Chapter XII: The Organs of Excretion (8)

As is shown in fig. 1, the notochord (_ch._) has now throughout the region of the embryo become separated from the subjacent hypoblast, and the lateral plates of mesoblast are distinctly divided into somatic and splanchnic layers. The medullary groove is continued as a deepish groove up to the opening of the neurenteric passage, which thus forms a perforation in the floor of the hinder end of the medullary groove (vide Series B, figs. 2, 3, and 4).

The passage itself is somewhat shorter than in the previous stage, and the whole of it is shown in a single section (fig. 4). This section must either have been taken somewhat obliquely, or else the passage have been exceptionally short in this embryo, since in an older embryo it could not all be seen in one section.

The front wall of the passage is continuous with the notochord, which for two sections or so in front remains attached to the hypoblast (figs. 2 and 3). Behind the perforation in the floor of the medullary groove is placed the primitive streak (fig. 5), where all the layers become fused together, as in the earlier stage. Into this part a narrow diverticulum from the end of the medullary groove is continued for a very short distance (vide fig. 5, _mc._).

The general features of the stage will best be understood by an examination of the diagrammatic longitudinal section, represented in woodcut, fig. 1. In front is shown the amnion (_am._), growing over the head of the embryo. The notochord (_ch._) is seen as an independent cord for the greater part of the length of the embryo, but falls into the hypoblast shortly in front of the neurenteric passage. The neurenteric passage is shown at _ne._, and behind it is shown the primitive streak.

In a still older stage, represented in surface view on Pl. 29, fig. C, the medullary folds have nearly met above, but have not yet united. The features of the passage from the neural groove to the hypoblast are precisely the same in the embryo just described, although the lumen of the passage has become somewhat narrower. There is still a short primitive streak behind the embryo.

The neurenteric passage persists but a very short time after the complete closure of the medullary canal. It is in no way connected with the allantois, as conjectured by Kupffer and Benecke, but the allantois is formed, as I have satisfied myself by longitudinal sections of a later stage, in the manner already described by Dobrynin, Gasser, and Kölliker for the bird and mammal.

The general results of Kupffer's and Benecke's observations, with the modifications introduced by my own observations, are as follows:--After the segmentation and the formation of the embryonic shield (area pellucida) the blastoderm becomes distinctly divided into epiblast and hypoblast[449]. At the hind end of the shield a somewhat triangular primitive streak is formed by the fusion of the epiblast and hypoblast with a number of cells between them, which are probably derived from the lower rows of the segmentation cells. At the front end of the streak a passage arises, open at both extremities, leading obliquely forwards through the epiblast to the space below the hypoblast. The walls of the passage are formed of a layer of columnar cells continuous both with epiblast and hypoblast. In front of the primitive streak the body of the embryo becomes first differentiated by the formation of a medullary plate, and at the same time there grows out from the primitive streak a layer of mesoblast, which spreads out in all directions between the epiblast and hypoblast. In the axis of the embryo the mesoblast plate is stated by Kupffer and Benecke to be continuous across the middle line, but this appears very improbable. In a slightly later stage the medullary plate becomes marked by a shallow groove, and the mesoblast of the embryo is then undoubtedly constituted of two lateral plates, one on each side of the median line. In the median line the notochord arises as a ridge-like thickening of the hypoblast, which becomes very soon quite separated from the hypoblast, except at the hind end, where it is continued into the front wall of the neurenteric passage. It is interesting to notice the remarkable relation of the notochord to the walls of the neurenteric passage. More or less similar relations are also well marked in the case of the goose and the fowl (Gasser)[450], and support the conclusion deducible from the lower forms of vertebrata, that the notochord is essentially hypoblastic.

Footnote 449: This appears to me to take place before the
formation of the embryonic shield.

Footnote 450: Gasser, _Der Primitivstreifen bei
Vogelembryonen_, Marburg, 1878.

The passage at the front end of the primitive streak forms the posterior boundary of the medullary plate, though the medullary groove is not at first continued back to it. The anterior wall of this passage connects together the medullary plate and the notochordal ridge of the hypoblast. In the succeeding stages the medullary groove becomes continued back to the opening of the passage, which then becomes enclosed in the medullary folds, and forms a true neurenteric passage. It becomes narrowed as the medullary folds finally unite to form the medullary canal, and eventually disappears.

I conclude this paper with a concise statement of what appears to me the probable nature of the much-disputed organ, the primitive streak, and of the arguments in support of my view.

In a paper on the primitive streak in the _Quart. Journ. of Mic. Sci._, in 1873 (p. 280) [This edition, p. 45], I made the following statement with reference to this subject:--"It is clear, therefore, that the primitive groove must be the rudiment of some ancestral feature.... It is just possible that it is the last trace of that involution of the epiblast by which the hypoblast is formed in most of the lower animals."

