Chapter X: Mammalia (1)

The classical researches of Bischoff on the embryology of several mammalian types, as well as those of other observers, have made us acquainted with the general form of the embryos of the Placentalia, and have shewn that, except in the earliest stages of development, there is a close agreement between them. More recently Hensen, Schäfer, Kölliker, Van Beneden and Lieberkühn have shed a large amount of light on the obscurer points of the earliest developmental periods, especially in the rabbit. For the early stages the rabbit necessarily serves as type; but there are grounds for thinking that not inconsiderable variations are likely to be met with in other species, and it is not at present easy to assign to some of the developmental features their true value. We have no knowledge of the early development of the Ornithodelphia or Marsupialia.

The ovum on leaving the ovary is received by the fimbriated extremity of the Fallopian tube, down which it slowly travels. It is still invested by the zona radiata, and in the rabbit an albuminous envelope is formed around it in its passage downwards. Impregnation takes place in the upper part of the Fallopian tube, and is shortly followed by the segmentation, which is remarkable amongst the Amniota for being complete.

Although this process (the details of which have been made known by the brilliant researches of Ed. van Beneden) has already been shortly dealt with as it occurs in the rabbit (Vol. II. p. 98) it will be convenient to describe it again with somewhat greater detail.

The ovum first divides into two nearly equal spheres, of which one is slightly larger and more transparent than the other. The larger sphere and its products will be spoken of as the epiblastic spheres, and the smaller one and its products as the hypoblastic spheres, in accordance with their different destinations.

Both the spheres are soon divided into two, and each of the four so formed into two again; and thus a stage with eight spheres ensues. At the moment of their first separation these spheres are spherical, and arranged in two layers, one of them formed of the four epiblastic spheres, and the other of the four hypoblastic. This position is not long retained, but one of the hypoblastic spheres passes to the centre; and the whole ovum again takes a spherical form.

In the next phase of segmentation each of the four epiblastic spheres divides into two, and the ovum thus becomes constituted of twelve spheres, eight epiblastic and four hypoblastic. The epiblastic spheres have now become markedly smaller than the hypoblastic.

The four hypoblastic spheres next divide, giving rise, together with the eight epiblastic spheres, to sixteen spheres in all; which are nearly uniform in size. Of the eight hypoblastic spheres four soon pass to the centre, while the eight superficial epiblastic spheres form a kind of cup partially enclosing the hypoblastic spheres. The epiblastic spheres now divide in their turn, giving rise to sixteen spheres which largely enclose the hypoblastic spheres. The segmentation of both epiblastic and hypoblastic spheres continues, and in the course of it the epiblastic spheres spread further and further over the hypoblastic, so that at the close of segmentation the hypoblastic spheres constitute a central solid mass almost entirely surrounded by the epiblastic spheres. In a small circular area however the hypoblastic spheres remain for some time exposed at the surface (fig. 134 A).

The whole process of segmentation is completed in the rabbit about seventy hours after impregnation. At its close the epiblast cells, as they may now be called, are clear, and have an irregularly cubical form; while the hypoblast cells are polygonal and granular, and somewhat larger than the epiblast cells.

The opening in the epiblastic layer where the hypoblast cells are exposed on the surface may for convenience be called with Van Beneden the blastopore, though it is highly improbable that it in any way corresponds with the blastopore of other vertebrate ova[76].

[76] It is stated by Bischoff that shortly after impregnation,
and before the commencement of the segmentation, the ova of the
rabbit and guinea-pig are covered with cilia and exhibit the
phenomenon of rotation. This has not been noticed by other
observers.

[FIG. 134. OPTICAL SECTIONS OF A RABBIT'S OVUM AT TWO STAGES CLOSELY
FOLLOWING UPON THE SEGMENTATION. (After E. van Beneden.)

_ep._ epiblast; _hy._ primary hypoblast; _bp._ Van Beneden's
blastopore. The shading of the epiblast and hypoblast is
diagrammatic.]

After its segmentation the ovum passes into the uterus. The epiblast cells soon grow over the blastopore and thus form a complete superficial layer. A series of changes next take place which result in the formation of what has been called the blastodermic vesicle. To Ed. van Beneden we owe the fullest account of these changes; to Hensen and Kölliker however we are also indebted for valuable observations, especially on the later stages in the development of this vesicle.

The succeeding changes commence with the appearance of a narrow cavity between the epiblast and hypoblast, which extends so as completely to separate these two layers except in the region adjoining the original site of the blastopore (fig. 134 B)[77]. The cavity so formed rapidly enlarges, and with it the ovum also; which soon takes the form of a thin-walled vesicle with a large central cavity. This vesicle is the blastodermic vesicle. The greater part of its walls are formed of a single row of flattened epiblast cells; while the hypoblast cells form a small lens-shaped mass attached to the inner side of the epiblast cells (fig. 135).

[77] Van Beneden regards it as probable that the blastopore is
situated somewhat excentrically in relation to the area of
attachment of the hypoblastic mass to the epiblast.

In the Vespertilionidæ Van Beneden and Julin have shewn that the ovum undergoes at the close of segmentation changes of a more or less similar nature to those in the rabbit; the blastopore would however appear to be wider, and to persist even after the cavity of the blastodermic vesicle has commenced to be developed.

[FIG. 135. RABBIT'S OVUM BETWEEN 70-90 HOURS AFTER IMPREGNATION.
(After E. van Beneden.)

_bv._ cavity of blastodermic vesicle (yolk-sack); _ep._ epiblast;
_hy._ primitive hypoblast; _Zp._ mucous envelope (zona pellucida).]

