Chapter X: Introduction (1)

The variations in the character of the embryonic development of the Amniota are far less important than in the case of the Ichthyopsida. There are, it is true, some very special features in the early developmental history of the Mammalia, but apart from these there is such a striking uniformity in the embryos of all the groups that it would, in many cases, be difficult to assign a young embryo to its proper class.

Amongst the Sauropsida the Aves have for obvious reasons received a far fuller share of attention than any other group; and an account of their embryology forms a suitable introduction to this part of our subject. For the convenience of the student many parts of their developmental history will be dealt with at greater length than in the case of the previous groups.

_The development of the Aves._

Comparatively few types of Birds have been studied embryologically. The common Fowl has received a disproportionately large share of attention; although within quite recent times the Duck, the Goose, the Pigeon, the Starling, and a Parrot (Melopsittacus undulatus) have also been studied. The result of these investigations has been to shew that the variations in the early development of different Birds are comparatively unimportant. In the sequel the common Fowl will be employed as type, attention being called when necessary to the development of the other forms.

[FIG. 85. YOLK ELEMENTS FROM THE EGG OF THE FOWL.

_A._ Yellow yolk. _B._ White yolk.]

The ovum of the Fowl, at the time when it is clasped by the expanded extremity of the oviduct, is a large yellow body enclosed in a vitelline membrane. It is mainly formed of spherules of food-yolk. Of these there are two varieties; one known as yellow yolk, and the other as white. The white yolk spherules form a small mass at the centre of the ovum, which is continued to the surface by a narrow stalk, and there expands into a somewhat funnel-shaped disc, the edges of which are continued over the surface of the ovum as a delicate layer. The major part of the ovum is formed of yellow yolk. The yellow yolk consists of large delicate spheres, filled with small granules (fig. 85 A); while the white yolk is formed of vesicles of a smaller size than the yellow yolk spheres, in which are a variable number of highly refractive bodies (fig. 85 B).

[FIG. 86. SECTION THROUGH THE GERMINAL DISC OF THE RIPE OVARIAN OVUM
OF A FOWL WHILE YET ENCLOSED IN ITS CAPSULE.

_a._ Connective-tissue capsule of the ovum; _b._ epithelium of the
capsule, at the surface of which nearest the ovum lies the vitelline
membrane; _c._ granular material of the germinal disc, which becomes
converted into the blastoderm. (This is not very well represented in
the woodcut. In sections which have been hardened in chromic acid it
consists of fine granules.) _w.y._ white yolk, which passes
insensibly into the fine granular material of the disc; _x._
germinal vesicle enclosed in a distinct membrane, but shrivelled up;
_y._ space originally completely filled up by the germinal vesicle,
before the latter was shrivelled up.]

In addition to the yolk there is present in the ovum a small protoplasmic region, containing the remains of the germinal vesicle, which forms the germinal disc (fig. 86). It overlies the funnel-shaped disc of white yolk, into which it is continued without any marked line of demarcation. It contains numerous minute spherules of the same nature as the smallest white yolk spherules.

Impregnation takes place at the upper extremity of the oviduct.

In its passage outwards the ovum gradually receives its accessory coverings in the form of albumen, shell-membrane, and shell (fig. 87).

[FIG. 87. DIAGRAMMATIC SECTION OF AN UNINCUBATED FOWL'S EGG.
(Modified from Allen Thomson.)

_bl._ blastoderm; _w.y._ white yolk. This consists of a central
flask-shaped mass and a number of layers concentrically arranged
around it. _y.y._ yellow yolk; _v.t._ vitelline membrane; _x._ layer
of more fluid albumen immediately surrounding the yolk; _w._ albumen
consisting of alternate denser and more fluid layers; _ch.l._
chalaza; _a.ch._ air-chamber at the broad end of the egg. This
chamber is merely a space left between the two layers of the
shell-membrane. _i.s.m._ internal layer of shell-membrane; _s.m._
external layer of shell-membrane; _s._ shell.]

[FIG 88. SURFACE VIEWS OF THE EARLY STAGES OF THE SEGMENTATION IN A
FOWL'S EGG. (After Coste.)

_a._ edge of germinal disc; _b._ vertical furrow; _c._ small central
segment; _d._ larger peripheral segment.]

[FIG. 89. SURFACE VIEW OF THE GERMINAL DISC OF FOWL'S EGG DURING A
LATE STAGE OF THE SEGMENTATION.

_c._ small central segmentation spheres; _b._ larger segments
outside these; _a._ large, imperfectly circumscribed, marginal
segments; _e._ margin of germinal disc.]

