562 



CHORD ATA 



chorion Jrondosnm, or diffuse placenta, which occurs in whales, pcrisso- 

 dactylcs and many artiodactyles. On the other hand, the vilH may Ije 

 restricted to certain places, becoming very strong there. This gives rise 

 to cotyledonary, discoidal, or zonary placenta;. To these correspond por- 

 tions of the uterine lining which are distinguished from the rest liy Ijc- 

 coming extremely vascular {uterine placenta). The cotvhdonary placenta 

 (fig. 6io) consists of many small placentar patches, the fy/y/fJoni- (most 



r'^^M^^x^ 



Fig. 6io. — Cotylcdonan,' placenta and embn'O of cow (from Balfour, after 

 Colin). C, cotyledons of' uterine; C", of foetal placenta; C7;, chorion; V, uterus; 

 y, vagina. 



ruminants). In the zonary placenta the villous area takes the shape of a 

 girdle or barrel (carnivores, Sircnia), while the discoidal (other mammals) 

 is, as its name indicates, discdike. By the restriction of the nutrient 

 structures to certain regions of the placenta, the corresponding part of the 

 uterus (the zonary or chscoid uterine placenta) is more moditied than is 

 the case with the diffuse or the cotyle<lonary type. AVhile in the latter 

 the placental villi are willnlrawn from the uterine walls without injury to 

 them (Indeciduata), in the case of the zonary or discoidal placenta the 

 superficial part of the mucous lining of the uterus, the dccidna, separates 

 (Deciduata), leaviitg the uterine placenta as a bleeding wound, the 

 hemorrhage beiiig stopped by the energetic muscular contractions of the 

 uterus. 



Besides the placental structures the higher mammals are characterized 

 by the disappearance of the cloaca, the unpaired vagina, and absence of 