Although by this stage, which occurs in the rabbit between seventy and ninety hours after impregnation, the blastodermic vesicle has by no means attained its greatest dimensions, it has nevertheless grown from about 0.09 mm.--the size of the ovum at the close of segmentation--to about 0.28. It is enclosed by a membrane formed from the zona radiata and the mucous layer around it. The blastodermic vesicle continues to enlarge rapidly, and during the process the hypoblastic mass undergoes important changes. It spreads out on the inner side of the epiblast and at the same time loses its lens-like form and becomes flattened. The central part of it remains however thicker, and is constituted of two rows of cells, while the peripheral part, the outer boundary of which is irregular, is formed of an imperfect layer of amoeboid cells which continually spread further and further within the epiblast. The central thickening of the hypoblast forms an opaque circular spot on the blastoderm, which constitutes the commencement of the embryonic area.

The history of the stages immediately following, from about the commencement of the fifth day to the seventh day, when a primitive streak makes its appearance, is imperfectly understood, and has been interpreted very differently by Van Beneden (No. 171) on the one hand and by Kölliker (184), Rauber (187) and Lieberkühn (186) on the other. I have myself in conjunction with my pupil, Mr Heape, also conducted some investigations on these stages, which have unfortunately not as yet led me to a completely satisfactory reconciliation of the opposing views.

Van Beneden states that about five days after impregnation the hypoblast cells in the embryonic area become divided into two distinct strata, an upper stratum of small cells adjoining the epiblast and a lower stratum of flattened cells which form the true hypoblast. At the edge of the embryonic area the hypoblast is continuous with a peripheral ring of the amoeboid cells of the earlier stage, which now form, except at the edge of the ring, a continuous layer of flattened cells in contact with the epiblast. During the sixth day the flattened epiblast cells are believed by Van Beneden to become columnar. The embryonic area gradually extends itself, and as it does so becomes oval. A central lighter portion next becomes apparent, which gradually spreads, till eventually the darker part of the embryonic area forms a crescent at the posterior part of the now somewhat pyriform embryonic area. The lighter part is formed of columnar epiblast and hypoblast only, while in the darker area a layer of the mesoblast, derived from the intermediate layer of the fifth day, is also found. In this darker area the primitive streak originates early on the seventh day.

Kölliker, following the lines originally laid down by Rauber, has arrived at very different results. He starts from the three-layered condition described by Van Beneden for the fifth day, but does not give any investigations of his own as to the origin of the middle layer. He holds the outer layer to be a provisional layer of protective cells, forming part of the wall of the original vesicle, the middle layer he regards as the true epiblast and the inner layer as the hypoblast.

During the sixth day he finds that the cells of the outer layer gradually cease to form a continuous layer and finally disappear; while the cells of the middle layer become columnar, and form the columnar epiblast present in the embryonic area at the end of the sixth day. The mesoblast first takes its origin in the region and on the formation of the primitive streak.

The investigations of Heape and myself do not extend to the first formation of the intermediate layer found on the fifth day. We find on the sixth day in germinal vesicles of about 2.2-2.5 millimetres in diameter with embryonic areas of about .8 mm. that the embryonic area (fig. 136) is throughout composed of:

(1) A layer of flattened hypoblast cells;
(2) A somewhat irregular layer of more columnar elements, in some
places only a single row deep and in other places two or more
rows deep.
(3) Flat elements on the surface, which do not, however, form
a continuous layer, and are intimately attached to the columnar
cells below.

Our results as to the structure of the blastoderm at this stage closely correspond therefore with those of Kölliker, but on one important point we have arrived at a different conclusion. Kölliker states that he has never found the flattened elements in the act of becoming columnar. We believe that we have in many instances been able to trace them in the act of undergoing this change, and have attempted to shew this in our figure.

Our next oldest embryonic areas were somewhat pyriform measuring about 1.19 mm. in length and .85 in breadth. Of these we have several, some from a rabbit in which we also met with younger still nearly circular areas. All of them had a distinctly marked posterior opacity forming a commencing primitive streak, though decidedly less advanced than in the blastoderm represented in fig. 140. In the younger specimens the epiblast in front of the primitive streak was formed of a single row of columnar cells (fig. 138 A), no mesoblast was present and the hypoblast formed a layer of flattened cells. In the region immediately in front of the primitive streak, an irregular layer of mesoblast cells was interposed between the epiblast and hypoblast. In the anterior part of the primitive streak itself (fig. 138 B) there was a layer of mesoblast with a considerable lateral extension, while in the median line there was a distinct mesoblastic proliferation of epiblast cells. In the posterior sections the lateral extension of the mesoblast was less, but the mesoblast cells formed a thicker cord in the axial line.

Owing to the unsatisfactory character of our data the following attempt to fill in the history of the fifth and sixth days must be regarded as tentative[78]. At the commencement of the fifth day the central thickening, of what has been called above the primitive hypoblast, becomes divided into two layers: the lower of these is continuous with the peripheral hypoblast and is formed of flattened cells, while the upper one is formed of small rounded elements. The superficial epiblast again is formed of flattened cells.

[78] The attempt made below to frame a consecutive history out of
the contradictory data at my disposal is not entirely
satisfactory. Should Kölliker's view turn out to be quite
correct, the origin of the middle layer of the fifth day, which
Kölliker believes to become the permanent epiblast, will have to
be worked out again, in order to determine whether it really
comes, as it is stated by Van Beneden to do, from the primitive
hypoblast.

During the fifth day remarkable changes take place in the epiblast of the embryonic area. It is probable that its constituent cells increase in number and become one by one columnar; and that in the process they press against the layer of rounded elements below them, so that the two layers cease to be distinguishable, and the _whole_ embryonic area acquires in section the characters represented in fig. 136[79]. Towards the end of the sixth day the embryonic area becomes oval, but the changes which next take place are not understood. In the front part of the area only two layers of cells are found, (1) an hypoblast, and (2) an epiblast of columnar cells probably derived from the flattened epiblast cells of the earlier stages. In the posterior part of the blastoderm a middle layer is present (Van Beneden) in addition to the two other layers; and this layer probably originates from the middle layer which extended throughout the area at the beginning of the fifth day, and then became fused with the epiblast. The middle layer does not give rise to the whole of the eventual mesoblast, but only to part of it. From its origin it may be called the hypoblastic mesoblast, and it is probably equivalent to the hypoblastic mesoblast already described in the chick (pp. 154 and 155). The stage just described has only been met with by Van Beneden[80].