The segmentation commences in the lower part of the oviduct, shortly before the shell has begun to be formed. It is meroblastic, being confined to the germinal disc, through the full depth of which however the earlier furrows do not extend. It is mainly remarkable for being constantly somewhat unsymmetrical (Kölliker)--a feature which is not represented in fig. 88, copied from Coste. Owing to the absence of symmetry the cells at one side of the germinal disc are larger than those at the other, but the relations between the disc and the axis of the embryo are not known. During the later stages the segmentation is irregular, and not confined to the surface; and towards its close the germinal disc becomes somewhat lenticular in shape; and is formed of segments, which are smallest in the centre and increase in size towards the periphery (figs. 89 and 90). The superficial segments in the centre of the germinal disc are moreover smaller than those below, and more or less separated as a distinct layer (fig. 90). As development proceeds the segmentation reaches its limits in the centre, but continues at the periphery; and thus eventually the masses at the periphery become of the same size as those at the centre. At the time when the ovum is laid (fig. 91) the uppermost layer of segments has given rise to a distinct membrane, the epiblast, formed of a single row of columnar nar cells (_ep_). The lower or hypoblast segments are larger, in some cases very much larger, than those of the epiblast, and are so granular that their nuclei can only with difficulty be seen. They form a somewhat irregular mass, several layers deep, and thicker at the periphery than at the centre: they rest on a bed of white yolk, from which they are in parts separated by a more or less developed cavity, which is probably filled with fluid yolk matter about to be absorbed. In the bed of white yolk nuclei are present, which are of the same character, and have the same general fate, as those in Elasmobranchii. They are generally more numerous in the neighbourhood of the thickened periphery of the blastoderm than elsewhere. Peculiar large spherical bodies are to be found amongst the lower layer cells, which superficially resemble the larger cells around them, and have been called formative cells [_vide_ Foster and Balfour (No. 126)]. Their real nature is still very doubtful, and though some are no doubt true cells, others are perhaps only nutritive masses of yolk. In a surface view the blastoderm, as the segmented germinal disc may now be called, appears as a circular disc; the central part of which is distinguished from the peripheral by its greater transparency, and forms what is known in the later stages as the area pellucida. The narrow darker ring of blastoderm, outside the area pellucida, is the commencing area opaca.

[FIG. 90. SECTION OF THE GERMINAL DISC OF A FOWL DURING THE LATER
STAGES OF SEGMENTATION.

The section, which represents rather more than half the breadth of
the blastoderm (the middle line being shewn at _c_), shews that the
upper and central parts of the disc segment faster than those below
and towards the periphery. At the periphery the segments are still
very large. One of the larger segments is shewn at _a_. In the
majority of segments a nucleus can be seen; and it seems probable
that the nucleus is present in them all. Most of the segments are
filled with highly refracting spherules, but these are more numerous
in some cells (especially the larger cells near the yolk) than in
others. In the central part of the blastoderm the upper cells have
commenced to form a distinct layer. No segmentation cavity is
present.

_a._ large peripheral cell; _b._ larger cells of the lower parts of
the blastoderm; _c._ middle line of blastoderm; _e._ edge of the
blastoderm adjoining the white yolk; _w._ white yolk.]

[FIG. 91. SECTION OF A BLASTODERM OF A FOWL'S EGG AT THE
COMMENCEMENT OF INCUBATION.

The thin epiblast _ep_ composed of columnar cells rests on the
incomplete lower layer _l_, composed of larger and more granular
hypoblast cells. The lower layer is thicker in some places than in
others, and is especially thick at the periphery. The line below the
under layer marks the upper surface of the white yolk. The larger
so-called formative cells are seen at _b_, lying on the white yolk.
The figure does not take in quite the whole breadth of the
blastoderm; but the reader must understand that both to the right
hand and to the left _ep_ is continued farther than _l_, so that at
the extreme edge it rests directly on the white yolk.]

As a result of incubation the blastoderm undergoes a series of changes, which end in the definite formation of three germinal layers, and in the establishment of the chief systems of organs of the embryo. The more important of these changes are accomplished in the case of the common Fowl during the first day and the early part of the second day of incubation.

There is hardly any question in development which has been the subject of so much controversy as the mode of formation of the germinal layers in the common Fowl. The differences in the views of authors have been caused to a large extent by the difficulties of the investigation, but perhaps still more by the fact that many of the observations were made at a time when the methods of making sections were very inferior to those of the present day. The subject itself is by no means of an importance commensurate with the attention it has received. The characters which belong to the formation of the layers in the Sauropsida are secondarily derived from those in the Ichthyopsida, and are of but little importance for the general questions which concern the nature and origin of the germinal layers. In the account in the sequel I have avoided as much as possible discussion of controverted points. My statements are founded in the main on my own observations, more especially on a recent investigation carried on in conjunction with my pupil, Mr Deighton. It is to Kölliker (No. 135), and to Gasser (No. 127) that the most important of the more recent advances in our knowledge are due. Kölliker, in his great work on Embryology, definitely established the essential connection between the primitive streak and the formation of the mesoblast; but while confirming his statement on this head, I am obliged to differ from him with reference to some other points.

Gasser's work, especially that part of it which relates to the passages leading from the neural to the alimentary canal, which he was the first to discover, is very valuable.

The blastoderm gradually grows in size, and extends itself over the yolk; the growth over the yolk being very largely effected by an increase in the size of the area opaca, which during this process becomes more distinctly marked off from the area pellucida. The area pellucida gradually assumes an oval form, and at the same time becomes divided into a posterior opaque region and an anterior transparent region. The posterior opacity is named by some authors the embryonic shield.

[FIG. 92. TRANSVERSE SECTION THROUGH THE BLASTODERM OF A CHICK
BEFORE THE APPEARANCE OF THE PRIMITIVE STREAK.

The epiblast is represented somewhat diagrammatically. The hyphens
shew the points of junction of the two halves of the section.]

During these changes the epiblast (fig. 92) becomes two layers deep over the greater part of the area pellucida, though still only one cell deep in the area opaca. The irregular hypoblast spheres of the unincubated blastoderm flatten themselves out, and unite into a definite hypoblastic membrane (fig. 92). Between this membrane and the epiblast there remain a number of scattered cells (fig. 92) which cannot however be said to form a definite layer altogether distinct from the hypoblast. They are almost entirely confined to the posterior part of the area pellucida, and give rise to the opacity of that part.

