. Text book of zoology. Zoology. 494 Vertebrata. termed the placenta fvetalis* or tliere are several regioBS upon which the villi are well-developed. This is the case in most Euminants, which possess a great number of small, very prominent placentee (cotyledons); elsewhere a large continuous placenta, either zonary (Carnivora, Seal, Elephant) or discoidal (in Man and otherst), occurs. That portion of the uterine wall which is connected with the placenta is termed the uterine placenta, p. uterina. In some cases the villi are simply withdrawn from the pits in the uterine wall at birth (Horse, Ru
. Text book of zoology. Zoology. 494 Vertebrata. termed the placenta fvetalis* or tliere are several regioBS upon which the villi are well-developed. This is the case in most Euminants, which possess a great number of small, very prominent placentee (cotyledons); elsewhere a large continuous placenta, either zonary (Carnivora, Seal, Elephant) or discoidal (in Man and otherst), occurs. That portion of the uterine wall which is connected with the placenta is termed the uterine placenta, p. uterina. In some cases the villi are simply withdrawn from the pits in the uterine wall at birth (Horse, Ruminants) ; in others (in all with zonary or discoidal placenta) a portion of the mucous membrane of the uterus remains attached to the embryonic membranes and is thrown off with them (decidua), so that the former has, to a great extent, to be regenerated. In tlie placental Mammals the serous membrane (c/., p. 354) is closely connected with, the allantois, and partially fuses with it; the vascular membrane thus formed is termed the chorion, and gives rise to the vascular papillse already mentioned. In older Mam- malian embi-yos the amnion is much extended, and often lies close to the allantois, and then it immediately suiTOunds the tubular peduncles of the allantois and of the yolk sac, as with a sheath. These peduncles (see Fig. 402), together with the sheath, are termed the umbilical cord. The circulation in an advanced embryo is in several respects very difPerent from that of the adult, the lungs of course are not yet functional; the oxygen which the embryo needs is received with the plasma from the pai-ent. The chief points of the circulation are the following: the arterial blood from the placenta mixes with the venous blood from the posterior part of the body and flows into the right auricle, which also receives venous blood from the anterior regions. Part of the blood from the right auricle flows into the right ventricle, thence to the pulmonary artery, and so partly into
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