[79] The section figured may perhaps hardly appear to justify
this view; the examination of a larger number of sections is,
however, more favourable to it, but it must be admitted that the
interpretation is by no means thoroughly satisfactory.

[80] Kölliker does not believe in the existence of this stage,
having never met with it himself. It appears to me, however, more
probable that Kölliker has failed to obtain it, than that Van
Beneden has been guilty of such an extraordinary blunder as to
have described a stage which has no existence.

[FIG. 136. SECTION THROUGH THE NEARLY CIRCULAR EMBRYONIC AREA OF A
RABBIT'S OVUM OF SIX DAYS, NINE HOURS AND .8 MM. IN DIAMETER.

The section shews the peculiar character of the upper layer with a
certain number of superficial flattened cells; and represents about
half the breadth of the area.]

A diagrammatic view of the whole blastodermic vesicle at about the beginning of the seventh day is given in fig. 137. The embryonic area is represented in white. The line _ge_ in B shews the extension of the hypoblast round the inner side of the vesicle. The blastodermic vesicle is therefore formed of three areas, (1) the embryonic area with three layers: this area is placed where the blastopore was originally situated. (2) The ring around the embryonic area where the walls of the vesicle are formed of epiblast and hypoblast. (3) The area beyond this again where the vesicle is formed of epiblast only[81].

[81] Schäfer describes the blastodermic vesicle of the cat as
being throughout in a bilaminar condition before the formation of
a definite primitive streak or of the mesoblast.

[FIG. 137. VIEWS OF THE BLASTODERMIC VESICLE OF A RABBIT ON THE
SEVENTH DAY WITHOUT THE ZONA. A. from above, B. from the side. (From
Kölliker.)

_ag._ embryonic area; _ge._ boundary of the hypoblast.]

The changes which next take place begin with the formation of a primitive streak, homologous with, and in most respects similar to, the primitive streak in Birds. The formation of the streak is preceded by that of a clear spot near the middle of the blastoderm, forming the nodal point of Hensen. This spot subsequently constitutes the front end of the primitive streak.

The history of the primitive streak was first worked out in a satisfactory manner by Hensen (No. 182), from whom however I differ in admitting the existence of a certain part of the mesoblast before its appearance.

Early on the seventh day the embryonic area becomes pyriform, and at its posterior and narrower end a primitive streak makes its appearance, which is due to a proliferation of rounded cells from the epiblast. At the time when this proliferation commences the layer of hypoblastic mesoblast is present, especially just in front of, and at the sides of, the anterior part of the streak; but no mesoblast is found in the anterior part of the embryonic area. These features are shewn in fig. 138 A and B. The mesoblast derived from the proliferation of the epiblast soon joins the mesoblast already present; though in many sections it seems possible to trace a separation between the two parts (fig. 139 B) of the mesoblast.

[FIG. 138. TWO SECTIONS THROUGH OVAL BLASTODERMS OF A RABBIT ON THE
SEVENTH DAY. THE LENGTH OF THE AREA WAS ABOUT 1.2 MM. AND ITS
BREADTH ABOUT .86 MM.

A. Through the region of the blastoderm in front of the primitive
streak; B. through the front part of the primitive streak; _ep._
epiblast; _m._ mesoblast; _hy._ hypoblast; _pr._ primitive streak.]

[FIG. 139. TWO TRANSVERSE SECTIONS THROUGH THE EMBRYONIC AREA OF AN
EMBRYO RABBIT OF SEVEN DAYS.

The embryo has nearly the structure represented in fig. 140.

A. is taken through the anterior part of the embryonic area. It
represents about half the breadth of the area, and there is no trace
of a medullary groove or of the mesoblast.

B. Is taken through the posterior part of the primitive streak.

_ep._ epiblast; _hy._ hypoblast.]

During the seventh day the primitive streak becomes a more pronounced structure, the mesoblast in its neighbourhood increases in quantity, while an axial groove--the primitive groove--is formed on its upper surface. The mesoblastic layer in front of the primitive streak becomes thicker, and, in the two-layered region in front, the epiblast becomes several rows deep (fig. 139 A).

In the part of the embryonic area in front of the primitive streak there arise during the eighth day two folds bounding a shallow median groove, which meet in front, but diverge behind, and enclose between them the foremost end of the primitive streak (fig. 141). These folds are the medullary folds and they constitute the first definite traces of the embryo. The medullary plate bounded by them rapidly grows in length, the primitive streak always remaining at its hinder end. While the lateral epiblast is formed of several rows of cells, that of the medullary plate is at first formed of but a single row (fig. 142, _mg_). The mesoblast, which appears to grow forward from the primitive streak, is stated to be at first a continuous sheet between the epiblast and hypoblast (Hensen). The evidence on this point does not however appear to me to be quite conclusive. In any case, as soon as ever the medullary groove is formed, the mesoblast becomes divided, exactly as in Lacerta and Elasmobranchii, into two independent lateral plates, which are not continuous across the middle line (fig. 142, _me_). The hypoblast cells are flattened laterally, but become columnar beneath the medullary plate (fig. 142).

[FIG. 140. EMBRYONIC AREA OF AN EIGHT DAYS' RABBIT. (After
Kölliker.)

_arg._ embryonic area; _pr._ primitive streak.]