At a later period, in July, 1876, after studying the development of Elasmobranch fishes, I enlarged the hypothesis in a review of the first part of Prof. Kölliker's _Entwicklungsgeschichte_. The following is the passage in which I speak of it[451]:

Footnote 451: _Journal of Anat. and Phys._, Vol. X. pp. 790
and 791. Compare also my _Monograph on Elasmobranch Fishes_,
note on p. 68 [This edition, p. 281].

"In treating of the exact relation of the primitive groove to the
formation of the embryo, Professor Kölliker gives it as his view that
though the head of the embryo is formed independently of the primitive
groove, and only secondarily unites with this, yet that the remainder
of the body is without doubt derived from the primitive groove. With
this conclusion we cannot agree, and the very descriptions of
Professor Kölliker appear to us to demonstrate the untenable nature of
his results. We believe that the front end of the primitive groove at
first occupies the position eventually filled by about the third pair
of protovertebræ, but that as the protovertebræ are successively
formed, and the body of the embryo grows in length, the primitive
groove is carried further and further back, so as always to be
situated immediately behind the embryo. As Professor Kölliker himself
has shewn it may still be seen in this position even later than the
fortieth hour of incubation.

"Throughout the whole period of its existence it retains a character
which at once distinguishes it in sections from the medullary groove.

"Beneath it the epiblast and mesoblast are _always fused_, though they
are always separate elsewhere; this fact, which was originally shewn
by ourselves, has been very clearly brought out by Professor
Kölliker's observations.

"The features of the primitive groove which throw special light on its
meaning are the following:--

"(1) It does not enter directly into the formation of the embryo.

"(2) The epiblast and mesoblast always become fused beneath it.

"(3) It is situated immediately behind the embryo.

"Professor Kölliker does not enter into any speculations as to the
meaning of the primitive groove, but the above-mentioned facts appear
to us clearly to prove that the primitive groove is a rudimentary
structure, the origin of which can only be completely elucidated by a
knowledge of the development of the Avian ancestors.

"In comparing the blastoderm of a bird with that of any anamniotic
vertebrate, we are met at the threshold of our investigations by a
remarkable difference between the two. Whereas in all the lower
vertebrates the embryo is situated at the _edge_ of the blastoderm, it
is in birds and mammals situated in the centre. This difference of
position at once suggests the view that the primitive groove may be in
some way connected with the change of position in the blastoderm which
the ancestors of birds must have undergone. If we carry our
investigations amongst the lower vertebrates a little further, we find
that the Elasmobranch embryo occupies at first the normal position at
the edge of the blastoderm, but that in the course of development the
blastoderm grows round the yolk far more slowly in the region of the
embryo than elsewhere. Owing to this, the embryo becomes left in a
bay, the two sides of which eventually meet and coalesce in a linear
fashion immediately behind the embryo, thus removing the embryo from
the edge of the blastoderm and forming behind it a linear streak not
unlike the primitive streak. We would suggest the hypothesis that the
primitive groove is a rudiment which gives the last indication of a
change made by the Avian ancestors in their position in the
blastoderm, like that made by Elasmobranch embryos when removed from
the edge of the blastoderm and placed in a central situation similar
to that of the embryo bird. On this hypothesis the situation of the
primitive groove immediately behind the embryo, as well as the fact of
its not becoming converted into any embryonic organ would be
explained. The central groove might probably also be viewed as the
groove naturally left between the coalescing edges of the blastoderm.

"Would the fusion of epiblast and mesoblast also receive its
explanation on this hypothesis? We are of opinion that it would. At
the edge of the blastoderm which represents the blastopore mouth of
Amphioxus all the layers become fused together in the anamniotic
vertebrates. So that if the primitive groove is in reality a rudiment
of the coalesced edges of the blastoderm, we might naturally expect
the layers to be fused there, and the difficulty presented by the
present condition of the primitive groove would rather be that the
hypoblast is not fused with the other layers than that the mesoblast
is indissolubly united with the epiblast. The fact that the hypoblast
is not fused with the other layers does not appear to us to be fatal
to our hypothesis, and in Mammalia, where the primitive and medullary
grooves present precisely the same relations as in birds, all three
layers are, according to Hensen's account, fused together. This,
however, is denied by Kölliker, who states that in Mammals, as in
Birds, only the epiblast and mesoblast fuse together. Our hypothesis
as to the origin of the primitive groove appears to explain in a
fairly satisfactory manner all the peculiarities of this very
enigmatical organ; it also relieves us from the necessity of accepting
Professor Kölliker's explanation of the development of the mesoblast,
though it does not, of course, render that explanation in any way
untenable."

At a somewhat later period Rauber arrived at a more or less similar conclusion, which, however, he mixes up with a number of opinions from which I am compelled altogether to dissent[452].

Footnote 452: "Primitivrinne u. Urmund," _Morphologisches
Jahrbuch_, Band II. p. 551.

