Fig.199. Human foetus, twelve weeks old, with its membranes. 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 foetus (at the end of the pregnancy, in its natural position, taken out of the uterine cavity). 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. 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.