In tracing the changes which take place in the relations of the layers, in passing from the region of the embryo to that of the primitive streak, it will be convenient to follow the account given by Schäfer for the guinea-pig (No. 190), which on this point is far fuller and more satisfactory than that of other observers. In doing so I shall leave out of consideration the fact (fully dealt with later in this chapter) that the layers in the guinea-pig are inverted. Fig. 143 represents a series of sections through this part in the guinea-pig. The anterior section (D) passes through the medullary groove near its hinder end. The commencement of the primitive streak is marked by a slight prominence on the floor of the medullary groove between the two diverging medullary folds (fig. 143 C, _ae_). Where this prominence becomes first apparent the epiblast and hypoblast are united together. The mesoblast plates at the two sides remain in the meantime quite free. Slightly further back, but before the primitive groove is reached, the epiblast and hypoblast are connected together by a cord of cells (fig. 143 B. _f_), which in the section next following becomes detached from the hypoblast and forms a solid keel projecting from the epiblast. In the following section the hitherto independent mesoblast plates become united with this keel (fig. 143 A); and in the posterior sections, through the part of the primitive streak with the primitive groove, the epiblast and mesoblast continue to be united in the axial line, but the hypoblast remains distinct. These peculiar relations may shortly be described by saying that in the axial line the hypoblast becomes _united with the epiblast at the posterior end of the embryo_; and that the cells which connect the hypoblast and epiblast are posteriorly continuous with the fused epiblast and mesoblast of the primitive streak, the hypoblast in the region of the primitive streak having become distinct from the other layers.

[FIG. 141. EMBRYONIC AREA OF A SEVEN DAYS' EMBRYO RABBIT. (From
Kölliker.)

_o._ place of future area vasculosa; _rf._ medullary groove; _pr._
primitive streak; _ag._ embryonic area.]

[FIG. 142. TRANSVERSE SECTION THROUGH AN EMBRYO RABBIT OF EIGHT
DAYS.

_ep._ epiblast; _me._ mesoblast; _hy._ hypoblast; _mg._ medullary
groove.]

[FIG. 143. A SERIES OF TRANSVERSE SECTIONS THROUGH THE JUNCTION OF
THE PRIMITIVE STREAK AND MEDULLARY GROOVE OF A YOUNG GUINEA-PIG.
(After Schäfer.)

A. is the posterior section.

_e._ epiblast; _m._ mesoblast; _h._ hypoblast; _ae._ axial epiblast
of the primitive streak; _ah._ axial hypoblast attached in B. and C.
to the epiblast at the rudimentary blastopore; _mg._ medullary
groove; _f._ rudimentary blastopore.]

The peculiar relations just described, which hold also for the rabbit, receive their full explanation by a comparison of the Mammal with the Bird and the Lizard, but before entering into this comparison, it will be well to describe the next stage in the rabbit, which is in many respects very instructive. In this stage the thickened axial portion of the hypoblast in the region of the embryo becomes separated from the lateral part as the notochord. Very shortly after the formation of the notochord, the hypoblast grows in from the two sides, and becomes quite continuous across the middle line. The formation of the notochord takes place from before backwards; and at the hinder end of the embryo the notochord is continued into the mass of cells which forms the axis of the primitive streak, becoming therefore at this point continuous with the epiblast. The notochord in fact behaves exactly as did the axial hypoblast in the earlier stage.

In comparison with Lacerta (pp. 203-205) it is obvious that the axial hypoblast and the notochord derived from it have exactly the same relations in Mammalia and Lacertilia. In both they are continued at the hind end of the embryo into the epiblast; and close to where they join it, the mesoblast and epiblast fuse together to form the primitive streak. The difference between the two types consists in the fact that in Reptilia there is formed a passage connecting the neural and alimentary canals, the front wall of which is constituted by the cells which form the above junction between the notochord and epiblast; and that in Mammalia this passage--which is only a rudimentary structure in Reptilia--has either been overlooked or else is absent. In any case the axial junction of the epiblast and hypoblast in Mammalia is shewn by the above comparison with Lacertilia to represent the dorsal lip of the true vertebrate blastopore. The presence of this blastopore seems to render it clear that the blastopore discovered by Ed. van Beneden cannot have the meaning he assigned to it in comparing it with the blastopore of the frog.

Kölliker adduces the fact that the notochord is continuous with the axial cells of the primitive streak as an argument against its hypoblastic origin. The above comparison with Lacertilia altogether deprives this argument of any force.

At the stage we have now reached the three layers are definitely established. The epiblast (on the view adopted above) clearly originates from epiblastic segmentation cells. The hypoblast without doubt originates from the hypoblastic segmentation spheres which give rise to the lenticular mass within the epiblast on the appearance of the cavity of the blastodermic vesicle; while, though the history of the mesoblast is still obscure, part of it appears to originate from the hypoblastic mass, and part is undoubtedly formed from the epiblast of the primitive streak.

While these changes have been taking place the rudiments of a vascular area become formed, and it is very possible that part of the hypoblastic mesoblast passes in between the epiblast and hypoblast, immediately around the embryonic area, to give rise to the area vasculosa. From Hensen's observation it seems at any rate clear that the mesoblast of the vascular area arises independently of the primitive streak: an observation which is borne out by the analogy of Birds.

_General growth of the Embryo._

We have seen that the blastodermic vesicle becomes divided at an early stage of development into an embryonic area, and a non-embryonic portion. The embryonic area gives rise to the whole of the body of the embryo, while the non-embryonic part forms an appendage, known as the umbilical vesicle, which becomes gradually folded off from the embryo, and has precisely the relations of the yolk-sack of the Sauropsida. It is almost certain that the Placentalia are descended from ancestors, the embryos of which had large yolk-sacks, but that the yolk has become reduced in quantity owing to the nutriment received from the wall of the uterus taking the place of that originally supplied by the yolk. A rudiment of the yolk-sack being retained in the umbilical vesicle, this structure may be called indifferently umbilical vesicle or yolk-sack.

