THE CHORION.
The outer wall of the ovum now consists of three livers—viz., the zona pellucida on the outside; inside that, the subzonal membrane or false amnion; and inside that again, the allantois. These three layers, origi nally distinct, soon become fused completely with one another, and are spoken of collectively as the chorion.
The Vascular System of the Embryo.
Blood-vessels are developed in the embryo at a very early period. The heart appears in an embryo of about the age represented in Fig. 98, 2 i.e., shortly after the commencement of the constriction separating the embryo from the yolk-sac, as a pair of tubes one on either side of the body; these soon meet one another and fuse underneath the throat to form a single tubular heart, in connection with which are vessels devel oped in the mesoblast both of the embryo and of the yolk-sac.
Circulation of the Yolk-sac.
The heart very soon commences beating: it drives the blood forwards through a series of aortic arches on either side, whereof one is situated in each of the visceral arches already described; from these some of the blood goes forwards to the head, but the greater part is sent back to the hinder part of the body along a couple of arteries, the dorsal aorta, which subsequently unite together to form the single dorsal aorta of the adult. From the dorsal aorta vitelline arteries take the blood to a network of vessels developed in the mesoblast of the yolk-sac, and from these it is returned to the posterior end of the heart by vitelline veins.
Circulation in the Allanlois.—The allantois is, from its earliest appear ance, very vascular, and its blood vessels increase very rapidly in size as it develops. Two very large branches from the posterior end of the dorsal aorta, the allantoic or umbilical arteries, carry blood to it from the em bryo, and a couple of umbilical veins return it back again to the heart of the embryo.
Umbilical Stalk.
The amnion, which at first invests the embryo tolerably closely, later on grows rapidly so as to leave a considerable space—the amnionic cavity —between the embryo and itself. As shown in Fig. 98, 5, the amnion, as it recedes from the embryo, forms an investment to the stalk of the allantois, and it is further evident from the figure that, in addition to the allantoic stalk, the yolk-stalk will also be included in this investment. The compound stalk formed in this way, which includes both the allan toic stalk with the placental vessels and the yolk-stalk with its vessels, and is ensheathed, as stated above, by the amnion, receives the name of umbilical stalk. It serves, as shown in Figs. 98, 5, and 2, to attach the
embryo to the placenta.
Bladder and Urachus.
It will be remembered that the allantois was at first a hollow bud, its cavity communicating with the alimentary canal, of which it was indeed a diverticulum (Fig. 98); the cavity soon becomes lost in the placenta itself, and if, indeed, it is ever present there it may persist in the umbilical stalk, more or less completely, throughout development; within the body of the embryo the portion of the cavity next to the alimentary canal becomes ultimately the urinary bladder, while the portion of the stalk extending from this part to the body wall of the embryo becomes the lunch us.
frizte of the Germinal Layers.
It will be convenient to give here a very brief account of the ultimate fate of each of the three germinal layers of which the embryonic area consisted at a very early stage, viz., epiblast, mesoblast, and hypoblast.
In the first place, it will be noticed that out of one or other of these three layers every portion of the body of the embryo or foetus is derived, directly or indirectly.
The epiblast, which is cloarly the most superficial layer of the three, gives rise to the epidermis covering the whole of the body, and also, as we have seen, to the whole of the central nervous system, both brain and spinal cord, and indeed to the nerves themselves, inasmuch as these arise as outgrowths from the central nervous system. It also forms the lining of the mouth and anus, which as already noticed are pittings-in from the exterior; and it takes a very important share in the formation of the organs of special sense., The hypoblast, or lowermost of the three layers, forms the epithelium lining the alimentary canal and its glands, and also that lining the bron chi and lungs, which arise as diverticula of the alimentary canal. It also forms a longitudinal solid rod—the notochord—which runs the whole length of the body underneath the central nervous system in the position afterwards held by the vertebral centre and the base of the skull.
The mesoblast forms all the rest of the body: muscles, bones, connec tive tissue, and blood-vessels, wherever they occur, are mesoblastic; also the peritoneal epithelium and the urinary and reproductive organs.