At the edge of the area pellucida the hypoblast becomes continuous with a thickened rim of material, underlying the epiblast, and derived from the original thickened edge of the blastoderm and the subjacent yolk. It is mainly formed of yolk granules, with a varying number of cells and nuclei imbedded in it. It is known as the germinal wall, and is spoken of more in detail on pp. 160 and 161.

[FIG. 93. DIAGRAMS ILLUSTRATING THE POSITION OF THE BLASTOPORE, AND
THE RELATION OF THE EMBRYO TO THE YOLK IN VARIOUS MEROBLASTIC
VERTEBRATE OVA.

A. Type of Frog. B. Elasmobranch type. C. Amniotic Vertebrate.

_mg._ medullary plate; _ne._ neurenteric canal; _bl._ portion of
blastopore adjoining the neurenteric canal. In B this part of the
blastopore is formed by the edges of the blastoderm meeting and
forming a linear streak behind the embryo; and in C it forms the
structure known as the primitive streak. _yk._ part of yolk not yet
enclosed by the blastoderm.]

The changes which next take place result in the complete differentiation of the embryonic layers, a process which is intimately connected with the formation of the structure known as the primitive streak. The meaning of the latter structure, and its relation to the embryo, can only be understood by comparison with the development of the forms already considered. The most striking peculiarity in the first formation of the embryo Bird, as also in that of the embryos of all Amniota, consists in the fact that _they do not occupy a position at the edge of the blastoderm, but are placed near its centre_. Behind the embryo there is however a peculiar structure--the primitive streak above mentioned--which is a linear body placed in the posterior region of the blastoderm. This body, the nature of which will be more fully explained in the chapter on the comparative development of Vertebrates, is really a rudimentary part of the blastopore, of the same nature as the linear streak behind the embryo in Elasmobranchii formed by the concrescence of the edges of the blastoderm (_vide_ p. 64); although there is no ontogenetic process in the Amniota, like the concrescence in Elasmobranchii. The relations of the blastopore in Elasmobranchii and Aves is shewn in figs. B and C of the diagram (fig. 93).

[FIG. 94. AREA PELLUCIDA OF A VERY YOUNG BLASTODERM OF A CHICK,
SHEWING THE PRIMITIVE STREAK AT ITS FIRST APPEARANCE.

_pr.s._ primitive streak; _ap._ area pellucida; _a.op._ area opaca.]

[FIG. 95. TRANSVERSE SECTION THROUGH A BLASTODERM OF ABOUT THE AGE
REPRESENTED IN FIG. 94, SHEWING THE FIRST DIFFERENTIATION OF THE
PRIMITIVE STREAK.

The section passes through about the middle of the primitive streak.
_pvs._ primitive streak; _ep._ epiblast; _hy._ hypoblast; _yk._ yolk
of the germinal wall.]

In describing in detail the succeeding changes we may at first confine our attention to the area pellucida. As this gradually assumes an oval form the posterior opacity becomes replaced by a very dark median streak, which extends forwards some distance from the posterior border of the area (fig. 94). This is the first rudiment of the primitive streak. In the region in front of it the blastoderm is still formed of two layers only, but in the region of the streak itself the structure of the blastoderm is greatly altered. The most important features in it are represented in fig. 95. This figure shews that the median portion of the blastoderm has become very much thickened (thus producing the opacity of the primitive streak), and that this thickening is caused by a proliferation of rounded cells from the epiblast. In the very young primitive streak, of which fig. 95 is a section, the rounded cells are still continuous throughout with the epiblast, but they form nevertheless the rudiment of the greater part of a sheet of mesoblast, which will soon arise in this region.

In addition to the cells clearly derived from the epiblast, there are certain other cells (_vide_ fig. 95), closely adjoining the hypoblast, which appear to me to be the derivatives of the cells interposed between the epiblast and hypoblast, which gave rise to the posterior opacity in the blastoderm during the previous stage. In my opinion these cells also have a share in forming the future mesoblast.

The number and distribution of these cells is subject to not inconsiderable variations. In a fair number of cases they are entirely congregated along the line of the primitive streak, leaving the sides of the blastoderm quite free. They then form a layer, which can only with difficulty be distinguished from the cells derived from the epiblast by slight peculiarities of staining, and by the presence of a considerable proportion of large granular cells. It is, I believe, by the study of such blastoderms that Kölliker has been led to deny to the intermediate cells of the previous stage any share in the formation of the mesoblast. In other instances, of which fig. 95 is a fairly typical example, they are more widely scattered. To follow with absolute certainty the history of these cells, and to prove that they join the mesoblast is not, I believe, possible by means of sections, and I must leave the reader to judge how far the evidence given in the sequel is sufficient to justify my opinions on this subject.

[FIG. 96. SURFACE VIEW OF THE AREA PELLUCIDA OF A CHICK'S BLASTODERM
SHORTLY AFTER THE FORMATION OF THE PRIMITIVE GROOVE.

_pr._ primitive streak with primitive groove; _af._ amniotic fold.

The darker shading round the primitive streak shews the extension of
the mesoblast.]

In the course of further growth the area pellucida soon becomes pyriform, the narrower extremity being the posterior. The primitive streak (fig. 96) elongates considerably, so as to occupy about two-thirds of the length of the area pellucida; but its hinder end in many instances does not extend to the posterior border of the area pellucida. The median line of the primitive streak becomes marked by a shallow groove, known as the primitive groove.

