Near the origin of the antennary nerves, and rather on the dorsal surface, there spring a few small twigs, which pass to the skin, and are presumably sensory. The largest of them is shewn in PI. 50, fig. 19 A. About one-third of the way back the two large optic nerves take their origin, also arising laterally, but rather from the dorsal surface (PL 50, fig. 19 D and E). Each of them joins a large ganglionic mass placed immediately behind the retina. Nearly on a level with the optic nerves and slightly nearer the middle dorsal line a pair of small nerves (fig. 19 D) spring from the brain and pass upwards, while nearly in the same line with the optic nerves and a little behind them a larger pair of nerves take their origin.
Behind all these nerves there arises from the line of suture between the two supra-cesophageal ganglia a large median nerve which appears to supply the integument of the dorsal part of the head (PL 48, fig. 8 ; PL 49, figs. 11 14 d it).
Sympathetic System. In addition to the nerves just described there are two very important nerves which arise near the median ventral line, close to the hind end of the supracesophageal ganglia. The origin of these two nerves is shewn in the surface view (fig. 22 sy, and in section in fig. n). They at first tend somewhat forwards and pass into the muscles near the epithelium lining the groove on each side of the tongue. Here they suddenly bend backwards again and follow the grooves into the pharynx.
The two grooves are continuous with the two dorsal angles of the pharynx ; and embedded in the muscles of the pharynx, in juxtaposition with the epithelium, these two nerves may easily be traced in sections. They pass backwards the whole length of the pharynx till the latter joins the oesophagus.
OF PERIPATUS CAPENSIS. 889
Here they at once approach and shortly meet in the median dorsal line (fig. 16). They can only be traced for a very short distance beyond their meeting point. These nerves are, without doubt, the homologues of the sympathetic system of Chaetopods, occupying as they do the exact position which Semper has shewn to be characteristic of the sympathetic nerves in that group, and arising from an almost identical part of the brain 1 .
Histology of the Nervous System.
Ventral Cords. The histology of the ventral cords and cesophageal commissures is very simple and uniform. They consist of a cord almost wholly formed of nerve-fibres, placed dorsally, and a ventral layer of ganglion cells (figs. 16 and 20).
The fibrous portion of the cord has the usual structure, being formed mainly of longitudinal fibres, each probably being a bundle of fibres of various sizes, enveloped in a sponge-work of connective tissue. The larger bundles of fibres are placed near the inner borders of the cords. In this part of the cord there are placed a very small number of ganglion cells.
The layer of ganglion cells is somewhat crescent-shaped in section, and, as shewn in figs. 16 and 20, envelopes the whole ventral aspect of the fibrous parts of the cord, and even creeps up slightly on to the dorsal side. It is thicker on the inner than on the outer side, and increases considerably in bulk at each ganglionic enlargement. The cells of which it is composed are for the most part of a nearly uniform size, but at the border of the fibrous matter a fair sprinkling of larger cells is found.
The tracheal vessels supplying the nervous system are placed amongst the larger cells, at the boundary between the ganglionic and fibrous regions of the cords.
With reference to the peripheral nerve-stems there is not much to be said. They have for the most part a similar structure to the fibrous parts of the main cord, but are provided with a somewhat larger number of cells.
1 Vide Spengel, " Oligognathus Boncllioc." Naples Mittheilungen, Bel. III. pi. iv. fig- 52 B. 57
890 ANATOMY AND DEVELOPMENT
Sheath of tlie Ventral Cords. The ventral cords are enveloped by a double sheath, the two layers of which are often in contact, while in other cases they may be somewhat widely separated from each other. The inner layer is extremely thin and always very closely envelopes the nerve-cords. The outer layer is thick and fibrous, and contains a fair sprinkling of nuclei.
Supra-cesophageal Ganglia. In the present state of our knowledge a very detailed description of the histology of the supracesophageal ganglia would be quite superfluous, and I shall confine myself to a description of the more obvious features in the arrangement of the ganglionic and fibrous portions (PI. 50, fig. 19 A G).
The ganglion cells are in the first place confined, for the most part, to the surface. Along the under side of each ganglion there is a very thick layer of cells, continuous behind, with the layer of ganglion cells which is placed on the under surface of the cesophageal commissures. These cells have, moreover, an arrangement very similar to that in the ventral cords, so that a section through the supra-cesophageal ganglia has an obvious resemblance to what would be the appearance of a section through the united ventral cords. On the outer borders of the ganglia the cells extend upwards, but they end on about the level of the optic nerve (fig. 19 D). Immediately dorsal to this point the fibrous matter of the brain is exposed freely on the surface (fig. 19 A, B, &c., a}. I shall call the region of fibrous matter so exposed the dorso-lateral horn of white matter.
Where the two ganglia separate in front the ganglion cells spread up the inner side, and arch over so as to cover part of the dorsal side. Thus, in the anterior part, where the two ganglia are separate, there is a complete covering of ganglionic substance, except for a narrow strip, where the dorso-lateral lobe of white matter is exposed on the surface (fig. 19 A). From the point where the two ganglia meet in front the nerve-cells extend backwards as a median strip on the dorsal surface (fig. 19 D and E). This strip, becoming gradually smaller behind, reaches nearly, though not quite, the posterior limit of the junction of the ganglia. Behind it there is, however, a region where
OF PERIPATUS CAPENSIS. 891
the whole dorsal surface of the ganglia is without any covering of nerve-cells.
This tongue of ganglion cells sends in, slightly behind the level of the eyes, a transverse vertical prolongation inwards into the white matter of the brain, which is shewn in the series of transverse sections in fig. 19 E, and also in the vertical longitudinal section (PL 51, fig. 21), and in horizontal section in PL 51, fig. 22.
On the ventral aspect of each lobe of the brain there is present a very peculiar, bluntly conical protuberance of ganglion cells (PL 51, fig. 22), which was first detected by Grube (No. 10), and described by him as "a white thick body of a regular tetrahedral form, and exhibiting an oval dark spot in the middle of two of the faces." He further states that it is united by a delicate nerve to the supra-cesophageal ganglion, and regards it as an organ of hearing.
In Peripatus capensis the organ in question can hardly be described as tetrahedral. It is rather, of a flattened oval form, and consists, as shewn in sections (PL 50, fig. 19 C and D, d\ mainly of ganglion cells. In its interior is a cavity with a distinct bounding membrane : the cells of which it is composed vary somewhat in size, being smallest near the point of attachment. At its free end is placed a highly refractive, somewhat oval body, probably forming what Grube describes as a dark spot, half embedded in its substance, and kept in place by the sheath of nervous matter surrounding it. This body appears to have fallen out in my sections. The whole structure is attached to the under surface of the brain by a very short stalk formed of a bundle of cells and nervous fibres.