The general correctness of my view, as explained in my second quotation, appears to me completely established by Gasser's beautiful researches on the early development of the chick and goose[453], and by my own observations just recorded on the lizard. While at the same time the parallel between the blastopore of Elasmobranchii and of the Sauropsida, is rendered more complete by the discovery of the neurenteric passage in the latter group, which was first of all made by Gasser.

Footnote 453: Gasser, _Der Primitivstreifen bei
Vogelembryonen_, Marburg, 1878.

The following paragraphs contain a detailed attempt to establish the above view by a careful comparison of the primitive streak and its adjuncts in the amniotic vertebrates with the blastopore in Elasmobranchii.

In Elasmobranchii the blastopore consists of the following parts:--(1), a section at the end of the medullary plate, which becomes converted into the neurenteric canal[454]; (2), a section forming what may be called the yolk blastopore, which eventually constitutes a linear streak connecting the embryo with the edge of the blastoderm (vide monograph on Elasmobranch fishes, pp. 281 and 296). In order to establish my hypothesis on the nature of the primitive streak, it is necessary to find the representatives of both these parts in the primitive streak of the amniotic vertebrates. The first section ought to appear as a passage from the neural to the enteric side of the blastoderm at the posterior end of the medullary plate. At its front edge the epiblast and hypoblast should be continuous, as they are at the hind end of the embryo in Elasmobranchii, and, finally, the passage should, on the closure of the medullary groove, become converted into the _neurenteric canal_. All these conditions are exactly fulfilled by the opening at the front end of the primitive streak of the lizard (vide woodcut, fig. 1, p. 647). In the chick there is at first no such opening, but, as I hope to shew in a future paper, it is replaced by the epiblast and hypoblast falling into one another at the front end of the primitive streak. At a later period, as has been shewn by Gasser[455], there is a distinct rudiment of the neurenteric canal in the chick, and a complete canal in the goose. Finally, in mammals, as has been shewn by Schäffer[456] for the guinea-pig, there is at the front end of the primitive streak a complete continuity between epiblast and hypoblast. The continuity of the epiblast and hypoblast at the hind end of the embryo in the bird and the mammal is a rudiment of the continuity of these layers at the dorsal lip of the blastopore in Elasmobranchii, Amphibia, &c. The second section of the blastopore in Elasmobranchii or yolk blastopore is, I believe, partly represented by the primitive streak. The yolk blastopore in Elasmobranchii is the part of the blastopore belonging to the yolk sac as opposed to that belonging to the embryo, and it is clear that the primitive streak cannot correspond to the whole of this, since the primitive streak is far removed from the edge of the blastoderm long before the yolk is completely enclosed. Leaving this out of consideration the primitive streak, in order that the above comparison may hold good, should satisfy the following conditions:

Footnote 454: I use this term for the canal connecting the
neural and alimentary tract, which was first discovered by
Kowalevsky.

Footnote 455: _Loc. cit._

Footnote 456: "A contribution to the history of the
development in the Guinea-pig," _Journal of Anat. and Phys._
Vol. XI. pp. 332-336.

1. It should connect the embryo with the edge of the blastoderm.

2. It should be constituted as if formed of the fused edges of the blastoderm.

3. The epiblast of it should eventually not form part of the medullary plate of the embryo, but be folded over on to the ventral side.

The first of these conditions is only partially fulfilled, but, considering the rudimentary condition of the whole structure, no great stress can, it seems to me, be laid on this fact.

The second condition seems to me very completely satisfied. Where the two edges of the blastoderm become united we should expect to find a complete fusion of the layers such as takes place in the primitive streak; and the fact that in the primitive streak the hypoblast does not so distinctly coalesce with the mesoblast as the mesoblast with the epiblast cannot be urged as a serious argument against me.

The growth outwards of the mesoblast from the axis of the primitive streak is probably a remnant of the invagination of the hypoblast and mesoblast from the lip of the blastopore in Amphibia, &c.

The groove in the primitive streak may with great plausibility be regarded as the indication of a depression which would naturally be found along the line where the thickened edges of the blastoderm became united.

With reference to the third condition, I will make the following observations. The neurenteric canal, as it is placed at the extreme end of the embryo, must necessarily, with reference to the embryo, be the hindermost section of the blastopore, and therefore the part of the blastopore apparently behind this can only be so owing to the embryo not being folded off from the yolk sac; and as the yolk sac is in reality a specialised part of the ventral wall of the body, the yolk blastopore must also be situated on the ventral side of the embryo.

Kölliker and other distinguished embryologists have believed that the epiblast of the whole of the primitive streak became part of the neural plate. If this view were correct, which is accepted even by Rauber, the hypothesis I am attempting to establish would fall to the ground. I have, however, no doubt that these embryologists are mistaken. The very careful observations of Gasser shew that the part of the primitive streak adjoining the embryo becomes converted into the tail-swelling, and that the posterior part is folded in on the ventral side of the embryo, and, losing its characteristic structure, forms part of the ventral wall of the body. On this point my own observations confirm those of Gasser. In the lizard the early appearance of the neurenteric canal at the front end of the primitive streak clearly shews that here also the primitive streak can take no share in forming the neural plate.