[FIG. 97. TRANSVERSE SECTION THROUGH THE FRONT END OF THE PRIMITIVE
STREAK OF A BLASTODERM OF THE SAME AGE AS FIG. 96.

_pv._ primitive groove; _m._ mesoblast; _ep._ epiblast; _hy._
hypoblast; _yh._ yolk of germinal wall.]

During these changes in external appearance there grow from the sides of the primitive streak two lateral wings of mesoblast cells, which gradually extend till they reach the sides of the area pellucida (fig. 97). The mesoblast still remains attached to the epiblast along the line of the primitive streak. During this extension many sections through the primitive streak give an impression of the mesoblast being involuted at the lips of a fold, and so support the view above propounded, that the primitive streak is the rudiment of the coalesced lips of the blastopore. The hypoblast below the primitive streak is always quite independent of the mesoblast above, though much more closely attached to it in the median line than at the sides. The part of the mesoblast, which I believe to be derived from the primitive hypoblast, can generally be distinctly traced. In many cases, especially at the front end of the primitive streak, it forms, as in fig. 97, a distinct layer of stellate cells, quite unlike the rounded cells of the mesoblastic involution of the primitive streak.

[FIG. 98. LONGITUDINAL SECTION THROUGH THE AXIAL LINE OF THE
PRIMITIVE STREAK, AND THE PART OF THE BLASTODERM IN FRONT OF IT, OF
AN EMBRYO CHICK SOMEWHAT YOUNGER THAN FIG. 99.

_pr.s._ primitive streak; _ep._ epiblast; _hy._ hypoblast of region
in front of primitive streak; _n._ nuclei; _yk._ yolk of germinal
wall.]

In the region in front of the primitive streak, where the first trace of the embryo will shortly appear, the layers at first undergo no important changes, except that the hypoblast becomes somewhat thicker. Soon, however, as shewn in longitudinal section in fig. 98, the hypoblast along the axial line becomes continuous behind with the front end of the primitive streak. Thus at this point, which is the future hind end of the embryo, the mesoblast, the epiblast, and the hypoblast all unite together; just as they do in all the types of Ichthyopsida.

Shortly afterwards, at a slightly later stage than that represented in fig. 96, an important change takes place in the constitution of the hypoblast in front of the primitive streak. The rounded cells, of which it is at first composed (fig. 98), break up into (1) a layer formed of a single row of more or less flattened elements below--the hypoblast--and (2) into a layer formed of several rows of stellate elements, between the hypoblast and the epiblast--the mesoblast (fig. 99). A separation between these two layers is at first hardly apparent, and before it has become at all well marked, especially in the median line, an axial opaque line makes its appearance in surface views, continued forwards from the front end of the primitive streak, but stopping short at a semicircular fold--the future head-fold--near the front end of the area pellucida. In section (fig. 100) this opaque line is seen to be due to a special concentration of cells in the form of a cord. This cord is the commencement of the notochord (_ch_). In some instances the commencing notochord remains attached to the hypoblast, while the mesoblast is laterally quite distinct (_vide_ fig. 100), and is therefore formed in the same manner as in most Ichthyopsida; while in other instances, and always apparently in the Goose (Gasser, No. 127), the notochord appears to become differentiated in the already separated layer of mesoblast. In all cases _the notochord and the hypoblast below it unite with the front end of the primitive streak_; with which also the two lateral plates of mesoblast become continuous.

[FIG. 99. TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE
BLASTODERM OF A CHICK SHORTLY PRIOR TO THE FORMATION OF THE
MEDULLARY GROOVE AND NOTOCHORD.

_m._ median line of the section; _ep._ epiblast; _ll._ lower layer
cells (primitive hypoblast) not yet completely differentiated into
mesoblast and hypoblast; _n._ nuclei of germinal wall.]

From what has just been said it is clear that in the region of the embryo the mesoblast originates as two lateral plates split off from the hypoblast, and that the notochord originates as a median plate, simultaneously with the mesoblast, with which it may sometimes be at first continuous.

Kölliker holds that the mesoblast of the region of the embryo is derived from a forward growth from the primitive streak. There is no theoretical objection to this view, and I think it would be impossible to shew for certain by sections whether or not there is a growth such as he describes; but such sections as that represented in fig. 99 (and I have series of similar sections from several embryos) appear to me to be conclusive in favour of the view that the mesoblast of the region of the embryo is to a large extent derived from a differentiation of the primitive hypoblast. I am however inclined to believe that some of the mesoblast cells of the embryonic region have the derivation which Kölliker ascribes to all of them.

[FIG. 100. TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE
BLASTODERM OF A CHICK AT THE TIME OF THE FORMATION OF THE NOTOCHORD,
BUT BEFORE THE APPEARANCE OF THE MEDULLARY GROOVE.

_ep._ epiblast; _hy._ hypoblast; _ch._ notochord; _me._ mesoblast;
_n._ nuclei of the germinal wall _yk._ yolk.]

As regards the mesoblast of the primitive streak, in a purely objective description like that given above, the greater part of it may fairly be described as being derived from the epiblast. But if it is granted that the primitive streak corresponds with the blastopore, it is obvious to the comparative embryologist that the mesoblast derived from it really originates from the lips of the blastopore, as in so many other cases; and that to describe it, without explanation, as arising from the epiblast, would give an erroneous impression of the real nature of the process.

[FIG. 101. TRANSVERSE SECTION OF A BLASTODERM INCUBATED FOR 18 HOURS.

The section passes through the medullary groove _mc._, at some
distance behind its front end.