It is difficult to offer any interpretation of the nature of this body. It is removed considerably from the surface of the animal, and is not, therefore, so far as I can see, adapted to serve as an organ of hearing.
The distribution of the white or fibrous matter of the ganglia is not very easy to describe.
There is a central lobe of white matter (fig. 19 E), which is continuous from ganglion to ganglion, where the two are united. It is smaller behind than in front. On its ventral side it exhibits fairly well-marked transverse commissural fibres, con 572
892 ANATOMY AND DEVELOPMENT
necting the two halves of the ganglion. Laterally and somewhat ventrally it is prolonged into a horn (fig. 19 D, E, b], which I propose calling the ventro-lateral horn. In front it is placed in a distinct protuberance of the brain, which is placed ventrally to and nearly in the same vertical plane as the optic nerve. This protuberance is best shewn in the view of the brain from below given in PL 51, fig. 22. This part of the horn is characterized by the presence of large vertically-directed bundles of nerve-fibres, shewn in transverse section in fig. 190. Posteriorly the diameter of this horn is larger than in front (fig. 19, E, F, G), but does not give rise to a protuberance on the surface of the brain owing to the smaller development of the median lobe behind.
The median lobe of the brain is also prolonged into a dorsolateral lobe (fig. 19, a], which, as already mentioned, is freely exposed on the surface. On its ventral border there springs the optic nerve, and several pairs of sensory nerves already described (fig. 19 D, E), while from its dorsal border a pair of sensory nerves also spring, nearly in the same vertical plane as the optic nerves.
Posteriorly where the dorsal surface of the brain is not covered in with ganglion cells the dorso-lateral horn and median lobe of the brain become indistinguishable.
In the front part of the brain the median lobe of white matter extends dorsalwards to the dorsal strip of ganglion cells, but behind the region of the transverse prolongation of these cells, into the white matter already described (p. 890), there is a more or less distinctly defined lobe of white matter on the dorsal surface, which I propose calling the postero-dorsal lobe of white matter. It is shewn in the transverse sections (fig. 19 F and G, c). It gradually thins away and disappears behind. It is mainly characterized by the presence on the ventral border of definite transverse commissural fibres.
OF PERIPATUS CAPENSIS. 893
THE SKIN.
The skin is formed of three layers.
1. The cuticle.
2. The epidermis or hypodermis.
3. The dermis.
The cuticle is a layer of about O'CO2 mm. in thickness. Its surface is not, however, smooth, but is everywhere, with the exception of the perioral region, raised into minute secondary papillae, the base of which varies somewhat in diameter, but is usually not far from O'O2 mm. On the ventral surface of the body these papillae are for the most part somewhat blunt, but on the dorsal surface they are more or less sharply pointed. In most instances they bear at their free extremity a somewhat prominent spine. The whole surface of each of the secondary papillae just described is in its turn covered by numerous minute spinous tubercles. In the perioral region, where the cuticle is smooth, it is obviously formed of two layers which easily separate from each other, and there is I believe a similar division elsewhere, though it is not so easy to see. It is to be presumed that the cuticle is regularly shed.
The epidermis, placed immediately within the cuticle, is composed of a single row of cells, which vary, however, a good deal in size in different regions of the body. The cells excrete the cuticle, and, as shewn in fig. 32, they stand in a very remarkable relation to the secondary papillae of the cuticle just described. Each epidermis cell is in fact placed within one of these secondary papillae, so that the cuticle of each secondary papilla is the product of a single epidermis cell. This relation is easily seen in section, while it may also be beautifully shewn by taking a part of the skin which is not too much pigmented, and, after staining it, examining from the surface.
In fig. 32 a region of the epidermis is figured, in which the cells are exceptionally columnar. The cuticle has, moreover, in the process of cutting the section, been somewhat raised and carried away from the subjacent cells. The cells of the epidermis are provided with large oval nuclei, which contain a well
894 ANATOMY AND DEVELOPMENT
developed reticulum, giving with low powers a very granular appearance to the nuclei. The protoplasm of the cells is also somewhat granular, and the granules are frequently so disposed as to produce a very well-marked appearance of striation on the inner end of the cells. The pigment which gives the characteristic colour to the skin is deposited in the protoplasm of the outer ends of the cells in the form of small granules. An attempt is made to shew this in fig. 32.
At the apex of most, if not all, the primary wart-like papillae there are present oval aggregations, or masses of epidermis cells, each such mass being enclosed in a thickish capsule (fig. 31). The cells of these masses appear to form the wall of a cavity which leads into the hollow interior of a long spine. These spines when carefully examined with high objectives present a rather peculiar structure. The base of the spine is enveloped by the normal cuticle, but the spine itself, which terminates in a very fine point, appears, as shewn in fig. 31, to be continuous with the inner layer of the cuticle. In the perioral region the outer layer of the cuticle, as well as the inner, appear to be continued to the end of the spines. Within the base of the spine there is visible a finely striated substance which may often be traced into the cavity enclosed by the cells, and appears to be continuous with the cells. Attached to the inner ends of most of the capsules of these organs a delicate fibrillated cord may be observed, and although I have not in any instance succeeded in tracing this cord into one of the nervestems, yet in the antennas, where the nerve-stems are of an enormous size, I have satisfied myself that the minute nerves leaving the main nerve-stems and passing out towards the skin are histologically not to be distinguished from these fibrillated cords. I have therefore but little hesitation in regarding these cords as nerves.
In certain regions of the body the oval aggregations of cells are extremely numerous ; more especially is this the case in the antennas, lips, and oral papillae. On the ventral surface of the peripheral rings of the thicker sections of the feet they are also very thick set (fig. 20 P). They here form a kind of pad, and have a more elongated form than in other regions. In the antennae they are thickly set side by side on the rings of skin
OF PERIPATUS CAPENSIS. 895
which give such an Arthropod appearance to these organs in Peripatus.
The arrangement of the cells in the bodies just described led me at first to look upon them as glands, but a further investigation induced me to regard them as a form of tactile organ. The arguments for this view are both of a positive and a negative kind.
The positive arguments are the following :
(1) The organs are supplied with large nerves, which is distinctly in favour of their being sense organs rather than glands.
(2) The peculiar striae at the base of the spines appear to me like the imperfectly preserved remains of sense hairs.
(3) The distribution of these organs favours the view that they are tactile organs. They are most numerous on the antennas, where such organs would naturally be present, especially in a case like that of Pe'ripatus, where the nerve passing to the antennas is simply gigantic. On the other hand, the antennae would not be a natural place to look for an enormous development of dermal glands.
The lips, oral papillae, and under surface of the legs, where these bodies are also very numerous, are situations where tactile organs would be of great use.
Under the head of negative arguments must be classed those which tell against these organs being glandular. The most important of these is the fact that they have no obvious orifice. Their cavities open no doubt into the spines, but the spines terminate in such extremely fine points that the existence of an orifice at their apex is hardly credible.
