MAMMALIA 409 



insectivores (Fig. 215), rodents, armadillos and man. The allantois 

 forms as in birds, but frequently remains rudimentary as in man 

 (Fig. 217); but in some cases, as in the rabbit (Fig. 216), it forms a 

 primitive type of allantoic placenta much like that seen in the mar- 

 supial, Perameles. 



The formation of the true chorionic placenta is a complicated 

 process. The mesodermic layer of the chorion, which becomes highly 

 vascular, and becomes connected with the embryonic circulation, 

 sends out branching processes, chorionic villi, into the uterine tissues, 

 which penetrate the uterine lymph cavities and absorb liquid nutri- 



FiG. 215. — Diagram of the formation of the amnion in the Insectivores. Black; 

 embryonic ectoderm; heavy stipples, trophoblast; light stipples, endoderm, 

 obhque ruling, mesoderm. A, before the appearance of the amniotic cavity; 

 inner cell mass differentiated into ectoderm and mesoderm; endoderm extending 

 completely around the wall of the vesicle. B, The amniotic cavity (a) appearing 

 in the ectoderm. C, Enlargement of the amniotic cavity. Mesoderm expanded 

 and split into somatic and splanchnic layers, separated by the ccelom. s, prim- 

 itive streak. (From Kellicott, after Keibel.) 



ment directly from the maternal supply. The maternal tissues be- 

 come thick and congested in these regions, and the fetal and maternal 

 tissues together constitute the definitive placenta. The entire 

 chorion is at first provided with simple villi, but later only certain 

 regions retain the villi and act as placental areas. Frequently the 

 placental area is discoidal in shape, as in the primates, in some of 

 the edentates, and in many of the rodents; sometimes the placental 

 area is band-like or zonary, as in the carnivores; and in the case of 

 the ungulates it is cotyledenous, in which case thick knots of villi 

 are scattered over almost the entire chorion, separated by extensive 

 non-villous areas. 

 Parturition or birth takes place at widely different stages of matu- 