A. epiblast. B. mesoblast. C. hypoblast.

_m.c._ medullary groove; _m.f._ medullary fold; _ch._ notochord.]

The differentiation of the embryo may be said to commence with the formation of the notochord and the lateral plates of mesoblast. Very shortly after the formation of these structures the axial part of the epiblast, above the notochord and in front of the primitive streak, which is somewhat thicker than the lateral parts, becomes differentiated into a distinct medullary plate, the sides of which form two folds--the medullary folds--enclosing between them a medullary groove (fig. 101).

In front the two medullary folds meet, while posteriorly they thin out and envelop between them the front end of the primitive streak. On the formation of the medullary folds the embryo assumes a form not unlike that of the embryos of many Ichthyopsida at a corresponding stage. The appearance of the embryo, and its relation to the surrounding parts is somewhat diagrammatically represented in fig. 102. The primitive streak now ends with an anterior swelling (not represented in the figure), and is usually somewhat unsymmetrical. In most cases its axis is more nearly continuous with the left, or sometimes the right, medullary fold than with the medullary groove. In sections its front end appears as a ridge on one side or on the middle of the floor of the widened end of the medullary groove.

[FIG. 102. SURFACE VIEW OF THE PELLUCID AREA OF A BLASTODERM OF 18
HOURS.

None of the opaque area is shewn, the pear-shaped outline indicating
the limits of the pellucid area.

At the hinder part of the area is seen the primitive groove _pr._,
with its nearly parallel walls, fading away behind, but curving
round and meeting in front so as to form a distinct anterior
termination to the groove, about halfway up the pellucid area.

Above the primitive groove is seen the medullary groove _m.c._, with
the medullary folds _A._ These, diverging behind, slope away on
either side of the primitive groove, while in front they curve round
and meet each other close upon a curved line which represents the
head-fold.

The second curved line in front of and concentric with the first is
the commencing fold of the amnion.]

The mesoblast and hypoblast, within the area pellucida, do not give rise to the whole of these two layers in the surrounding area opaca; but the whole of the hypoblast of the area opaca, and a large portion of the mesoblast, and possibly even some of the epiblast, take their origin from the peculiar material already spoken of, which forms the germinal wall, and is continuous with the hypoblast at the edge of the area opaca (_vide_ figs. 91, 94, 97, 98, 99, 100).

The exact nature of this material has been the subject of many controversies. Into these controversies it is not my purpose to enter, but subjoined are the results of my own examination. The germinal wall first consists, as already mentioned, of the lower cells of the thickened edge of the blastoderm, and of the subjacent yolk material with nuclei. During the period before the formation of the primitive streak the epiblast extends itself over the yolk, partly, it appears, at the expense of the cells of the germinal wall, and possibly even of cells formed around the nuclei in this part. This mode of growth of the epiblast is very similar to that in the epibolic gastrulas of many Invertebrata, of the Lamprey, etc.; but how far this process is continued in the subsequent extension of the epiblast I am unable to say. The cells of the germinal wall, which are at first well separated from the yolk below, become gradually absorbed in the growth of the hypoblast, and the remaining cells and yolk then become mingled together, and constitute a compound structure, continuous at its inner border with the hypoblast. This structure is the germinal wall usually so described. It is mainly formed of yolk granules with numerous nuclei, and a somewhat variable number of largish cells imbedded amongst them. The nuclei typically form a special layer immediately below the epiblast, some of which are probably enclosed by a definite cell-body. A special mass of nuclei (_vide_ figs. 98 and 100, _n_) is usually present at the junction of the hypoblast with the germinal wall.

The germinal wall at this stage corresponds in many respects with the granular material, forming a ring below the edge of the blastoderm in Teleostei.

It retains the characters above enumerated till near the close of the first day of incubation, _i.e._ till several mesoblastic somites have become established. It then becomes more distinctly separated from the subjacent yolk, and its component parts change very considerably in character. The whole wall becomes much less granular. It is then mainly formed of large vesicles, which often assume a palisade-like arrangement, and contain granular balls, spherules of white yolk, and in an early stage a good deal of granular matter (_vide_ fig. 115). These bodies have some resemblance to cells, and have been regarded as such by Kölliker (No. 135) and Virchow (No. 150): they contain however nothing which can be considered as a nucleus. Between them however nuclei[62] may easily be seen in specimens hardened in picric acid, and stained with hæmatoxylin (these nuclei are not shewn in fig. 115). These nuclei are about the same size as those of the hypoblast cells, and are surrounded by a thin layer of granular protoplasm, which is continuous with a mesh-work of granular protoplasm enveloping the above described vesicles. The germinal wall is still continuous with the hypoblast at its edge; and close to the junction of the two the hypoblast at first forms a layer of moderately columnar cells, one or two deep and directly continuous with the germinal wall, and at a later period usually consists of a mass of rounder cells lying above the somewhat abrupt inner edge of the germinal wall.

[62] The presence of numerous nuclei in the germinal wall was, I
believe, first clearly proved by His (No. 132). I cannot however
accept the greater number of his interpretations.

The germinal wall certainly gives rise to the hypoblast cells, which mainly grow at its expense. They arise at the edge of the area pellucida, and when first formed are markedly columnar, and enclose in their protoplasm one of the smaller vesicles of the germinal wall.