Another argument, from the distribution of these organs over the body is practically the converse of that already used. The distribution being as unfavourable to the view that they are glands, as it is favourable to that of their being sense organs.
THE TRACHEAL SYSTEM.
The apertures of the tracheal system are placed in the depressions between the papillae or ridges of the skin. Each of them leads into a tube, which I shall call the tracheal pit (fig. 30), the walls of which are formed of epithelial cells bounded
896 ANATOMY AND DEVELOPMENT
towards the lumen of the pit by a very delicate cuticular membrane continuous with the cuticle covering the surface of the body. The pits vary somewhat in depth; the pit figured was about O'CX) mm. It perforates the dermis and terminates in the subjacent muscular layer. The investigation of the inner end of the pit gave me some little trouble.
Transverse sections (fig. 30) through the trunk containing a tracheal opening shew that the walls of the pit expanded internally in a mushroom-like fashion, the narrow part being, however, often excentric in relation to the centre of the expanded part.
Although it was clear that the tracheae started from the expanded region of the walls of the pit, I could not find that the lumen of the pit dilated into a large vesicle in this part, and further investigation proved that the tracheae actually started from the slightly swollen inner extremity of the narrow part of the pit, the expanded walls of the pit forming an umbrella-like covering for the diverging bundles of tracheae.
I have, in fig. 30, attempted to make clear this relation between the expanded walls of the tracheal pits and the tracheae. In longitudinal sections of the trunk the tracheal pits do not exhibit the lateral expansion which I have just described, which proves that the divergence of the bundles of tracheae only takes place laterally and not in an antero-posterior direction. Cells similar in general character to those of the walls of the tracheal pits are placed between the branches of tracheae, and somewhat similar cells, though generally with more elongated nuclei, accompany the bundles of tracheae as far as they can be followed in my sections. The structure of these parts in the adult would, in fact, lead one to suppose that the tracheae had originated at the expense of the cells of pits of the epidermis, and that the cells accompanying the bundles of tracheae were the remains of cords of cells which sprouted out from the blind ends of the epidermis pits and gave rise in the first instance to the tracheae.
The tracheae themselves are extremely minute, unbranched (so far as I could follow them) tubes. Each opening by a separate aperture into the base of the tracheal pit, and measuring about O-QO2 mm. in diameter. They exhibit a faint transverse striation, which I take to be the indication of a spiral fibre.
OF PERIPATUS CAPENSIS. 897
[Moseley (Phil. Trans., 1874, PI. 73, fig. i) states that the tracheae branch, but only exceptionally.]
Situation of the tracheal apertures. Moseley states (No. 13) that the tracheae arise from the skin all over the surface of the body, but are especially developed in certain regions. He finds "a row of minute oval openings on the ventral surface of the body," the openings being "situate with tolerable regularity in the centres of the interspaces between the pairs of members, but additional ones occurring at irregular intervals. Other similar openings occur in depressions on the inner side of the conical foot protuberance." It is difficult in preserved specimens to make out the exact distributions of the tracheal apertures, but I have been able to make out certain points about them.
There is a double row of apertures on each side of the median dorsal line, forming two sub-dorsal rows of apertures. The apertures are considerably more numerous than the legs. There is also a double row of openings, again more numerous than the legs, on each side of the median ventral line between the insertions of the legs. Moseley speaks of a median row in this position. I think this must be a mistake.
Posteriorly the two inner rows approach very close to each other in the median ventral line, but I have never seen them in my section opening quite in the middle line. Both the dorsal and ventral rows are very irregular.
I have not found openings on the ventral or dorsal side of the feet but there are openings at the anterior and posterior aspects of the feet. There are, moreover, a considerable number of openings around the base of the feet.
The dorsal rows of tracheal apertures are continued into the head and give rise in this situation to enormous bundles of tracheae.
In front of the mouth there is a very large median ventral tracheal pit, which gives off tracheae to the ventral part of the nervous system, and still more in front a large number of such pits close together. The tracheae to the central nervous system in many instances enter the nervous system bound up in the same sheath as the nerves.
898 ANATOMY AND DEVELOPMENT
THE MUSCULAR SYSTEM.
The general muscular system consists of (i) the general wall of the body; (2) the muscles connected with the mouth, pharynx, and jaws; (3) the muscles of the feet; (4) the muscles of the alimentary tract.
The muscular wall of the body is formed of (i) an external layer of circular fibres; (2) an internal layer of longitudinal muscles; (3) a layer of transverse fibres.
The layer which I have spoken of as formed of circular fibres is formed of two strata of fibres which girth the body somewhat obliquely (PI. 51, fig. 25). In the outer stratum the rings are arranged so that their ventral parts are behind, while the ventral parts of the rings of the inner stratum are most forward. Both in the median dorsal and ventral lines the layer of circular fibres become somewhat thinner, and where the legs are attached the regularity of both strata is somewhat interfered with, and they become continuous with a set of fibres inserted in the wall of the foot.
The longitudinal muscles are arranged as five bands (vide fig. 1 6), viz. two dorsal, two lateral, and three ventral. The three ventral may be spoken of as the latero-ventral and medioventral bands.
The transverse fibres consist of (i) a continuous sheet on each side inserted dorsally in the cutis, along a line opposite the space between the dorsal bands of longitudinal fibres, and ventrally between the ventro-median and ventro-lateral bands. Each sheet at its insertion slightly breaks up into separate bands. They divide the body-cavity into three regions a median, containing the alimentary tract, slime glands, &c., and two lateral, which are less well developed, and contain the nervous system, salivary glands, segmental organs, &c.
(2) Inserted a little dorsal to the transverse band just described is a second band which immediately crosses the first, and then passes on the outer side of the nervous cord and salivary gland, where such is present, and is inserted ventrally in the space between the ventro-lateral and lateral longitudinal band.
OF PERIPATUS CAPENSIS. 899
Where the feet are given off the second transverse band becomes continuous with the main retractor muscular fibres in the foot, which are inserted both on to the dorsal side and ventral side.
Muscular system of the feet. This consists of the retractors of the feet connected with the outer transverse muscle and the circular layer of muscles. In addition to these muscles there are intrinsic transverse muscles which cross the cavity of the feet in various directions (PI. 51, fig. 20). There is no special circular layer of fibres.
Histology of the muscle, The main muscles of the body are unstriated and divided into fibres, each invested by a delicate membrane. Between the membrane and muscle are scattered nuclei, which are never found inside the muscle fibres. The muscles attached to the jaws form an exception in that they are distinctly transversely striated.
THE BODY-CAVITY AND VASCULAR SYSTEM.
The body-cavity, as already indicated, is formed of three compartments one central and two lateral. The former is by far the largest, and contains the alimentary tract, the generative organs, and the mucous glands. It is lined by a delicate endothelial layer, and is not divided into compartments nor traversed by muscular fibres.