The yolk which fills the yolk-sack in Birds is replaced in Mammals by a coagulable fluid; while the gradual extension of the hypoblast round the wall of the blastodermic vesicle, which has already been described, is of the same nature as the growth of the hypoblast round the yolk-sack in Birds.

The whole embryonic area would seem to be employed in the formation of the body of the embryo. Its long axis has no very definite relation to that of the blastodermic vesicle. The first external trace of the embryo to appear is the medullary plate, bounded by the medullary folds, and occupying at first the anterior half of the embryonic area (fig. 141). The two medullary folds diverge behind and enclose the front end of the primitive streak. As the embryo elongates, the medullary folds nearly meet behind and so cut off the front portion of the primitive streak, which then appears as a projection in the hind end of the medullary groove. In an embryo rabbit, eight days after impregnation, the medullary groove is about 1.80 mm. in length. At this stage a division may be clearly seen in the lateral plates of mesoblast into a vertebral zone adjoining the embryo and a more peripheral lateral zone; and in the vertebral zone indications of two somites, about 0.37 mm. from the hinder end of the embryo, become apparent. The foremost of these somites marks the junction, or very nearly so, of the cephalic region and trunk. The small size of the latter as compared with the former is very striking, but is characteristic of Vertebrates generally. The trunk gradually elongates relatively to the head, by the addition behind of fresh somites. The embryo has not yet begun to be folded off from the yolk-sack. In a slightly older embryo of nine days there appears (Hensen, Kölliker) round the embryonic area a delicate clear ring which is narrower in front than behind (fig. 144 A, _ap_). This ring is regarded by these authors as representing the peripheral part of the area pellucida of Birds, which does not become converted into the body of the embryo. Outside the area pellucida, an area vasculosa has become very well defined. In the embryo itself (fig. 144 A) the disproportion between head and trunk is less marked than before; the medullary plate dilates anteriorly to form a spatula-shaped cephalic enlargement; and three or four somites are established. In the lateral parts of the mesoblast of the head there may be seen on each side a tube-like structure (_hz_). Each of these is part of the heart, which arises as two independent tubes. The remains of the primitive streak (_pr_) are still present behind the medullary groove.

In somewhat older embryos (fig. 144 B) with about eight somites, in which the trunk considerably exceeds the head in length, the first distinct traces of the folding-off of the head end of the embryo become apparent, and somewhat later a fold also appears at the hind end. In the formation of the hind end of the embryo the primitive streak gives rise to a tail swelling and to part of the ventral wall of the postanal gut. In the region of the head the rudiments of the heart (_h_) are far more definite. The medullary groove is still open for its whole length, but in the head it exhibits a series of well-marked dilatations. The foremost of these (_vh_) is the rudiment of the fore-brain, from the sides of which there project the two optic vesicles (_ab_); the next is the mid-brain (_mh_), and the last is the hind-brain (_hh_), which is again divided into smaller lobes by successive constrictions. The medullary groove behind the region of the somites dilates into an embryonic sinus rhomboidalis like that of the Bird. Traces of the amnion (_af_) are now apparent both in front of and behind the embryo.

[FIG. 144. EMBRYO RABBITS OF ABOUT NINE DAYS FROM THE DORSAL SIDE.
(From Kölliker.)

A. magnified 22 times, and B. 21 times.

_ap._ area pellucida; _rf._ medullary groove; _h´._ medullary plate
in the region of the future fore-brain; _h´´._ medullary plate in the
region of the future mid-brain; _vh._ fore-brain; _ab._ optic
vesicle; _mh._ mid-brain; _hh._ and _h´´._ hind-brain; _uw._
mesoblastic somite; _stz._ vertebral zone; _pz._ lateral zone; _hz._
and _h._ heart; _ph._ pericardial section of body cavity; _vo._
vitelline vein; _af._ amnion fold.]

The structure of the head and the formation of the heart at this age are illustrated in fig. 145. The widely-open medullary groove (_rf_) is shewn in the centre. Below it the hypoblast is thickened to form the notochord _dd´_; and at the sides are seen the two tubes, which, on the folding-in of the foregut, give rise to the unpaired heart. Each of these is formed of an outer muscular tube of splanchnic mesoblast (_ahh_), not quite closed towards the hypoblast, and an inner epithelioid layer (_ihh_); and is placed in a special section of the body cavity (_ph_), which afterwards forms the pericardial cavity.

[FIG. 145. TRANSVERSE SECTION THROUGH THE HEAD OF A RABBIT OF THE
SAME AGE AS FIG. 144 B. (From Kölliker.)

B. is a more highly magnified representation of part of A.

_rf._ medullary groove; _mp._ medullary plate; _rw._ medullary fold;
_h._ epiblast; _dd._ hypoblast; _dd´._ notochordal thickening of
hypoblast; _sp._ undivided mesoblast; _hp._ somatic mesoblast;
_dfp._ splanchnic mesoblast; _ph._ pericardial section of body
cavity; _ahh._ muscular wall of heart; _ihh._ epithelioid layer of
heart; _mes._ lateral undivided mesoblast; _sw._ fold of hypoblast
which will form the ventral wall of the pharynx; _sr._ commencing
throat.]

Before the ninth day is completed great external changes are usually effected. The medullary groove becomes closed for its whole length with the exception of a small posterior portion. The closure commences, as in Birds, in the region of the mid-brain. Anteriorly the folding-off of the embryo proceeds so far that the head becomes quite free, and a considerable portion of the throat, ending blindly in front, becomes established. In the course of this folding the, at first widely separated, halves of the heart are brought together, coalesce on the ventral side of the throat, and so give rise to a median undivided heart. The fold at the tail end of the embryo progresses considerably, and during its advance the allantois is formed in the same way as in Birds. The somites increase in number to about twelve. The amniotic folds nearly meet above the embryo.

[FIG. 146. ADVANCED EMBRYO OF A RABBIT (ABOUT TWELVE DAYS)[82].