The above considerations appear to me sufficient to establish my hypothesis with reference to the nature of the primitive streak, which has the merit of explaining, not only the structural peculiarities of the primitive streak, but also the otherwise inexplicable position of the embryo of the amniotic vertebrates in the centre of the blastoderm.

DESCRIPTION OF PLATE 29.

COMPLETE LIST OF REFERENCE LETTERS.

_am._ Amnion. _ch._ Notochord. _ch´._ Notochordal thickening of hypoblast. _ep._ Epiblast. _hy._ Hypoblast. _m.g._ Medullary groove. _me.p._ Mesoblastic plate. _ne._ Neurenteric canal (blastopore). _pr._ Primitive streak.

SERIES A. Sections through an embryo shortly after the formation of the medullary groove. x 120[457].

Footnote 457: The spaces between the layers in these sections
are due to the action of the hardening reagent.

Fig. 1. Section through the trunk of the embryo.

Figs. 2-5. Sections through the neurenteric canal.

Fig. B. Surface view of a somewhat older embryo than that from which Series A is taken. x 30.

SERIES B. Sections through the embryo represented in Fig. B. x 120.

Fig. 1. Section through the trunk of the embryo.

Figs. 2, 3. Sections through the hind end of the medullary groove.

Fig. 4. Section through the neurenteric canal.

Fig. 5. Section through the primitive streak.

Fig. C. Surface view of a somewhat older embryo than that represented in Fig. B. x 30.

XV. ON CERTAIN POINTS IN THE ANATOMY OF PERIPATUS CAPENSIS[458].

Footnote 458: From the _Proceedings of the Cambridge
Philosophical Society_, Vol. III. 1879.

The discovery by Mr Moseley[459] of a tracheal system in Peripatus must be reckoned as one of the most interesting results obtained by the naturalists of the "Challenger." The discovery clearly proves that the genus Peripatus, which is widely distributed over the globe, is the persisting remnant of what was probably a large group of forms, from which the present tracheate Arthropoda are descended.

Footnote 459: "On the Structure and Development of _Peripatus
Capensis_," _Phil. Trans._, Vol. CLXIV. 1874.

The affinities of Peripatus render any further light on its anatomy a matter of some interest; and through the kindness of Mr Moseley I have had an opportunity of making investigations on some well preserved examples of _Peripatus capensis_, a few of the results of which I propose to lay before the Society.

I shall confine my observations to three organs. (1) The segmental organs, (2) the nervous system, (3) the so-called fat bodies of Mr Moseley.

In all the segments of the body, with the exception of the first two or three postoral ones, there are present glandular bodies, apparently equivalent to the segmental organs of Annelids.

These organs have not completely escaped the attention of previous observers. The anterior of them were noticed by Grube[460], but their relations were not made out. By Saenger[461], as I gather from Leuckart's _Bericht_ for the years 1868-9, these structures were also noticed, and they were interpreted as segmental organs. Their external openings were correctly identified. They are not mentioned by Moseley, and no notice of them is to be found in the text-books. The observations of Grube and Saenger seem, in fact, to have been completely forgotten.

Footnote 460: "Bau von _Perip. Edwardsii_," _Archiv f. Anat.
u. Phys._ 1853.

Footnote 461: _Moskauer Naturforscher Sammlung_, Abth. Zool.
1869.

The organs are placed at the bases of the feet in two lateral divisions of the body-cavity shut off from the main central median division of the body-cavity by longitudinal septa of transverse muscles.

Each fully developed organ consists of three parts:

(1) A dilated vesicle opening externally at the base of a foot.

(2) A coiled glandular tube connected with this and subdivided again into several minor divisions.

(3) A short terminal portion opening at one extremity into the coiled tube (2) and at the other, as I believe, into the body-cavity. This section becomes very conspicuous in stained preparations by the intensity with which the nuclei of its walls absorb the colouring matter.

The segmental organs of Peripatus, though formed on a type of their own, more nearly resemble those of the Leech than of any other form with which I am acquainted. The annelidan affinities shewn by their presence are of some interest. Around the segmental organs in the feet are peculiar cells richly supplied with tracheæ, which appear to me to be similar to the fat bodies in insects. There are two glandular bodies in the feet in addition to the segmental organs.

The more obvious features of the nervous system have been fully made out by previous observers, who have shewn that it consists of large paired supra-oesophageal ganglia connected with two widely separated ventral cords--stated by them not to be ganglionated. Grube describes the two cords as falling into one another behind the anus--a feature the presence of which is erroneously denied by Saenger. The lateral cords are united by numerous (5 or 6 for each segment) transverse cords.

The nervous system would appear at first sight to be very lowly organised, but the new points I believe myself to have made out, as well as certain previously known features in it appear to me to shew that this is not the case.