The lateral divisions are much smaller than the central, and are shut .off from it by the inner transverse band of muscles. They are almost entirely filled with the nerve-cord and salivary gland in front and with the nerve-cord alone behind, and their lumen is broken up by muscular bands. They further contain the segmental organs which open into them. They are prolonged into the feet, as is the embryonic body-cavity of most Arthropoda.
The vascular system is usually stated to consist of a dorsal heart. I find between the dorsal bands of longitudinal fibres a vessel in a space shut off from the body-cavity by a continuation of the endothelial. lining of the latter (fig. 16). The vessel has definite walls and an endothelial lining, but I could not make out whether the walls were muscular. The ventral
9OO ANATOMY AND DEVELOPMENT
part of it is surrounded by a peculiar cellular tissue, probably, as suggested by Moseley, equivalent to the fat bodies of insects. It is continued from close to the hind end of the body to the head, and is at its maximum behind. In addition to this vessel there is present a very delicate ventral vessel, by no means easy to see, situated between the cutis and the outer layer of circular muscles.
SEGMENTAL ORGANS.
A series of glandular organs are found in Peripatus which have their external openings situated on the ventral surface of a certain number of the legs, and which, to the best of my belief, end internally by opening into the lateral compartments of the body-cavity. These organs are probably of an excretory nature, and I consider them homologous with the nephridia or segmental organs of the Chaetopoda.
In Peripatus capensis they are present in all the legs. In all of them (except the first three) the following parts may be recognized :
(1) A vesicular portion opening to the exterior by a narrow passage.
(2) A coiled portion, which is again subdivided into several sections.
(3) A terminal section ending by a somewhat enlarged opening into the lateral compartment of the body-cavity.
The last twelve pairs of these organs are all constructed in a very similar manner, while the two pairs situated in the fourth and fifth pairs of legs are considerably larger than those behind, and are in some respects very differently constituted.
It will be convenient to commence with one of the hinder nephridia. Such a nephridium from the ninth pair of legs is represented in fig. 28. The external opening is placed at the outer end of a transverse groove placed at the base of one of the feet, while the main portion of the organ lies in the body-cavity in the base of the leg, and extends into the trunk to about the level of the outer edge of the nerv.e-cord of its side. The external opening (p s) leads into a narrow tube (s d\ which gradually dilates into a large sack (s).
OF PERIPATUS CAPENS1S. QOI
The narrow part is lined by small epithelial cells, which are directly continuous with and perfectly similar to those of the epidermis (fig. 20). It is provided with a superficial coating of longitudinal muscular fibres, which thins out where it passes over the sack, along which it only extends for a short distance.
The sack itself, which forms a kind of bladder or collecting vesicle for the organ, is provided with an extremely thin wall, lined with very large flattened cells. These cells are formed of granular protoplasm, and each of them is provided with a large nucleus, which causes a considerable projection into the lumen of the sack (figs. 20, 29 s). The epithelial wall of the sack is supported by a membrana propria, over which a delicate layer of the peritoneal epithelium is reflected.
The coiled tube forming the second section of the nephridium varies in length, and by the character of the epithelium lining it may be divided into four regions. It commences with a region lined by a fairly columnar epithelium with smallish nuclei (fig. 28 s c i). The boundaries of the cells of this epithelium are usually very indistinct, and the protoplasm contains numerous minute granules, which are usually arranged in such a manner as to give to optical or real sections of the wall of this part of the tube a transversely striated appearance. These granules are very probably minute balls of excretory matter.
The nuclei of the cells are placed near their free extremities, contrary to what might have been anticipated, and the inner ends of the cells project for very different lengths into the interior, so causing the inner boundary of the epithelium of this part of the tube to have a very ragged appearance. This portion of the coiled tube is continuous at its outer end with the thin-walled vesicle. At its inner end it is continuous with region No. 2 of the coiled tube (fig. 28 s c 2), which is lined by small closely-packed columnar cells. This portion is followed by region No. 3, which has a very characteristic structure (fig. 28 s c 3). The cells lining this part are very large and flat, and contain large disc-shaped nuclei, which are usually provided with large nucleoli, and often exhibit a beautiful reticulum. They may frequently be observed in a state of division. The protoplasm of this region is provided with similar granules to that in the first region, and the boundaries of the cells are usually
902 ANATOMY AND DEVELOPMENT
very indistinct. The fourth region is very short (fig. 28 s c 4), and is formed of small columnar cells. It gradually narrows till it opens suddenly into the terminal section (s o t], which ends by opening into the body-cavity, and constitutes the most distinct portion of the whole organ. Its walls are formed of columnar cells almost filled by oval nuclei, which absorb colouring matters with very great avidity, and thus renders this part extremely conspicuous. The nuclei are arranged in several rows.
The study of the internal opening of this part gave me some trouble. No specimens ever shew it as rounded off in the characteristic fashion of tubes ending in a cul-de-sac. It is usually somewhat ragged and apparently open. In the best preserved specimens it expands into a short funnel-shaped mouth, the free edge of which is turned back. Sections confirm the results of dissections. Those passing longitudinally through the opening prove its edges are turned back, forming a kind of rudimentary funnel. This is represented in fig. 29, from the last leg of a female. I have observed remains of what I consider to be cilia in this section of the organ. The fourth region of the organ is always placed close to the thin-walled collecting vesicle (figs. 28 and 29). In the whole of the coiled tube just described the epithelium is supported by a membrana propria, which in its turn is invested by a delicate layer of peritoneal epithelium.
The fourth and fifth pairs are very considerably larger than those behind, and are in other respects peculiar. The great mass of each organ is placed behind the leg, on which the external opening is placed, immediately outside one of the lateral nerve-cords. Its position is shewn in fig. 8.
The external opening, instead of being placed near the base of the leg, is placed on the ventral side of the third ring (counting from the outer end) of the thicker portion of the leg. It leads (fig. 27) into a portion which clearly corresponds with the collecting vesicle of the hinder nephridia. This part is not, however, dilated into a vesicle in the same sort of way, and the cells which form the lining epithelium have not the same characteristic structure, but are much smaller. Close to the point where the vesicle joins the coiled section of the nephridium the
OF PERIPATUS CAPENSIS. 903
former has a peculiar nick or bend in it. At this nick it is firmly attached to the ventral side of the foot by muscles and tracheae, and when cut away from its attachment the muscles and tracheae cannot easily be detached from it. The main part of the coils are formed by region No. i, and the epithelial cells lining this part present very characteristically the striated appearance which has already been spoken of. The large-celled region of the coiled tube (fig. 2 ; ") is also of considerable dimensions, and the terminal portion is wedged in between this and the commencing part of the coiled tube. The terminal portion with its internal opening is in its histological characters exactly similar to the homologous region in the hinder nephridia.