_mb_. mid-brain; _th._ thalamencephalon; _ce._ cerebral hemisphere;
_op._ eye; _iv.v._ fourth ventricle; _mx._ maxillary process; _md._
mandibular arch; _hy._ hyoid arch; _fl._ fore-limb; _hl._ hind-limb;
_um._ umbilical stalk.]

[82] This figure was drawn for me by my pupil, Mr Weldon.

The later stages in the development proceed in the main in the same manner as in the Bird. The cranial flexure soon becomes very marked, the mid-brain forming the end of the long axis of the embryo (fig. 146). The sense organs have the usual development. Under the fore-brain appears an epiblastic involution giving rise both to the mouth and to the pituitary body. Behind the mouth are three well-marked pairs of visceral arches. The first of these is the mandibular arch (fig. 146, _md_), which meets its fellow in the middle line, and forms the posterior boundary of the mouth. It sends forward on each side a superior maxillary process (_mx_) which partially forms the anterior margin of the mouth. Behind the mandibular arch are present a well-developed hyoid (_hy_) and a first branchial arch (not shewn in fig. 146). There are four clefts, as in other Amniota, but the fourth is not bounded behind by a definite arch. Only the first of these clefts persists as the tympanic cavity and Eustachian tube.

At the time when the cranial flexure appears, the body also develops a sharp flexure immediately behind the head, which is thus bent forwards upon the posterior straight part of the body (fig. 146). The amount of this flexure varies somewhat in different forms. It is very marked in the dog (Bischoff). At a later period, and in some species even before the stage figured, the tail end of the body also becomes bent (fig. 146), so that the whole dorsal side assumes a convex curvature, and the head and tail become closely approximated. In most cases the embryo, on the development of the tail, assumes a more or less definite spiral curvature (fig. 146); which however never becomes nearly so marked a feature as it commonly is in Lacertilia and Ophidia. With the more complete development of the lower wall of the body the ventral flexure partially disappears, but remains more or less persistent till near the close of intra-uterine life. The limbs are formed as simple buds in the same manner as in Birds. The buds of the hind-limbs are directed somewhat forwards, and those of the fore-limb backwards.

_Embryonic membranes and yolk-sack._

The early stages in the development of the embryonic membranes are nearly the same as in Aves; but during the later stages in the Placentalia the allantois enters into peculiar relations with the uterine walls, and the two, together with the interposed portion of the subzonal membrane or false amnion, give rise to a very characteristic Mammalian organ--the placenta--into the structure of which it will be necessary to enter at some length. The embryonic membranes vary so considerably in the different forms that it will be advantageous to commence with a description of their development in an ideal case.

We may commence with a blastodermic vesicle, closely invested by the delicate remnant of the zona radiata, at the stage in which the medullary groove is already established. Around the embryonic area a layer of mesoblast would have extended for a certain distance; so as to give rise to an area vasculosa, in which however the blood-vessels would not have become definitely established. Such a vesicle is represented diagrammatically in fig. 147, 1. Somewhat later the embryo begins to be folded off, first in front and then behind (fig. 147, 2). These folds result in a constriction separating the embryo and the yolk-sack (_ds_), or as it is known in Mammalian embryology, the umbilical vesicle. The splitting of the mesoblast into a splanchnic and a somatic layer has taken place, and at the front and hind end of the embryo a fold (_ks_) of the somatic mesoblast and epiblast begins to rise up and grow over the head and tail of the embryo. These two folds form the commencement of the amnion. The head and tail folds of the amnion are continued round the two sides of the embryo, till they meet and unite into a continuous fold. This fold grows gradually upwards, but before it has completely enveloped the embryo, the blood-vessels of the area vasculosa become fully developed. They are arranged in a manner not very different from that in the chick.

The following is a brief account of their arrangement in the Rabbit:--

The outer boundary of the area, which is continually
extending further and further round the umbilical vesicle,
is marked by a venous sinus terminalis (fig. 147, _st_).
The area is not, as in the chick, a nearly complete circle,
but is in front divided by a deep indentation extending
inwards to the level of the heart. In consequence of this
indentation the sinus terminalis ends in front in two
branches, which bend inwards and fall directly into the
main vitelline veins. The blood is brought from the dorsal
aortæ by a series of lateral vitelline arteries, and not by
a single pair as in the chick. These arteries break up into
a more deeply situated arterial network, from which the
blood is continued partly into the sinus terminalis, and
partly into a superficial venous network. The hinder end of
the heart is continued into two vitelline veins, each of
which divides into an anterior and a posterior branch. The
anterior branch is a limb of the sinus terminalis, and the
posterior and smaller branch is continued towards the hind
part of the sinus, near which it ends. On its way it
receives, on its outer side, numerous branches from the venous
network, which connect by their anastomoses the posterior branch
of the vitelline vein and the sinus terminalis.

[FIG. 147. FIVE DIAGRAMMATIC FIGURES ILLUSTRATING THE FORMATION OF
THE FOETAL MEMBRANES OF A MAMMAL. (From Kölliker.)

In 1, 2, 3, 4 the embryo is represented in longitudinal section.

1. Ovum with zona pellucida, blastodermic vesicle, and embryonic
area.
2. Ovum with commencing formation of umbilical vesicle and amnion.
3. Ovum with amnion about to close, and commencing allantois.
4. Ovum with villous subzonal membrane, larger allantois, and mouth
and anus.
5. Ovum in which the mesoblast of the allantois has extended round
the inner surface of the subzonal membrane and united with it to
form the chorion. The cavity of the allantois is aborted. This fig.
is a diagram of an early human ovum.