The following is a summary of the fresh points I have observed in the nervous system:

(1) Immediately underneath the oesophagus the oesophageal commissures dilate and form a pair of ganglia equivalent to the annelidan and arthropodan sub-oesophageal ganglia. These ganglia are closely approximated and united by 5 or 6 commissures. They give off large nerves to the oral papillæ.

(2) The ventral nerve cords are covered on their ventral side by a thick ganglionic layer[462], and at each pair of feet they dilate into a small but distinct _ganglionic swelling_. From each ganglionic swelling are given off a pair of large nerves[463] to the feet; and the ganglionic swellings of the two cords are connected together by _a pair of commissures containing ganglion cells_[464]. The other commissures connecting the two cords together do not contain ganglion cells.

Footnote 462: This was known to Grube, _loc. cit._

Footnote 463: These nerves were noticed by Milne-Edwards, but
Grube failed to observe that they were much larger than the
nerves given off between the feet.

Footnote 464: These commissures were perhaps observed by
Saenger, _loc. cit._

The chief feature in which Peripatus was supposed to differ from normal Arthropoda and Annelida, viz. the absence of ganglia on the ventral cords, does not really exist. In other particulars, as in the amount of nerve cells in the ventral cords and the completeness of the commissural connections between the two cords, &c., the organisation of the nervous system of Peripatus ranks distinctly high. The nervous system lies within the circular and longitudinal muscles, and is thus not in proximity with the skin. In this respect also Peripatus shews no signs of a primitive condition of the nervous system.

A median nerve is given off from the posterior border of the supra-oesophageal ganglion to the oesophagus, which probably forms a rudimentary sympathetic system. I believe also that I have found traces of a paired sympathetic system.

The organ doubtfully spoken of by Mr Moseley as a fat body, and by Grube as a lateral canal, is in reality a glandular tube, lined by beautiful columnar cells containing secretion globules, which opens by means of a non-glandular duct into the mouth. It lies close above the ventral nerve cords in a lateral compartment of the body-cavity, and extends backwards for a varying distance.

This organ may perhaps be best compared with the simple salivary gland of Julus. It is not to be confused with the slime glands of Mr Moseley, which have their opening in the oral papillæ. If I am correct in regarding it as homologous with the salivary glands so widely distributed amongst the Tracheata, its presence indicates a hitherto unnoticed arthropodan affinity in Peripatus.

XVI. ON THE MORPHOLOGY AND SYSTEMATIC POSITION OF THE SPONGIDA[465].

Footnote 465: From the _Quarterly Journ. of Microscopical
Science_, Vol. XIX. 1879.

Professor Schulze's[466] last memoir on the development of Calcareous Sponges, confirms and enlarges Metschnikoff's[467] earlier observations, and gives us at last a fairly complete history of the development of one form of Calcareous Sponge. The facts which have been thus established have suggested to me a view of the morphology and systematic position of the Spongida, somewhat different to that now usually entertained. In bringing forward this view, I would have it understood that it does not claim to be more than a mere suggestion, which if it serves no other function may, perhaps, be of use in stimulating research.

Footnote 466: "Untersuchungen über d. Bau u. d. Entwicklung
der Spongien," _Zeit. f. wiss. Zool._ Bd. XXXI. 1878.

Footnote 467: "Zur Entwicklungsgeschichte der Kalkschwämme,"
_Zeit. f. wiss. Zool._ Bd. XXIV. 1874.

To render clear what I have to say, I commence with a very brief statement of the facts which may be considered as established with reference to the development of _Sycandra raphanus_, the form which was studied by both Metschnikoff and Schulze. The segmentation of the ovum, though in many ways remarkable, is of no importance for my present purpose, and I take up the development at the close of the segmentation, while the embryo is still encapsuled in the parental tissues. It is at this stage lens-shaped, with a central segmentation cavity. An equatorial plane divides it into two parts, which have equal shares in bounding the segmentation cavity. One of these halves is formed of about thirty-two large, round, granular cells, the other of a larger number of ciliated clear columnar cells. While the embryo is still encapsuled a partial invagination of the granular cells takes place, reducing the segmentation cavity to a mere slit; this invagination is, however, quite temporary and unimportant, and on the embryo becoming free, which shortly takes place, no trace of it is visible; but, on the contrary, the segmentation cavity becomes larger, and the granular cells project very much more prominently than in the encapsuled state.

Two free stages in the development of _Sycandra raphanus_ (copied from Schulze).

A. Amphiblastula stage; B. a later stage after the ciliated cells have commenced to become invaginated; _cs._ segmentation cavity; _ec._ granular cells which will form the ectoderm; _en._ ciliated cells which become invaginated to form the entoderm.]

