562 



CHORDATA 



cliorion frondosum, or diffuse placenta, which occurs in whales, perisso- 

 dactyles and many artiodactyles. On the other hand, the villi may be 

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

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

 tions of the uterine lining which are distinguished from the rest by be- 

 coming extremely vascular (uterine placenta). The cotyledonary placenta 

 (fig. 610) consists of many small placentar patches, the cotyledons (most 



C 1 



C 1 



Ch 



FIG. 6 10. Cotyledonary placenta and embryo of cow (from Balfour, after 

 Colin). C 1 , cotyledons of uterine; C 2 , of foetal placenta; Ch, chorion; U, uterus; 

 V, vagina. 



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

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

 is, as its name indicates, disc-like. By the restriction of the nutrient 

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

 uterus (the zonary or discoid uterine placenta) is more modified than is 

 the case with the diffuse or the cotyledonary type. While in the latter 

 the placental villi are withdrawn from the uterine walls without injury to 

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

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

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

 hemorrhage being 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 



