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 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 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 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 (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.