The larva, after it has left the parental tissues, has an oval form and is transversely divided into two areas (fig. 1, A). One of these areas is formed of the elongated, clear, ciliated cells, with a small amount of pigment near the inner ends (_en_), and the other and larger area of the thirty-two granular cells already mentioned (_ec_). Fifteen or sixteen of these are arranged as a special ring on the border of the clear cells. In the centre of the embryo is a segmentation cavity (_cs_) which lies between the granular and the clear cells, but is mainly bounded by the vaulted inner surface of the latter. This stage is known as the amphiblastula stage. After the larva has for some time enjoyed a free existence, a remarkable series of changes takes place, which result in the invagination of the half of it formed of the clear cells, and form a prelude to the permanent attachment of the larva. The entire process of invagination is completed in about half an hour. The whole embryo first becomes flattened, but especially the ciliated half which gradually becomes less prominent (fig. 1, B), and still later the cells composing it undergo a true process of invagination. As a result of this invagination the segmentation cavity is obliterated and the larva assumes a compressed plano-convex form with a central gastrula cavity, and a blastopore in the middle of the flattened surface. The two layers of the gastrula may now be spoken of as ectoderm and entoderm. The blastopore becomes gradually narrowed by the growth over it of the outer row of granular cells. When it has become very small the attachment of the larva takes place by the flat surface where the blastopore is situated. It is effected by protoplasmic processes of the outer ring of ectoderm cells, which, together with the other ectoderm cells, now become amoeboid. At the same time they become clearer and permit a view of the interior of the gastrula. Between the ectoderm cells and the entoderm cells which line the gastrula cavity there arises a hyaline structureless layer, which is more closely attached to the ectoderm than to the entoderm, and is probably derived from the former. A view of the gastrula stage after the larva has become fixed is given in fig. 2.

Fixed Gastrula stage of _Sycandra raphanus_ (copied from Schulze).

The figure shews the amoeboid ectoderm cells (_ec_) derived from the granular cells of the earlier stage, and the columnar entoderm cells, lining the gastrula cavity, derived from the ciliated cells of the earlier stage. The larva is fixed by the amoeboid cells on the side on which the blastopore is situated.]

The young of _Sycandra raphanus_ shortly after the development of the _spicula_ (copied from Schulze).

A. View from the side; B. view from the free extremity; _os._ osculum; _ec._ ectoderm; _en._ entoderm composed of collared ciliated cells. The terminal osculum and lateral pores are represented as oval white spaces.]

After invagination the cilia of the entoderm cells can no longer be seen, and are probably absorbed, and their disappearance is nearly coincident with the complete obliteration of the blastopore, an event which takes place shortly after the attachment of the larva. After the formation of the structureless layer between the ectoderm and entoderm, calcareous spicules make their appearance in it as delicate unbranched rods pointed at both extremities. The larva when once fixed rapidly grows in length and assumes a cylindrical form (fig. 3, A). The sides of the cylinder are beset with calcareous spicules which project beyond the surface, and in addition to the unbranched forms, spicules are developed with three and four rays as well as some with a blunt extremity and serrated edge. The extremity of the cylinder opposite the attached surface is flattened, and though surrounded by a ring of four-rayed spicules is itself free from them. At this extremity a small perforation is formed leading into the gastric cavity which rapidly increases in size and forms an exhalent osculum (_os_). A series of inhalent apertures are also formed at the sides of the cylinder. The relative times of appearance of the single osculum and smaller apertures is not constant for the different larvæ. On the central gastrula cavity of the sponge becoming placed in communication with the external water, the entoderm cells lining it become ciliated afresh (fig. 3, B, _en_) and develop the peculiar collar characteristic of the entoderm cells of the Spongida. When this stage of development is reached we have a fully developed sponge of the type made known by Haeckel as Olynthus.

Till the complete development of other forms of Spongida has been worked out it is not possible to feel sure how far the phenomena observable in Sycandra hold good in all cases. Quite recently the Russian embryologist, M. Ganin[468], has given an account, without illustrations, of the development of _Spongilla fluviatilis_, which does not appear reconcileable with that of Sycandra. Considering the difficulties of observation it appears better to assume for this and some other descriptions that the observations are in error rather than that there is a fundamental want of uniformity in development amongst the Spongida.

Footnote 468: "Zur Entwicklung d. Spongilla fluviatilis,"
_Zoologischer Anzeiger_, Vol. I. No. 9, 1878.

The first point in the development of Sycandra which deserves notice is the character of the free swimming larva. The peculiar larval form, with one half of the body composed of amoeboid granular cells and the other of clear ciliated cells is nearly constant amongst the Calcispongiæ, and widely distributed in a somewhat modified condition amongst the Fibrospongiæ and Myxospongiæ. Does this larva retain the characters of an ancestral type of the Spongida, and if so what does its form mean? It is, of course, possible that it has no ancestral meaning but has been secondarily acquired; I prefer myself to think that this is not the case, more especially as it appears to me that the characters of the larva may be plausibly explained by regarding it as a transitional form between the Protozoa and Metazoa. According to this view the larva is to be considered as a colony of Protozoa, one half of the individuals of which have become differentiated into nutritive forms, and the other half into locomotor and respiratory forms. The granular amoeboid cells represent the nutritive forms, and the ciliated cells represent the locomotor and respiratory forms. That the passage from the Protozoa to the Metazoa may have been effected by such a differentiation is not improbable on _à priori_ grounds, and fits in very well with the condition of the free swimming larva of Spongida, though another and perhaps equally plausible suggestion as to this passage has been put forward by my friend Professor Lankester[469].