The three pairs of nephridia in the three foremost pairs of legs are very rudimentary, consisting, so far as I have been able to make out, solely of the collecting vesicle and the duct leading from them to the exterior. The external opening is placed on the ventral side of the base of the feet, in the same situation as that of the posterior nephridia, but the histological
part of the body to which it belongs), does not acquire the
normal relations of a blastopore, but presents only those
rudimentary features (deep groove connected with origin of
mesoblast) which the whole blastopore of other tracheates
presents.
We think it probable that the larval anus eventually shifts to the hind end of the body, and gives rise to the adult anus. We reserve the account of the internal structure of these embryos (Stages A E) and of the later stages for a subsequent memoir.
We may briefly summarise the more important facts of the early development of Peripatus capensis, detailed in the preceding account.
1. The greater part of the mesoblast is developed from the walls of the archenteron.
2. The embryonic mouth and anus are derived from the respective ends of the original blastopore, the middle part of the blastopore closing up.
3. The embryonic mouth almost certainly becomes the adult mouth, i.e. the aperture leading from the buccal cavity into the pharynx, the two being in the same position. The embryonic anus is in front of the position of the adult anus, but in all probability. shifts back, and persists as the adult anus.
4. The anterior pair of mesoblastic somites gives rise to the swellings of the praeoral lobes, and to the mesoblast of the head 1 .
There is no need for us to enlarge upon the importance of these facts. Their close bearing upon some of the most important problems of morphology will be apparent to all, and we may with advantage quote here some passages from Balfour's Comparative Embryology, which shew that he himself long ago had anticipated and in a sense predicted their discovery.
"Although the mesoblastic groove of insects is not a gastrula, it is quite possible that it is the rudiment of a blastopore, the gastrula corresponding to which has now vanished
1 We have seen nothing in any of our sections which we can identify as of socalled mesenchymatous origin.
OF PERIPATUS CAPENSIS. 913
from development." (Comparative Embryology, Vol. I. p. 378, the original edition 1 .)
"TRACHEATA. Insecta. It (the mesoblast) grows inwards from the lips of the germinal groove, which probably represents the remains of a blastopore." (Comparative Embryology, Vol. II. p. 291, the original edition 2 .)
"It is, therefore, highly probable that the paired ingrowths of the mesoblast from the lips of the blastopore may have been, in the first instance, derived from a pair of archenteric diverticula." (Comparative Embryology, Vol. II. p. 294, the original edition 3 .)
The facts now recorded were discovered in June last, only a short time before Balfour started for Switzerland ; we know but little of the new ideas which they called up in his mind. We can only point to passages in his published works which seem to indicate the direction which his speculations would have taken.
After speculating as to the probability of a genetic connection between the circumoral nervous system of the Ccelenterata, and the nervous system of Echinodermata, Platyelminthes, Chaetopoda, Mollusca, &c., he goes on to say :
" A circumoral nerve-ring, if longitudinally extended, might give rise to a pair of nerve-cords united in front and behind exactly such a nervous system, in fact, as is present in many Nemertines (the Enopla and Pelagonemertes), in Peripatus and in primitive molluscan types (Chiton, Fissurella, &c.). From the lateral parts of this ring it would be easy to derive the ventral cord of the Chaetopoda and Arthropoda. It is especially deserving of notice, in connection with the nervous system of the above-mentioned Nemertines and Peripatus, that the commissure connecting the two nerve-cords behind is placed on the dorsal side of the intestines. As is at once obvious, by referring to the diagram (fig. 231 B), this is the position this commissure ought, undoubtedly, to occupy if derived from part of a nervering which originally followed more or less closely the ciliated edge of the body of the supposed radiate ancestor." (Comparative Embryology, Vol. II. pp. 311, 312, the original edition 4 .)
1 This edition, Vol. n. p. 457. 2 This edition, Vol. III. p. 352.
3 This edition, Vol. m. p. 356. 4 This edition, Vol. in. pp. 378, 379.
9 14 ANATOMY AND DEVELOPMENT OF PERIPATUS CAPENSIS.
The facts of development here recorded give a strong additional support to this latter view, and seem to render possible a considerable extension of it along the same lines.]
LIST OF MEMOIRS ON PERIPATUS.
1. M. Lansdown Guilding. "An Account of a New Genus of Mollusca," Zoological Journal, Vol. II. p. 443, 1826.
2. M. Andouin and Milne-Edwards. " Classific. des Anndlides et description de celles qui habitent les cotes de France," p. 411, Ann. Scien. Nat. ser. I. Vol. xxx. 1833.
3. M. Gervais. "Etudes p. servir a 1'histoire naturelle des Myriapodes," Ann. Scien. Nat. ser. n. Vol. vn. 1837, p. 38.
4. Wiegmann. Wiegmann's Archiv, 1837.
5. H. Milne-Edwards. "Note sur le Peripate juluforme" Ann. Scien. Nat. ser. n. Vol. xvm. 1842.
6. Blanchard. "Sur Forganisation des Vers," chap. IV. pp. 137 141, Ann. Scien. Nat. ser. in. Vol. Vlll. 1847.
7. Quatrefages. " Anat. des Hermelles, note on," p. 57, Ann. Scien. Nat. ser. in. Vol. x. 1848.
8. Quatrefages. Hist. Nat. des Anneles, 1865, Appendix, pp. 675 6.
9. De Blainville. SuppL au Diet, des Sc. Nat. Vol. I.
10. Ed. Grube. " Untersuchungen lib. d. Bau von Peripatus Edwardsii? Archiv fur Anat. und Physiol. 1853.
11. Saenger. " Moskauer Naturforscher Sammlung," Abth. Zool. 1869.
12. H. N. Moseley. "On the Structure and Development of Peripatus capensis? Proc. Roy. Soc. N.O. 153, 1874.
13. H. N. Moseley. " On the Structure and Development of Peripatus capensis," Phil. Trans. Vol. CLXIV. 1874.
14. H. N. Moseley. "Remarks on Observations by Captain Hutton, Director of the Otago Museum, on Peripatus novce zealandice," Ann. and Mag. of Nat. History, Jan. 1877.
15. Captain Hutton. " Observations on Peripatus novce sealandice," Ann. and Mag. of Nat. History, Nov. 1876.
16. F. M. Balfour. "On Certain Points in the Anatomy of Peripatus capensis" Quart. Journ. of Micr. Science, Vol. xix. 1879.
17. A. Ernst. Nature, March loth, 1881.
EXPLANATION OF PLATES. 915
EXPLANATION OF PLATES 4653!.
COMPLETE LIST OF REFERENCE LETTERS.
