3. Development of the Fœtus.
a. Anatomy.
2981. The germ may be regarded as a vesicle, full of nutritive matter or albumen, situated within the cavity of the uterus, the walls of which act upon it.
2982. As the fundus of the uterus is the arterial pole, so it oxydizes the vesicle and repels that part which lies close upon it. Through this originates a saccular inversion, as in the mesentery of the peritoneum, and the vesicle separates into three divisions. In itself it is amnion, the inverted part is the integument of the embryo, the tube which unites these the umbilical cord.
2983. The amnion is thus the root or primary cyst of the integument.
2984. Through continuous oxydation blood-vessels are developed upon the surface of the amnion, which finally withdraw to constitute a special integument, which is called chorion. Its vessels are in like manner repelled from the fundus uteri, and prolonged into the inversion of the umbilical cord and the embryo. The chorion is thus the root or primary cyst of the vascular system.
2985. These two bladders, sacs, or cysts are the only general ones which circumscribe or invest the entire embryo, because there are only two general vegetative systems, namely, the tegumentary and vascular.
2986. The embryo has not originated freely in these shut sacs, but only through their introversion; it is itself a portion of these sacs.
2987. The embryo properly lies external to its envelopes, as the intestine does in respect to the mesentery.
2988. Just as the two general vegetative systems have been developed from primary cysts, so also are there sacs for the two special vegetative systems, or the intestinal and sexual; but, on that very account, these cannot be general sacs, nor any longer envelop the embryo.