Footnote 469: "Notes on Embryology and Classification."
_Quarterly Journal of Microscopical Science_, Vol. XVII. 1877.
It seems not impossible, if the speculations in this paper have
any foundation that while the views here put forward as to the
passage from the Protozoon to the Metazoon condition may hold
true for the Spongida, some other mode of passage may have
taken place in the case of the other Metazoa.

While the above view seems fairly satisfactory for the free swimming stage of the larval Sponge there arises in the subsequent development a difficulty which appears at first sight fatal to it. This difficulty is the invagination of the ciliated cells instead of the granular ones. If the granular cells represent the nutritive individuals of the colony, they and not the ciliated cells ought most certainly to give rise to the lining of the gastrula cavity, according to the generally accepted views of the morphology of the Spongida. The suggestion which I would venture to put forward in explanation of this paradox involves a completely new view of the nature and functions of the germinal layers of adult Sponges.

It is as follows:--When the free swimming ancestor of the Spongida became fixed, the ciliated cells by which its movements used to be effected must have to a great extent become functionless. At the same time the amoeboid nutritive cells would need to expose as large a surface as possible. In these two considerations there may, perhaps, be found a sufficient explanation of the invagination of the ciliated cells, and the growth of the amoeboid cells over them. Though respiration was, no doubt, mainly effected by the ciliated cells, it is improbable that it was completely localised in them, but the continuation of their function was provided for by the formation of an osculum and pores. The ciliated collared cells which line the ciliated chambers, or in some cases the radial tubes, are undoubtedly derived from the invaginated cells, and if there is any truth in the above suggestion, the collared cells in the adult Sponge must be mainly respiratory and not digestive in function, while the normal epithelial cells which cover the surface of the sponge, and in most cases line the greater part of the passages through its substance, must carry on the digestion[470]. If the reverse is the case the whole theory falls to the ground. It has not, so far as I know, been definitely made out where the digestion is carried on. Lieberkühn would appear to hold the view that the amoeboid lining cells of the passages are mainly concerned with digestion, while Carter holds that digestion is carried on by the collared cells of the ciliated chambers.

Footnote 470: That the flat cells which line the greater part
of the passages of most Sponges are really derived from
ectodermic invaginations appears to me clearly proved by
Schulze's and Barrois' observations on the young fixed stages
of Halisarea. Ganin appears, however, to maintain a contrary
view for Spongilla.

If it is eventually proved by actual experiments on the nutrition of Sponges, that digestion is carried on by the general cells lining the passages, and not by the ciliated cells, it is clear that neither the ectoderm nor entoderm of Sponges will correspond with the similarly named layers in the Coelenterata and the Metazoa. The invaginated entoderm will be the respiratory layer and the ectoderm the digestive and sensory layer; the sensory function being probably mainly localised in the epithelium on the surface, and the digestive one in the epithelium lining the passages. Such a fundamental difference in the germinal layers between the Spongida and the other Metazoa, would necessarily involve the creation of a special division of the Metazoa for the reception of the former group.

XVII. NOTES ON THE DEVELOPMENT OF THE ARANEINA[471].

Footnote 471: From the _Quarterly Journ. of Microscopical
Science_, Vol. XX. 1880.

(With Plates 30, 31, 32.)

The following observations do not profess to contain a complete history of the development even of a single species of spider. They are the result of investigations carried on at intervals during rather more than two years, on the ova of _Agelena labyrinthica_; and I should not have published them now, if I had any hope of being able to complete them before the appearance of the work I am in the course of publishing on Comparative Embryology. It appeared to me, however, desirable to publish in full such parts of my observations as are completed before the appearance of my treatise, since the account of the development of the Araneina is mainly founded upon them.

My investigations on the germinal layers and organs have been chiefly conducted by means of sections. To prepare the embryos for sections, I employed the valuable method first made known by Bobretzky. I hardened the embryos in bichromate of potash, after placing them for a short time in nearly boiling water. They were stained as a whole with hæmatoxylin after the removal of the membranes, and embedded for cutting in coagulated albumen.

The number of investigators who have studied the development of spiders is inconsiderable. A list of them is given at the end of the paper.

The earliest writer on the subject is Herold (No. 4); he was followed after a very considerable interval of time by Claparède (No. 3), whose memoir is illustrated by a series of beautiful plates, and contains a very satisfactory account of the external features of development.