A. Anus. a. Dorso-lateral horn of white matter in brain, a.g. Accessory gland of male (modified accessory leg gland), at. Antenna, at. n. Antennary nerve, b. Ventro-lateral horn of white matter of brain. b. c. Body-cavity. bl. Blastopore. C. Cutis. c. Postero-dorsal lobe of white matter of brain. e.g. Supracesophageal ganglia, cl. Claw. c. m. Circular layer of muscles, co. Commissures between the ventral nerve-cords, co. i. Second commissure between the ventral nerve-cords. co 1 . 2. Mass of cells developed on second commissure, cor. Cornea, c. s. d. Common duct for the two salivary glands. . cu. Cuticle, d. Ventral protuberance of brain. d. 1. m. Dorsal longitudinal muscle of pharynx. d. n. Median dorsal nerve to integument from supraoesophageal ganglia, d. o. Muscular bands passing from the ventro-lateral wall of the pharynx at the region of its opening into the buccal cavity. E. Eye. E. Central lobe of white matter of brain, e. n. Nerves passing outwards from the ventral cords, ep. Epidermis, ep.c. Epidermis cells. F. i, F. a, &c. First and second pair of feet, c. f. Small accessory glandular tubes of the male generative apparatus. F.^. Ganglionic enlargement on ventral nerve-cord, from which a pair of nerves to foot pass off. f. gl. Accessory foot-gland. F. n. Nerves to feet. g. co. Commissures between the ventral nerve-cords containing ganglion cells, g. o. Generative orifice. H. Heart, h. Cells in lateral division of body-cavity. hy. Hypoblast, i.j. Inner jaw. j. Jaw. j. n. Nerves to jaws. L. Lips. /. Lens. /. b. c. Lateral compartment of body-cavity, le. Jaw lever (cuticular prolongation of inner jaw lying in a backwardly projecting diverticulum of the buccal cavity). /. m. Bands of longitudinal muscles. M. Buccal cavity. M 1 . Median backward diverticulum of mouth or common salivary duct which receives the salivary ducts, me. Mesenteron. mes. Mesoblastic somite, m. 1. Muscles of jaw lever, m. s. Sheets of muscle passing round the side walls of pharynx to dorsal body wall. od. Oviduct, ce. OZsophagus. a's. co. OZsophageal commissures, o.f. g. Orifice of duct of foot-gland, o.j. Outer jaw. op. Optic ganglion, op. n. Optic nerve, or.g. Ganglionic enlargements for oral papillae, o r. n. Nerves to oral papillae, or. p. Oral papillas. o. s. Orifice of duct of segmental organ, ov. Ovary, p. Pads on ventral side of foot. p. Common duct into which the vasa deferentia open. p. c. Posterior lobe of brain. /. d. c. Posterior commissure passing dorsal to rectum. /./. Internal opening of nephridium into body cavity, ph. Pharynx, pi. Pigment in outer ends of epidermic cells, pi. r. Retinal pigment, p. n. Nerves to feet. p.p. Primary papilla, pr. Prostate. R. Rectum. Re. Retinal rods. R. m. Muscle of claw. s. Vesicle of nephridium. j 1 . Part of 4th or 5th nephridium which corresponds to vesicle of other nephridia.
1 The explanations of the figures printed within inverted commas are by Professor Balfour, the rest are by the Editors.
91 6 EXPLANATION OF PLATES.
s. c. i. Region No. i of coiled tube of nephridium. s. c. 2. Region No. i of ditto. s. c. 3. Region No. 3 of ditto. s. c. 4. Region No. 4 of ditto, s. d. Salivary duct. s. g. Salivary gland, si. d. Reservoir of slime gland, sl.g. Tubules of slime gland. s. o. i, 2, 3, &c. Nephridia of ist, 2nd, &c., feet. s. o.f. Terminal portion of nephridium. s.p. Secondary papilla, st. Stomach, sf. e. Epithelium of stomach, sy. Sympathetic nerve running in muscles of tongue and pharynx, sy 1 . Origin of pharyngeal sympathetic nerves. T. Tongue, t. Teeth on tongue, te. Testis. tr. Trach.e0e. tr. c. Cells found along the course of the tracheae. tr. o. Tracheal stigma, tr. p. Tracheal pit. tit. Uterus, v. c. Ventral nerve cord. v. d. Vas deferens. v. g. Imperfect ganglia of ventral cord.
PLATE 46.
Fig. i. Peripatus capensis, x 4 ; viewed from the dorsal surface. (From a drawing by Miss Balfour. )
PLATE 47.
Fig. 2. A left leg of Peripatus capensis, viewed from the ventral surface ; x 30. (From a drawing by Miss Balfour.)
P'ig. 3. A right leg of Peripatus capensis, viewed from the front side. (From a drawing by Miss Balfour.)
Fig. 4. .The last left (i7th) leg of a male Peripatus capensis, viewed from the ventral side to shew the papilla at the apex of which the accessory gland of the male, or enlarged crural gland, opens to the exterior. (From a drawing by Miss Balfour.) Prof. Balfour left a rough drawing (not reproduced) shewing the papilla, to which is appended the following note. " Figure shewing the accessory genital gland of male, which opens on the last pair of legs by a papilla on the ventral side. The papilla has got a slit-like aperture at its extremity."
Fig. 5. Ventral view of head and oral region of Peripatus capensis. (From a drawing by Miss Balfour.)
PLATE 48.
Figs. 6 and 7 are from one drawing.
Fig. 6. Peripatus capensis dissected so as to shew the alimentary canal, slime glands, and salivary glands ; x 3. (From a drawing by Miss Balfour.)
Fig. 7. The anterior end of Fig. 6 enlarged ; x 6. (From a drawing by Miss Balfour.) The dissection is viewed from the ventral side, and the lips, L., have been cut through in the middle line behind and pulled outwards, so as to expose the jaws, /., which have been turned outwards, and the tongue, T. , bearing a median row of chitinous teeth, which branches behind into two. The junction of the salivary ducts, j. d., and the opening of the median duct so formed into the buccal cavity is also shewn. The muscular pharynx, extending back into the space between the ist and 2nd pairs of legs, is followed by a short tubular oesophagus. The latter opens into the large stomach with plicated walls, extending almost to the hind end of the animal. The stomach at its point of junction with the rectum presents an S-shaped ventrodorsal curve.
A. Anus. at. Antenna. F. i, K. 2. First and second feet. /. Jaws. L. Lips.
ae. OZsophagus. or. p. Oral papilla, ph. Pharynx. R. Rectum, s. d. Salivary
duct. s. g. Salivary gland, si. d. Slime reservoir, si. g. Portion of tubules of slime
gland, st. Stomach. T. Tongue in roof of mouth.
Fig. 8. Peripatus capensis, X4; male. (From a drawing by Miss Balfour.) Dissected so as to shew the nervous system, slime glands, ducts of the latter passing into the oral papilla, accessory glands opening on the last pair of legs (enlarged crural glands), and segmental organs, viewed from dorsal surface. The first three pairs of segmental organs consist only of the vesicle and duct leading to the exterior. The fourth and fifth pairs are larger than the succeeding, and open externally to the crural glands. The ventral nerve-cords unite behind dorsal to the rectum.
