Fig. 199—Human fœtus, twelve weeks old, with its membranes. The umbilical cord goes from its navel to the placenta. b amnion, c chorion, d placenta, d apostrophe, relics of villi on smooth chorion, f internal or reflex decidua, g external or true decidua. (From B. Schultze.)
Fig. 200—Mature human fœtus (at the end of pregnancy, in its natural position, taken out of the uterine cavity). On the inner surface of the latter (to the left) is the placenta, which is connected by the umbilical cord with the child’s navel. (From Bernhard Schultze.)
The first blood-vessels of the mammal embryo have been considered by us previously, and we shall study the development of the heart in the second volume.
In every vertebrate it lies at first in the ventral wall of the fore-gut, or in the ventral (or cardiac) mesentery, by which it is connected for a time with the wall of the body. But it soon severs itself from the place of its origin, and lies freely in a cavity—the cardiac cavity. For a short time it is still connected with the former by the thin plate of the mesocardium. Afterwards it lies quite free in the cardiac cavity, and is only directly connected with the gut-wall by the vessels which issue from it.
Fig. 201—Vitelline vessels in the germinative area of a chick-embryo, at the close of the third day of incubation. (From Balfour.) The detached germinative area is seen from the ventral side: the arteries are dark, the veins light. H heart, AA aorta-arches, Ao aorta, R.of.A right omphalo-mesenteric artery, S.T. sinus terminalis, L.Of and R.Of right and left omphalo-mesenteric veins, S.V. sinus venosus, D.C. ductus Cuvieri, S.Ca.V. and V.Ca. fore and hind cardinal veins.