_d._ zona radiata; _d´._ processes of zona; _sh._ subzonal membrane;
_ch._ chorion; _ch.z._ chorionic villi; _am._ amnion; _ks._
head-fold of amnion; _ss._ tail-fold of amnion; _a._ epiblast of
embryo; _a´._ epiblast of non-embryonic part of the blastodermic
vesicle; _m._ embryonic mesoblast; _m´._ non-embryonic mesoblast;
_df._ area vasculosa; _st._ sinus terminalis; _dd._ embryonic
hypoblast; _i._ non-embryonic hypoblast; _kh._ cavity of
blastodermic vesicle, the greater part of which becomes the cavity
of the umbilical vesicle _ds._; _dg._ stalk of umbilical vesicle;
_al._ allantois; _e._ embryo; _r._ space between chorion and amnion
containing albuminous fluid; _vl._ ventral body wall; _hh._
pericardial cavity.]

While the above changes have been taking place the whole blastodermic vesicle, still enclosed in the zona, has become attached to the walls of the uterus. In the case of the typical uterus with two tubular horns, the position of each embryo, when there are several, is marked by a swelling in the walls of the uterus, preparatory to the changes which take place on the formation of the placenta. In the region of each swelling the zona around the blastodermic vesicle is closely embraced, in a ring-like fashion, by the epithelium of the uterine wall. The whole vesicle assumes an oval form, and it lies in the uterus with its two ends free. The embryonic area is placed close to the mesometric attachment of the uterus. In many cases peculiar processes or villi grow out from the ovum (fig. 147, 4, _sz_), which fit into the folds of the uterine epithelium. The nature of these processes requires further elucidation, but in some instances they appear to proceed from the zona (the Rabbit) and in other instances from the subzonal membrane (the Dog). In any case the attachment between the blastodermic vesicle and the uterine wall becomes so close at the time when the body of the embryo is first formed out of the embryonic area, that it is hardly possible to separate them without laceration; and at this period--from the 8th to the 9th day in the Rabbit--it requires the greatest care to remove the ovum from the uterus without injury. It will be understood of course that the attachment above described is at first purely superficial and not vascular.

Shortly after the establishment of the circulation of the yolk-sack the folds of the amnion meet and coalesce above the embryo (fig. 147, 3 and 4, _am_). After this the inner or true amnion becomes severed from the outer or false amnion, though the two sometimes remain connected by a narrow stalk. Between the true and false amnion is a continuation of the body cavity. The true amnion consists of a layer of epiblastic epithelium and generally also of somatic mesoblast, while the false amnion consists, as a rule, of epiblast only; though it is possible that in some cases (the Rabbit?) the mesoblast may be continued along its inner face.

[FIG. 147*. DIAGRAM OF THE FOETAL MEMBRANES OF A MAMMAL. (From
Turner.)

Structures which either are or have been at an earlier period of
development continuous with each other are represented by the same
character of shading.

_pc._ zona with villi; _sz._ subzonal membrane; _E._ epiblast of
embryo; _am._ amnion; _AC._ amniotic cavity; _M._ mesoblast of
embryo; _H._ hypoblast of embryo; _UV._ umbilical vesicle; _al._
allantois; _ALC._ allantoic cavity.]

Before the two limbs of the amnion are completely severed, the epiblast of the umbilical vesicle becomes separated from the mesoblast and hypoblast of the vesicle (fig. 147, 3), and, together with the false amnion (_sh_), with which it is continuous, forms a complete lining for the inner face of the zona radiata. The space between this membrane and the umbilical vesicle with the attached embryo is obviously continuous with the body cavity (_vide_ figs. 147, 4 and 147*). To this membrane Turner has given the appropriate name of subzonal membrane: by Von Baer it was called the serous envelope. It soon fuses with the zona radiata, or at any rate the zona ceases to be distinguishable.

While the above changes are taking place in the amnion, the allantois grows out from the hind gut as a vesicle lined by hypoblast, but covered externally by a layer of splanchnic mesoblast (fig. 147, 3 and 4, _al_)[83]. The allantois soon becomes a flat sack, projecting into the now largely developed space between the subzonal membrane and the amnion, on the dorsal side of the embryo (fig. 147*, _ALC_). In some cases it extends so as to cover the whole inner surface of the subzonal membrane; in other cases again its extension is much more limited. Its lumen may be retained or may become nearly or wholly aborted. A fusion takes place between the subzonal membrane and the adjoining mesoblastic wall of the allantois, and the two together give rise to a secondary membrane round the ovum, known as the chorion. Since however the allantois does not always come in contact with the whole inner surface of the subzonal membrane, the term chorion is apt to be somewhat vague; and in the rabbit, for instance, a considerable part of the so-called chorion is formed by a fusion of the wall of the yolk-sack with the subzonal membrane (fig. 148). The placental region of the chorion may in such cases be distinguished as the true chorion, from the remaining part which will be called the false chorion.

[83] The hypoblastic element in the allantois is sometimes very
much reduced, so that the allantois may be mainly formed of a
vascular layer of mesoblast.

The mesoblast of the allantois, especially that part of it which assists in forming the chorion, becomes highly vascular; the blood being brought to it by two allantoic arteries continued from the terminal bifurcation of the dorsal aorta, and returned to the body by one, or rarely two, allantoic veins, which join the vitelline veins from the yolk-sack. From the outer surface of the true chorion (fig. 147, 5, _d_, 148) villi grow out and fit into crypts or depressions which have in the meantime made their appearance in the walls of the uterus[84]. The villi of the chorion are covered by an epithelium derived from the subzonal membrane, and are provided with a connective tissue core containing an artery and vein and a capillary plexus connecting them. In most cases they assume a more or less arborescent form, and have a distribution on the surface of the chorion varying characteristically in different species. The walls of the crypts into which the villi are fitted also become highly vascular, and a nutritive fluid passes from the maternal vessels of the placenta to the foetal vessels by a process of diffusion; while there is probably also a secretion by the epithelial lining of the walls of the crypts, which becomes absorbed by the vessels of the foetal villi. The above maternal and foetal structures constitute together the organ known as the placenta. The maternal portion consists essentially of the vascular crypts in the uterine walls, and the foetal portion of more or less arborescent villi of the true chorion fitting into these crypts.