In the later stages (fourth day and onwards) the whole germinal wall is stated to break up into columnar hypoblast cells, each of them mainly formed of one of the vesicles just spoken of. After the commencing formation of the embryo the mesoblast becomes established at the inner edge of the area opaca, between the germinal wall and the epiblast; and gives rise to the tissue which eventually forms the area vasculosa. It seems probable that the mesoblast in this situation is mainly derived from cells formed around the nuclei of the germinal wall, which are usually specially aggregated close below the epiblast. Disse (No. 122) has especially brought evidence in favour of this view, and my own observations also support it.

The mesoblastic somites begin to be formed in the lateral plates of the mesoblast before the closure of the medullary folds. The first somite arises close to the foremost extremity of the primitive streak, but the next is stated to arise in front of this, so that the first formed somite corresponds to the second permanent vertebra[63]. The region of the embryo in front of the second formed somite--at first the largest part of the embryo--is the cephalic region. The somites following the second are formed in the regular manner, from before backwards, out of the unsegmented posterior part of the embryo, which rapidly grows in length to supply the necessary material (fig. 103). As the somites retain during the early stages of development an approximately constant breadth, their number is a fair test of the length of the trunk. With the growth of the embryo the primitive streak is continually carried back, the lengthening of the embryo always taking place between the front end of the primitive streak and the last somite; and during this process the primitive streak undergoes important changes both in itself and in its relation to the embryo. Its anterior thicker part, which is enveloped in the diverging medullary folds, soon becomes distinguished in structure from the part behind this, and placed symmetrically in relation to the axis of the embryo (fig. 103, _a.pr_), and at the same time the medullary folds, which at first simply diverge on each side of the primitive streak, bend in again and meet behind so as completely to enclose the front part of the primitive streak. The region of the embryo bird, where the medullary folds diverge, is known as the sinus rhomboidalis, though it has no connection with the similarly named structure in the adult. By the time that ten somites are formed the sinus rhomboidalis is completely established, and the medullary groove has become converted into a tube till close up to the front end of the sinus. In the following stages the closure of the medullary canal extends to the sinus rhomboidalis, and the folding off of the hind end of the embryo from the yolk commences. Coincidently with the last-named changes the sides of the front part of the primitive streak become thickened, and give rise to conspicuous caudal swellings; in which the layers of the embryo are indistinguishably fused. The apparently hinder part of the primitive streak becomes, as more particularly explained in the sequel, folded downwards and forwards on the ventral side.

[63] Further investigations in confirmation of this widely
accepted statement are very desirable.

[FIG. 103. DORSAL VIEW OF THE HARDENED BLASTODERM OF A CHICK WITH
FIVE MESOBLASTIC SOMITES. THE MEDULLARY FOLDS HAVE MET FOR PART OF
THEIR EXTENT, BUT HAVE NOT UNITED.

_a.pr._ anterior part of the primitive streak; _p.pr._ posterior
part of the primitive streak.]

This is a convenient place to notice remarkable appearances which present themselves close to the junction of the neural plate and the primitive streak. These are temporary passages leading from the hinder end of the neural tube into the alimentary canal. They vary somewhat in different species of birds, and it appears that in the same species there may be several openings of the kind, which appear one after the other and then close again. They were first discovered by Gasser (No. 127). In all cases[64] they lead round the posterior end of the notochord, or through the point where the notochord falls into the primitive streak.

[64] This does not appear to be the case with the anterior
opening in Melopsittacus undulatus, though its relations are not
clear from Braun's description (No. 120).

If the primitive streak is, as I believe, formed of the lips of the blastopore, there can be but little doubt that these structures are disappearing, and functionless rudiments of the opening of the blastopore, and they thus lend support to my view as to the nature of the primitive streak. That, in part, they correspond with the neurenteric canal of the Ichthyopsida is clear from the detailed statements below. Till their relations have been more fully worked out it is not possible to give a more definite explanation of them.

According to Braun (No. 120) three independent communications are to be distinguished in Birds. These are best developed in the Duck. The first of these is a small funnel-shaped diverticulum leading from the neural groove through the hypoblast. It is visible when eight mesoblastic somites are present, and soon disappears. The second, which is the only one I have myself investigated, is present in the embryo duck with twenty-six mesoblastic somites, and is represented in the series of sections (fig. 104). The passage leads obliquely backwards and ventralwards from the hind end of the neural tube into the notochord, where the latter joins the primitive streak (B). A narrow diverticulum from this passage is continued forwards for a short distance along the axis of the notochord (A, _ch_). After traversing the notochord, the passage is continued into a hypoblastic diverticulum, which opens ventrally into the future lumen of the alimentary tract (C). Shortly behind the point where the neurenteric passage communicates with the neural tube the latter structure opens dorsally, and a groove on the surface of the primitive streak is continued backwards from it for a short distance (C). The first part of this passage to appear is the hypoblastic diverticulum above mentioned.

This passage does not long remain open, but after its closure, when the tail-end of the embryo has become folded off from the yolk, a third passage is established, and leads round the end of the notochord from the closed medullary canal into the postanal gut. It is shewn diagrammatically in fig. 106, _ne_, and, as may be gathered from that figure, has the same relations as the neurenteric canal of the Ichthyopsida.

In the goose a passage has been described by Gasser, which appears when about fourteen or fifteen somites are present, and lasts till twenty-three are formed. Behind its opening the medullary canal is continued back as a small diverticulum, which follows the course of the primitive groove and is apparently formed by the conversion of this groove into a canal. It is at first open to the exterior, but soon becomes closed, and then atrophies.

[FIG. 104. FOUR TRANSVERSE SECTIONS THROUGH THE NEURENTERIC PASSAGE
AND ADJOINING PARTS IN A DUCK EMBRYO WITH TWENTY-SIX MESOBLASTIC
SOMITES.