A. Anus. a. g. Accessory generative gland, or enlarged crural gland of the iyth leg. at. Antenna, c. g. Supra-oesophageal ganglia with eyes. co. Commissures between the ventral nerve-cords, d. n. Large median nerve to dorsal integument from hinder part of brain. F. i, i, &c. Feet. g. o. Generative orifice, <x. (Esophagus. KS. co. QEsophageal commissures, or. p. Oral papilla, p.d.c. Posterior dorsal commissure between the ventral nerve-cords, ph. Pharynx, p. n. Nerves to feet, one pair from each ganglionic enlargement. si. d. Reservoir of slime gland. si. g. Tubules of slime gland. s. o. i, 2, 3, &c. Segmental organs. v. c. Ventral nervecords, "v. g. Imperfect ganglia of ventral cords.
Figs. 9 and 10. Left jaw of Peripatus capensis (male), shewing reserve jaws. (From a drawing by Miss Balfour.)
Fig. 9. Inner jaw. Fig. 10. Outer jaw.
PLATE 49.
Figs, ii 16. A series of six transverse sections through the head of Peripatus capensis.
Fig. n. The section is taken immediately behind the junction of the supracesophageal ganglia, c. g., and passes through the buccal cavity, M., and jaws, o.j. and i.j.
Fig. 12. The section is taken through the hinder part of the buccal cavity at the level of the opening of the mouth into the pharynx and behind the jaws. The cuticular rod-like continuation (le.) of the inner jaw lying in a backwardly directed pit of the buccal cavity is shewn; on the right hand side the section passes through the opening of this pit.
Fig. 1 3. The section passes through the front part of the pharynx, and shews the opening into the latter of the median backward diverticulum of the mouth (M 1 ), which receives the salivary ducts. It also shews the commencement of the ventral nerve-cords, and the backwardly projecting lobes of the brain.
Fig. 14. The section passes through the anterior part of the pharynx at the level of the second commissure (co. 2), between the ventral nerve-trunks, and shews the mass of cells developed on this commissure, which is in contact with the epithelium of the backward continuation of the buccal cavity (M 1 ).
QI 8 EXPLANATION OF PLATES.
Fig. 15. Section through the point of junction of the salivary ducts with the median oral diverticulum.
Fig. 1 6. Section behind the pharynx through the oesophagus.
b. c. Body-cavity. C. Cutis. c. b. c. Central compartment of body-cavity, c. g. Supra-oesophageal ganglia, c. m. Layer of circular muscles, co. Commissure between ventral nerve-cords. co. i. Second commissure between the ventral nerve-cords. co 1 . i. Mass of cells developed on second commissure (probably sensory), c. s. d. Common duct for the two salivary glands, d. /. m. Dorsal longitudinal muscles of pharynx, d. o. Muscles serving to dilate the opening of the pharynx. Ep. Epidermis, e. n. Nerve passing outwards from ventral nerve-cord. H. Heart, i.j. Inner jaw. j. p. Jaw papillae. L. Lips of buccal cavity. /. b. c. Lateral compartment of body-cavity, le. Rod-like cuticular continuation of inner jaw, lying in a pit of the buccal cavity. /. m. Bands of longitudinal muscles. M. Buccal cavity. M 1 . Median backward continuation of buccal cavity, m. 1. Muscles of jaw lever, m. s. Muscular sheets passing from side walls of pharynx to dorsal body wall. ce. CEsophagus. ces. co. CEsophageal commissures. o.j. Outer jaw. ph. Pharynx, s. d. Salivary duct. s. g. Salivary gland, si. d. Reservoir of slime gland, sy. Sympathetic nerves running in muscles of tongue or pharynx, sy 1 . Origin of sympathetic nerves to pharynx. T. Tongue, v. c. Ventral nerve-cords.
Figs. 17, 1 8. Two longitudinal horizontal sections through the head of Peripatus capensis. Fig. 17 is the most ventral. They are both taken ventral to the cerebral ganglia. In Fig. 17 dorsal tracheal pits are shewn with tracheae passing off from them. (Zeiss a a, Hartnack's camera.) C. Cutis. c. s. d. Common salivary duct. ep. Epidermis, i.j. Inner jaw. M. Buccal cavity. M 1 . Median backward diverticulum of mouth, o.j. Outer jaw. s. d. Salivary ducts. T. Tongue, t. Teeth on tongue, tr. Tracheae, tr. p. Tracheal pits.
PLATE 50.
Fig. 19. "A, B, c, D, E, F, G. Seven transverse sections illustrating the structure of the- supra- cesophageal ganglia. (Zeiss A, Hartnack's camera.) a. Dorso-lateral horn of white matter. b. Ventro-lateral horn of white matter, c. Postero-dorsal lobe of white matter, d. Ventral protuberance of brain, e. Central lobe of white matter, o.p. Optic ganglion.
" A. Section through anterior portions of ganglia close to the origin of the antennary nerve. B. Section a little in front of the point where the two ganglia unite, c. Section close to anterior junction of two ganglia. D. Section through origin of optic nerve on the right side. E. Section shewing origin of the optic nerve on the left side. F. Section through the dorso-median lobe of white matter. G. Section near the termination of the dorsal tongue of ganglion cells."
PLATE 51.
Fig. 10. Portion of a transverse section through the hinder part of Peripatus capensis (male). The section passes through a leg, and shews the opening of the segmental organ (p. s.), and of a crural gland, o.f.g., and the forward continuation of the enlarged crural gland of the i7th leg (/ g!.). (Zeiss a a, Hartnack's camera.) a-g. accessory gland of male (modified crural gland of last leg), c. Cutis. cL Claw. cu. Cuticle, ep. Epidermis, f.gl. Crural gland, h. Cells in lateral compartment of body cavity, o.f. g. Orifice of accessory foot gland, o. s. Opening of segmental organ, p. Three spinous pads on ventral surface of foot. pr. Prostate. R. M. Retractor muscle of claw. s. Vesicle of nephridium. s. c. i. Region No. i of coiled part of nephridium. si. g. Tubule of slime gland, s. o. t. Terminal portion of nephridium. st. Stomach, st. e. Epithelium of stomach, v. c. Ventral nerve-cord, v. d. Vas deferens.
Fig. 21. "Longitudinal vertical section through the supra-oesophageal ganglion and oesophageal commissures of Peripatus capensis. (Zeiss a a, Hartnack.)" at. Antenna, e. Central lobe of white matter. /. Part of jaw. s. g. Salivary gland.
Fig. 22: drawn by Miss Balfour. Brain and anterior part of the ventral nervecords of Peripatus capensis enlarged and viewed from the ventral surface. The paired appendages (d) of the ventral surface of the brain are seen, and the pair of sympathetic nerves (sy 1 ) arising from the ventral surface of the hinder part.