[84] These crypts have no connection with the openings of glands
in the walls of the uterus. They are believed by Ercolani to be
formed to a large extent by a regeneration of the lining tissue
of the uterine walls.

While the placenta is being developed, the folding-off of the embryo from the yolk-sack becomes more complete; and the yolk-sack remains connected with the ileal region of the intestine by a narrow stalk, the vitelline duct (fig. 147, 4 and 5 and fig. 147*), consisting of the same tissues as the yolk-sack, viz. hypoblast and splanchnic mesoblast. While the true splanchnic stalk of the yolk-sack is becoming narrow, a somatic stalk connecting the amnion with the walls of the embryo is also formed, and closely envelops the stalk both of the allantois and the yolk-sack. The somatic stalk together with its contents is known as the umbilical cord. The mesoblast of the somatopleuric layer of the cord develops into a kind of gelatinous tissue, which cements together the whole of the contents. The allantoic arteries in the cord wind in a spiral manner round the allantoic vein. The yolk-sack in many cases atrophies completely before the close of intra-uterine life, but in other cases it is only removed with the other embryonic membranes at birth. The intra-embryonic portion of the allantoic stalk gives rise to two structures, viz. to (1) the urinary bladder formed by a dilatation of its proximal extremity, and to (2) a cord known as the urachus connecting the bladder with the wall of the body at the umbilicus. The urachus, in cases where the cavity of the allantois persists till birth, remains as an open passage connecting the intra- and extra-embryonic parts of the allantois. In other cases it gradually closes, and becomes nearly solid before birth, though a delicate but interrupted lumen would appear to persist in it. It eventually gives rise to the ligamentum vesicæ medium.

At birth the foetal membranes, including the foetal portion of the placenta, are shed; but in many forms the interlocking of the foetal villi with the uterine crypts is so close that the uterine mucous membrane is carried away with the foetal part of the placenta. It thus comes about that in some placentæ the maternal and foetal parts simply separate from each other at birth, and in others the two remain intimately locked together, and both are shed together as the afterbirth. These two forms of placenta are distinguished as non-deciduate and deciduate, but it has been shewn by Ercolani and Turner that no sharp line can be drawn between the two types; moreover, a larger part of the uterine mucous membrane than that forming the maternal part of the placenta is often shed in the deciduate Mammalia, and in the non-deciduate Mammalia it is probable that the mucous membrane (not including vascular parts) of the maternal placenta either peels or is absorbed.

_Comparative history of the Mammalian foetal membranes._

Two groups of Mammalia--the Monotremata and the Marsupialia--are believed not to be provided with a true placenta.

The nature of the foetal membranes in the Monotremata is not known. Ova, presumably in an early stage of development, have been found free in the uterus of Ornithorhyncus by Owen. The lining membrane of the uterus was thickened and highly vascular. The females in which these were found were killed early in October[85].

[85] The following is Owen's account of the young after birth
(_Comp. Anat. of Vertebrates_, Vol. III. p. 717):
"On the eighth of December Dr Bennet discovered in the
subterranean nest of Ornithorhyncus three living young, naked,
not quite two inches in length." On the 12th of August, 1864, "a
female Echidna hystrix was captured ... having a young one with
its head buried in a mammary or marsupial fossa. This young one
was naked, of a bright red colour, and one inch two lines in
length."

Marsupialia. Our knowledge of the foetal membranes of the Marsupialia is almost entirely due to Owen. In Macropus major he found that birth took place thirty-eight days after impregnation. A foetus at the twentieth day of gestation measured eight lines from the mouth to the root of the tail. The foetus was enveloped in a large subzonal membrane, with folds fitting into uterine furrows, but _not adhering to the uterus, and without villi_. The embryo was enveloped in an amnion reflected over the stalk of the yolk-sack, which was attached by a filamentary pedicle to near the end of the ileum. The yolk-sack was large and vascular, and was connected with the foetal vascular system by a vitelline artery and two veins. The yolk-sack was partially adherent, especially at one part, to the subzonal membrane. No allantois was observed. In a somewhat older foetus of ten lines in length there was a small allantois supplied by two allantoic arteries and one vein. The allantois was quite free and not attached to the subzonal membrane. The yolk-sack was more closely attached to the subzonal membrane than in the younger embryo[86].

[86] Owen quotes in the _Anatomy of Vertebrates_, Vol. III. p.
721, a description from Rengger of the development of Didelphis
azaræ, which would seem to imply that a vascular adhesion arises
between the uterine walls and the subzonal membrane, but the
description is too vague to be of any value in determining the
nature of the foetal membranes.

All Mammalia, other than the Monotremata and Marsupialia, have a true allantoic placenta. The placenta presents a great variety of forms, and it will perhaps be most convenient first to treat these varieties in succession, and then to give a general exposition of their mutual affinities[87].

[87] Numerous contributions to our knowledge of the various types
of placenta have been made during the last few years, amongst
which those of Turner and Ercolani may be singled out, both from
the variety of forms with which they deal, and the important
light they have thrown on the structure of the placenta.

Amongst the existing Mammals provided with a true placenta, the most primitive type is probably retained by those forms in which the placental part of the chorion is confined to a comparatively restricted area on the dorsal side of the embryo; while the false chorion is formed by the vascular yolk-sack fusing with the remainder of the subzonal membrane. In all the existing forms with this arrangement of foetal membranes, the placenta is deciduate. This, however, was probably not the case in more primitive forms from which these are descended[88]. The placenta would appear from Ercolani's description to be simpler in the mole (Talpa) than in other species. The Insectivora, Cheiroptera, and Rodentia are the groups with this type of placenta; and since the rabbit, amongst the latter, has been more fully worked out than other species, we may take it first.