Illustration: Figure 140Fig. 140. embryonic area of an eight days' Rabbit.(After Kölliker.)arg.embryonic area;pr.primitive streak.
Fig. 140. embryonic area of an eight days' Rabbit.(After Kölliker.)arg.embryonic area;pr.primitive streak.
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).
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. 143represents 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. 143C,ae). Where this prominence becomes first apparent the epiblast and hypoblast are united together. The mesoblast plates at the two sides remainin 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. 143B.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. 143A); 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 becomesunited 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.
Illustration: Figure 141Fig. 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. 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.
Illustration: Figure 142Fig. 142. Transverse section through an embryo Rabbit of eight days.ep.epiblast;me.mesoblast;hy.hypoblast;mg.medullary groove.
Fig. 142. Transverse section through an embryo Rabbit of eight days.ep.epiblast;me.mesoblast;hy.hypoblast;mg.medullary groove.
Illustration: Figure 143Fig. 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.
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 stagethe 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 foldsnearly 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 1.80mm.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 0.37mm.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. 144A,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. 144A) 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. 144B) 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 inthe 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.
Illustration: Figure 144Fig. 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.andh´´´.hind-brain;uw.mesoblastic somite;stz.vertebral zone;pz.lateral zone;hz.andh.heart;ph.pericardial section of body cavity;vo.vitelline vein;af.amnion fold.
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.andh´´´.hind-brain;uw.mesoblastic somite;stz.vertebral zone;pz.lateral zone;hz.andh.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 infig. 145. The widely-open medullary groove (rf) is shewn in the centre. Below it the hypoblast is thickened to form the notochorddd´; 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.
Illustration: Figure 145Fig. 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.
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 farthat 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.
Illustration: Figure 146Fig. 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.
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.
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 superiormaxillary 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 infig. 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 infig. 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.
Illustration: Figure 147Fig. 147. Five diagrammatic figures illustrating the formation of the fœtal 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 vesicleds.;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.
Fig. 147. Five diagrammatic figures illustrating the formation of the fœtal 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 vesicleds.;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-sackthe 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.
Illustration: Figure 147 asteriskFig. 147*. Diagram of the fœtal 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.
Fig. 147*. Diagram of the fœtal 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 (videfigs.147, 4 and147*). 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.
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 theirappearance 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 fœtal 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 fœtal villi. The above maternal and fœtal structures constitute together the organ known as the placenta. The maternal portion consists essentially of the vascular crypts in the uterine walls, and the fœtal portion of more or less arborescent villi of the true chorion fitting into these crypts.
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 andfig. 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 bladderformed 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 fœtal membranes, including the fœtal portion of the placenta, are shed; but in many forms the interlocking of the fœtal villi with the uterine crypts is so close that the uterine mucous membrane is carried away with the fœtal part of the placenta. It thus comes about that in some placentæ the maternal and fœtal 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 fœtal membranes.
Two groups of Mammalia—the Monotremata and the Marsupialia—are believed not to be provided with a true placenta.
The nature of the fœtal 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].
Marsupialia. Our knowledge of the fœtal 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 fœtus at the twentieth day of gestation measured eight lines from the mouth to the root of the tail. The fœtus was enveloped in a large subzonal membrane, with folds fitting into uterine furrows, butnot 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 fœtal 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 fœtus 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].
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].
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 thevascular yolk-sack fusing with the remainder of the subzonal membrane. In all the existing forms with this arrangement of fœtal 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.
The Rabbit. In the pregnant female Rabbit several ova are generally found in each horn of the uterus. The general condition of the egg-membranes at the time of their full development is shewn infig. 148.
Illustration: Figure 148Fig. 148. diagrammatic longitudinal section of a Rabbit’s ovum at an advanced stage of pregnancy.(From Kölliker after Bischoff.)e.embryo;a.amnion;a.urachus;al.allantois with blood-vessels;sh.subzonal membrane;pl.placental villi;fd.vascular layer of yolk-sack;ed.hypoblastic layer of yolk-sack;ed´.inner portion of hypoblast, anded´´.outer portion of hypoblast lining the compressed cavity of the yolk-sack;ds.cavity of yolk-sack;st.sinus terminalis;r.space filled with fluid between the amnion, the allantois and the yolk-sack.
Fig. 148. diagrammatic longitudinal section of a Rabbit’s ovum at an advanced stage of pregnancy.(From Kölliker after Bischoff.)e.embryo;a.amnion;a.urachus;al.allantois with blood-vessels;sh.subzonal membrane;pl.placental villi;fd.vascular layer of yolk-sack;ed.hypoblastic layer of yolk-sack;ed´.inner portion of hypoblast, anded´´.outer portion of hypoblast lining the compressed cavity of the yolk-sack;ds.cavity of yolk-sack;st.sinus terminalis;r.space filled with fluid between the amnion, the allantois and the yolk-sack.
The embryo is surrounded by the amnion, which is comparatively small. The yolk-sack (ds) is large and attached to the embryo by a long stalk. It has the form of a flattened sack closely applied to about two-thirds of the surface of the subzonal membrane. The outer wall of this sack, adjoining the subzonal membrane, is formed of hypoblast only; but the inner wall is covered by the mesoblast of the area vasculosa, as indicated by the thick black line (fd). The vascular area is bordered by the sinus terminalis (st). In an earlier stage of development the yolk-sack had not the compressed form represented in the figure. It is, however, remarkable that the vascular area never extends over the whole yolk-sack; but the inner vascular wall of the yolk-sack fuses with the outer, and with the subzonal membrane, and so forms a false chorion, which receives its blood supply from the yolk-sack. This part of the chorion does not develop vascular villi.
The allantois (al) is a simple vascular sack with a large cavity. Part of its wall is applied to the subzonal membrane, and gives rise tothe true chorion, from which there project numerous vascular villi. These fit into corresponding uterine crypts. It seems probable, from Bischoff’s and Kölliker’s observations, that the subzonal membrane in the area of the placenta becomes attached to the uterine wall, by means of villi, even before its fusion with the allantois. In the later periods of gestation the intermingling of the maternal and fœtal parts of the placenta becomes very close, and the placenta is truly deciduate. The cavity of the allantois persists till birth. Between the yolk-sack, the allantois, and the embryo, there is left a large cavity filled with an albuminous fluid.
The Hare does not materially differ in the arrangement of its fœtal membranes from the Rabbit.