From the commencement of the cesophageal commissures (as. co. ) pass off on each side a pair of nerves to the jaws (/. .).
The three anterior commissures between the ventral nerve-cords are placed close together; immediately behind them the nerve-cords are swollen, to form the ganglionic enlargements from which pass off to the oral papillce a pair of large nerves on each side (or. n. )
Behind this the cords present a series of enlargements, one pair for each pair of feet, from which a pair of large nerves pass off on each side to the feet (p. n). at. n. Antennary nerves, co. Commissures between ventral cords, d. Ventral appendages of brain. E. Eye. e. n. Nerves passing outwards from ventral cord. F-g- Ganglionic enlargements from which nerves to feet pass off. j. n. Nerves to jaws. or. g. Ganglionic enlargement from which nerves to oral papillce pass off. or. n. Nerves to oral papillae, p.c. Posterior lobe of brain, p. n. Nerves to feet. s.y. Sympathetic nerves.
Fig. 23. "Longitudinal horizontal section through the head of Peripatus capensis, shewing the structure of the brain, the antennary and optic nerves, &c. (Zeiss a a, Hartnack's camera.)" at. Antenna, at. n. Antennary nerve, cor. Cornea, e. Central mass of white matter. /. Lens. op. n. Optic nerve, ph. Pharynx, p.p. Primary papilla covered with secondary papillte and terminating in a long spine, sy. Pharyngeal sympathetic nerves.
Fig. 24. "Eye of Peripatus capensis, as shewn in a longitudinal horizontal section through the head. The figure is so far diagrammatic that the lens is represented as filling up the whole space between the rods and the cornea. In the actual section there is a considerable space between the parts, but this space is probably artificial, being in part caused by the shrinkage of the lens and in part by the action of the razor. (Zeiss c, Hartnack's camera.)" (It appears that the ganglionic region of the eye is covered by a thin capsule, which is omitted in the figure.)
cor. Cornea. /. Lens. op. Optic ganglion, op-, n. Optic nerve. //'. r. Pigment. Re. rods. s. p. Secondary papillae.
Fig. 25. Longitudinal horizontal section through the dorsal skin, shewing the
peculiar arrangement of the circular muscular fibres. (Zeiss A, Hartnack's camera.)
PLATE 52.
Fig. 26. Portion of ventral cord of Peripatus capensis enlarged, shewing two ganglionic enlargements and the origin of the nerves and commissures. (From a drawing by Miss Balfour.)
co. Commissures. E. n. Nerves passing out from ventral cords. F. n. Nerves to feet. g. co. Commissures between the ventral cords containing ganglion cells, v. g. Ganglionic enlargements.
Fig. 27. Segmental organ from the 5th pair of legs of Peripatus capensis. This nephridium resembles those of the 4th legs, and differs from all the others in its large size and in the absence of any dilatation giving rise to a collecting vesicle on its external portion (enlarged). The terminal portion has the same histological characters as in the case of the hinder segmental organs. (From a drawing by Miss Balfour. )
Fig. 28. Segmental organ or nephridium from the 9th pair of legs of Peripatus capensis^ shewing the external opening, the vesicle, the coiled portion and the terminal portion with internal opening (enlarged). (From a drawing by Miss Balfour.)
o. s. External opening of segmental organ, p.f. Internal opening of nephridium into the body-cavity (lateral compartment). s. Vesicle of segmental organ, j 1 . Portion of segmental organ of 4th and 5th legs, corresponding to vesicle of the other nephridia. s. c. i. First or external portion of coiled tube of nephridium, lined by columnar epithelium with small nuclei ; the cells project for very different distances, giving the inner boundary of this region a ragged appearance, s. c. 2. Region No. 2 of coiled tube of nephridium, lined by small closely-packed columnar cells, s. c. 3. Region No. 3 of coiled tube of segmental organ, lined by large flat cells with large disc-shaped nuclei, s. c. 4. Region No. 4 of coiled tube of nephridium ; this region is very short and lined by small columnar cells, s. o. t. Terminal portion of nephridium.
Fig. 29. " Portion of nephridium of the hindermost leg of Peripatus capensis, seen in longitudinal and vertical section. The figure is given to shew the peritoneal funnel of the nephridium. Portions of the collecting sack (s.) and other parts are also represented. (Zeiss B, Hartnack's camera.)"
p.f. Peritoneal funnel, s. Vesicle, s.c.i, s.c.i, s.c.$. Portions of coiled tube.
Fig. 30. " Section through a tracheal pit and diverging bundles of tracheal tubes" taken transversely to the long axis of the body. (Zeiss E, oc. 2.) (From a rough drawing by Prof. Balfour.)
tr. Tracheae, shewing rudimentary spiral fibre, tr. c. Cells resembling those lining the tracheal pits, which occur at intervals along the course of the trachere. tr. s. Tracheal stigma, tr. p. Tracheal pit.
Fig- 31. "Sense organs and nerves attached from antenna of Peripatus capensis (Zeiss, immersion 2, oc. 2.)" (From a rough drawing by Prof. Balfour.) The figure shews the arrangement of the epidermis cells round the base of the spine. The spine is seen to be continuous with the inner layer of the cuticle.
EXPLANATION OF PLATE 53. 92 1
Fig. 32. Section through the skin of Peripatus capensis ; it shews the secondary papillae covered with minute spinous tubercles and the relation of the epidermis to them. (The cuticle in the process of cutting has been torn away from the subjacent cells.) The cells of the epidermis are provided with large oval nuclei, and there is a deposit of pigment in the outer ends of the cells. The granules in the protoplasm of the inner ends of the cells are arranged in lines, so as to give a streaked appearance. (Zeiss E, oc. 2.) (From a rough drawing by Prof. Balfour.)
c. Dermis. cu. Cuticle, ep. c. Epidermis cells, pi. Deposit of pigment in outer ends of epidermis cells, s.p. Secondary papillae.
Fig. 33. Female generative organs of Peripatus capensis, x 5. (From a rough drawing by Prof. Balfour.) The following note was appended to this drawing: "Ovary rather to dorsal side, lying in a central compartment of body-cavity and attached to one of the longitudinal septa, dividing this from the lateral compartment between the penultimate pair of legs and that next in front. The oviducts cross before opening to the exterior, the right oviduct passing under the rectum and the left over it. They meet by opening into a common vestibule, which in its turn opens below the anus. On each side of it are a pair of short papillae (aborted feet ?)."
F. 16, 17. Last two pairs of legs. od. Oviduct, ov. Ovary, ut. Uterus, v. c. Nerve-cord.
PLATE 53.
Cite this page: Hill, M.A. (2024, April 19) Embryology The Works of Francis Balfour 1-24. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/The_Works_of_Francis_Balfour_1-24
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