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

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

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

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

Each yolk-segment is formed of a number of large clear somewhat oval yolk-spherules. In hardened specimens the yolk-spherules become polygonal, and in ova treated with hot water prior to preservation are not unfrequently broken up. Amongst the yolk-segments are placed a fair number of nucleated bodies of a very characteristic appearance. Each of them is formed of (1) a large, often angular, nucleus, filled with deeply staining bodies (nucleoli?); (2) a layer of protoplasm surrounding the nucleus, prolonged into a protoplasmic reticulum. The exact relation of these nucleated bodies to the yolk-segments is not very easy to make out, but the general tendency of my observations is to shew (1) that each nucleated body belongs to a yolk-sphere, and (2) that it is generally placed not at the centre, but to one side of a yolk-sphere. If the above conclusions are correct each complete yolk-segment is a cell, and each such cell consists of a normal nucleus, protoplasm, and yolk-spherules. There is a special layer of protoplasm surrounding the nucleus, while the remainder of the protoplasm consists of a reticulum holding together the yolk-spherules. Yolk-cells of this character are seen in Pls. 31 and 32, figs. 10-21.

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

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

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

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

After a short time the cumulus elongates itself on one side, and becomes connected by a streak with a white patch, which appears on the surface of the egg, about 90° from the cumulus. This patch gradually enlarges, and soon covers the whole surface of the ovum except the region where the cumulus is placed. It becomes the ventral plate or germinal streak of the embryo, its extremity adjoining the cumulus is the anal extremity, and its opposite extremity the cephalic one. The cumulus itself is placed in a depression on the dorsal surface of the ovum. Claparède compares the cumulus to the dorsal organ of many Crustacea.

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

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

The first change that I find in the embryo (when examined after it has been hardened)[472] is the appearance of a small, whitish spot, which is at first very indistinct. A section through such an ovum (Pl. 31, fig. 10) shews that the cells of about one half of the ovum have become more columnar than those of the other half, and that there is a point (_pr.c._) near one end of the thickened half where the cells are more columnar, and about two layers or so deep. It appears to me probable that this point is the whitish spot visible in the hardened ovum. In a somewhat later stage (Pl. 30, fig. 1) the whitish spot becomes more conspicuous (_pc._), and appears as a distinct prominence, which is, without doubt, the primitive cumulus, and from it there proceeds on one side a whitish streak. The prominence, as noticed by Claparède and Balbiani, is situated on the flatter side of the ovum. Sections at this stage shew the same features as the previous stage, except that (1) the cells throughout are smaller, (2) those of the thickened hemisphere of the ovum more columnar, and (3) the cumulus is formed of several rows of cells, though not divided into distinct layers. In the next stage the appearances from the surface are rather more obscure, and in some of my best specimens a coagulum, derived from the fluid surrounding the ovum, covers the most important part of the blastoderm. In Pl. 30, fig. 2, I have attempted to represent, as truly as I could, the appearances presented by the ovum. There is a well-marked whitish side of the ovum, near one end of which is a prominence (_pc._), which must, no doubt, be identified with the cumulus of the earlier stages. Towards the opposite end, or perhaps rather nearer the centre of the white side of the ovum, is an imperfectly marked triangular white area. There can be no doubt that the line connecting the cumulus with the triangular area is the future long axis of the embryo, and the white area is, without doubt, the procephalic lobe of Balbiani.

Footnote 472: I was unfortunately too much engaged, at the
time when the eggs were collected, to study them in the fresh
condition; a fact which has added not a little to my
difficulties in elucidating the obscure points in the early
stages.

A section of the ovum at this stage is represented in Pl. 31, fig. 11. It is not quite certain in what direction the section is taken, but I think it probable it is somewhat oblique to the long axis. However this may be, the section shews that the whitish hemisphere of the blastoderm is formed of columnar cells, for the most part two or so layers deep, but that there is, not very far from the middle line, a wedge-shaped internal thickening of the blastoderm where the cells are several rows deep. With what part visible in surface view this thickened portion corresponds is not clear. To my mind it most probably corresponds to the larger white patch, in which case I have not got a section through the terminal prominence. In the other sections of the same embryo the wedge-shaped thickening was not so marked, but it, nevertheless, extended through all the sections. It appears to me probable that it constitutes a longitudinal thickened ridge of the blastoderm. In any case, it is clear that the white hemisphere of the blastoderm is a thickened portion of the blastoderm, and that the thickening is in part due to the cells being more columnar, and, in part, to their being more than one row deep, _though they have not become divided into two distinct germinal layers_. It is further clear that the increase in the number of cells in the thickened part of the blastoderm is, _in the main, a result of the multiplication of the original single row of cells_, while a careful examination of my sections proves that it is also partly due to cells, derived from the yolk, having been added to the blastoderm.

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

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

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

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

Though Claparède and Balbiani differ somewhat as to the position of the organ, they both make it last longer than I do: I feel certainly inclined to doubt whether Claparède is right in considering a body he figures after six segments are present, to be the same as the dorsal organ of the embryo before the formation of any segments, especially as all the stages between the two appear to have escaped him. In Agelena there is undoubtedly no organ in the position he gives when six segments are found.

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

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