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 villi are simply withdrawn from the pits in the uterine wall at birth 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 

 the lungs, partly through the ductus Botalli into the aorta ; another portion 

 of blood from the right auricle goes through the opening in the auricular 

 septum into the left auricle, and fx-om this through the left ventricle into 

 the main arterial trunk. A very considerable mixing of arterial and venous 

 blood, thei'efore, occm-s in the embryo. 



The length of time during which the placental embryo remains 

 in the uterus (uterine gestation) varies considerably for different 

 forms, though it is fairly constant for each species. As a general 

 rule large Mammals have a long gestation (a year or more), and 



Fig. 402. Placenta of a Mammal, 

 diagrammatic, am amnion, al allantois, 

 h yolk sac ; the outermost line is the 

 serous membrane. The external layer 

 of the allantois is fused with the serous 

 membrane forming the chorion which is 

 beset with branched processes. — Orig. 



* If the villi are uniformly distributed over tlie whole membrane, the animal 

 (e.g., the Horse) is said to possess a diffuse placenta; strictly speaking there is 

 none. 



■f Apes, Bats, Inseotivores, and Ebdents, 