A. Section in front of the neurenteric canal shewing a lumen in the
notochord.

B. Section through the passage from the medullary canal into the
notochord.

C. Section shewing the hypoblastic opening of the neurenteric canal,
and the groove on the surface of the primitive streak, which opens
in front into the medullary canal.

D. Primitive streak immediately behind the opening of the
neurenteric passage.

_mc._ medullary canal; _ep._ epiblast; _hy._ hypoblast; _ch._
notochord; _pr._ primitive streak.]

In the chick there is a perforation on the floor of the neural canal, which is not so marked as those in the goose or duck, and never results in a complete continuity between the neural and alimentary tracts; but simply leads from the floor of the neural canal into the tissues of the tail swelling, and thence into a cavity in the posterior part of the notochord. The hinder diverticulum of the neural canal along the line of the primitive groove is, moreover, very considerable in the chick, and is not so soon obliterated as in the goose. The incomplete passage in the chick arises when about twelve somites are present. It is regarded by Braun as equivalent to the first formed passage in the duck, but I very much doubt whether there is a very exact equivalence between the openings in different types, and think it more probable that they are variable remnants of a primitive neurenteric canal, which in the ancestors of those forms persisted through the whole period of the early development. The third passage is formed in the chick (Kupffer) during the third day of incubation. In Melopsittacus undulatus the two first communications are stated by Braun (No. 120) to be present at the same time, the one in front of the other.

It is probable, from the above description, that the front portion of the primitive streak in the bird corresponds with that part of the lips of the blastopore in Elasmobranchii which becomes converted into the tail swelling and the lining of the neurentic canal; while the original groove of the front part of the primitive streak appears to be converted into the posterior diverticulum of the neural canal. The hinder part of the primitive streak of the bird corresponds, in a very general way, with the part of the blastopore in Elasmobranchii, which shuts off the embryo from the edge of the blastoderm (_vide_ p. 64), though there is of course no genetic relation between the two structures. When the anterior part of the streak is becoming converted into the tail swelling, the groove of the posterior part gradually shallows and finally disappears. The hinder part itself atrophies from behind forwards, and in the course of the folding off of the embryo from the yolk the part of the blastoderm where it was placed becomes folded in, so as to form part of the ventral wall of the embryo. The apparent hinder part of the primitive streak is therefore in reality the ventral and anterior part[65].

[65] This nomenclature may seem a little paradoxical. But on
reflection it will appear that so long as the embryo is simply
extended on the yolk-sphere, the point where the ventral surface
begins has to be decided on purely morphological grounds. That
point may fairly be considered to be close to the junction of the
medullary plate and primitive streak. To use a mathematical
expression the sign will change when we pass from the dorsal to
the ventral surface, so that in strict nomenclature we ought in
continuing round the egg in the same direction to speak of
passing backwards along the medullary, but forwards along the
primitive streak. Thus the apparent hind end of the primitive
streak is really the front end, and _vice versâ_. I have avoided
using this nomenclature to simplify my description, but it is of
the utmost importance that the morphological fact should be
grasped. If any reader fails to understand my point, a reference
to fig. 52 B will, I trust, make everything quite clear. The
heart of Acipenser (_ht_) is there seen apparently in front of
the head. It is of course really ventral, and its apparent
position is due to the extension of the embryo on a sphere. The
apparent front end of the heart is really the hind end, and _vice
versâ_.

It has generally been maintained that the primitive streak and groove become wholly converted into the dorsal portion of the trunk of the embryo, _i.e._ into the posterior part of the medullary plate and subjacent structures. This view appears to me untenable in itself, and quite incompatible with the interpretation of the primitive streak given above. To shew how improbable it is, apart from any theoretical considerations, I have compiled two tables of the relative lengths of the primitive streak and the body of the embryo, measured by the number of sections made through them, in a series of examples from the data in Gasser's important memoir (No. 127). In these tables each horizontal line relates to a single embryo. The first column shews the number of somites, and the second the number of sections through the primitive streak. Where the primitive streak becomes divided into two parts the sections through the two parts are given separately: the left column (A) referring to the anterior part of the streak; the right column (P) to the posterior part. The third column gives the number of sections through the embryo. The first table is for fowl embryos, the second for goose embryos.

+--------+--------------+----------+
| No. of | No. of | No. of |
|Somites.| sections | sections |
| | through | through |
| | the | the |
| | Primitive | Embryo. |
| | Streak. | |
+--------+--------------+----------+
| 0 | 29 | 7 |
| 0 | 45 | 10 |
| 0 | 39 | 23 |
| 2 | 30 | 30 |
| 4 | 30 | 30 |
| | A P | |
| 5 or 6 | 10 + 17 = 27 | |
| 8 | 12 + 20 = 32 | 48 |
| 12 | 13 + 10 = 23 | |
| 14 | 9 + 12 = 21 | |
| 18 | 10 + 7 = 17 | 70 |
| | 8 + 4 = 12 | |
| | 8 + 3 = 11 | |
+--------+--------------+----------+

+--------+--------------+----------+
| No. of | No. of | No. of |
|Somites.| sections | sections |
| | through | through |
| | the | the |
| | Primitive | Embryo. |
| | Streak. | |
+--------+--------------+----------+
| 0 | 10 | 4 |
| 0 | 28 | 5 |
| 0 | 44 | 12 |
| 2 | 36 | 32 |
| 4 | 24 | 42 |
| | A P | |
| 9 | 10 + 10 = 20 | 61 |
| 14 | 8 + 10 = 18 | 68 |
| 17 | 8 + 5 = 13 | |
| 22 | 9 + 6 = 15 | |
| 26 | 6 + 5 = 11 | |
+--------+--------------+----------+

An inspection of these two tables shews that an actual diminution in the length of the primitive streak takes place just about the time when the first somites are being formed, but there is no ground for thinking that the primitive streak becomes then converted into the medullary plate. Subsequently the primitive streak does not for a considerable time become markedly shorter, and certainly its curtailment is not really sufficient to account for the increased length of the embryo--an increase in length, which (with the exception of the head) takes place entirely by additions at the hind end. At the stage with fourteen somites the primitive streak is still pretty long. In the later stages, as is clearly demonstrated by the tables, the diminution in the length of the primitive streak mainly concerns the posterior part and not that adjoining the embryo.

_General history of the germinal layers._

The epiblast. The epiblast of the body of the embryo, though several rows of cells deep, does not become divided into two strata till late in embryonic life; so that the organs of sense formed from the epiblast, which are the same as in the types already described, are not specially formed from an inner nervous stratum. The medullary canal is closed in the same manner as in Elasmobranchii, the Frog, etc., by the simple conversion of an open groove into a closed canal. The closure commences first of all in the region of the mid-brain, and extends rapidly backwards and more slowly forwards. It is completed in the Fowl by about the time that twelve mesoblastic somites are formed.

The mesoblast. The general changes of this layer do not exhibit any features of special interest--the division into lateral and vertebral plates, etc., being nearly the same as in the lower forms.

[FIG. 105. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE AXIS OF AN
EMBRYO BIRD.

The section is supposed to be made at a time when the head-fold has
commenced but the tail-fold has not yet appeared.

_F.So._ head-fold of the somatopleure. _F.Sp._ head-fold of the
splanchnopleure.

_pp._ pleuroperitoneal cavity; _Am._ commencing (head-) fold of the
amnion; _D._ alimentary tract; _N.C._ neural canal; _Ch._ notochord;
_A._ epiblast; _B._ mesoblast; _C._ hypoblast.]

[FIG. 106. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR
END OF AN EMBRYO BIRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.

_ep._ epiblast; _Sp.c._ spinal canal; _ch._ notochord; _n.e._
neurenteric canal; _hy._ hypoblast; _p.a.g._ postanal gut; _pr._
remains of primitive streak folded in on the ventral side; _al._
allantois; _me._ mesoblast; _an._ point where anus will be formed;
_p.c._ perivisceral cavity; _am._ amnion; _so._ somatopleure; _sp._
splanchnopleure.]

The hypoblast. The closure of the alimentary canal is entirely effected by a process of tucking in or folding off of the embryo from the yolk-sack. The general nature of the process is seen in the diagrams figs. 105 and 121. The folds by which it is effected are usually distinguished as the head-, the tail- and the lateral folds. The head-fold (fig. 105) is the first to appear; and in combination with the lateral folds gives rise to the anterior part of the mesenteron (_D_) (including the oesophagus, stomach and duodenum), which by its mode of formation clearly ends blindly in front. The tail-fold, in combination with the two lateral folds, gives rise to the hinder part of the alimentary tract, including the cloaca, which is a true part of the mesenteron. At the junction between the two folds there is present a circular opening leading into the yolk-sack, which becomes gradually narrowed as development proceeds. The opening is completely closed long before the embryo is hatched. Certain peculiarities in reference to the structure of the tail-fold are caused by the formation of the allantois, and are described with the embryonic appendages. The stomodæum and proctodæum are formed by epiblastic invaginations. The communication between the stomodæum and the mesenteron is effected comparatively early (on the 4th day in the chick), while that between the proctodæum and mesenteron does not take place till very late (15th day in the chick). The proctodæum gives rise to the bursa Fabricii, as well as to the anus. Although the opening of the anus is so late in being formed, the proctodæum itself is very early apparent. Soon after the hinder part of the primitive streak becomes tucked in on the ventral side of the embryo, an invagination may be noticed where the tail of the embryo is folded off. This gradually becomes deeper, and finally comes into contact with the hypoblast at the front (primitively the apparent hind) border of the posterior section of the primitive streak. An early stage in the invagination is shewn in the diagram (fig. 106, _an_). It deserves to be noted that the anus lies some way in front of the blind end of the mesenteron, so that there is in fact a well-developed postanal section of the gut (fig. 106, _p.a.g_), which corresponds with that in the Ichthyopsida. For a short period, as mentioned above (p. 163), a neurenteric canal is present connecting the postanal gut with the medullary tube in the duck, fowl, and other birds. On the ventral wall of the postanal gut there are at first two prominences. The posterior of these is formed of part of the tail swelling, and is therefore derived from the apparent anterior part of the primitive streak. The anterior is formed from what was originally the apparent posterior part of the primitive streak. The postanal gut becomes gradually less and less prominent, and finally atrophies.

_General development of the Embryo._

It will be convenient to take the Fowl as a type for the general development of the Sauropsida.

The embryo occupies a fairly constant position with reference to the egg-shell. Its long axis is placed at right angles to that of the egg, and the broad end of the egg is on the left side of the embryo. The general history of the embryo has already been traced up to the formation of the first formed mesoblastic somites (fig. 107). This stage is usually reached at about the close of the first day. After this stage the embryo rapidly grows in length, and becomes, especially in front and to the sides, more and more definitely folded off from the yolk-sack.