REPRODUCTION 



119 



hatching the amnion and the slender neck of the allantois are ruptured at 

 the umbilicus. As the young animal emerges into free life the amnion 

 and chorion and the extra-embryonic part of the allantois are abandoned. 

 The proximal portion of the allantois, remaining within the body, becomes 

 enlarged and serves as the urinary bladder of such adult reptiles as 

 possess that organ. In birds, the adult having no urinary bladder, the 

 proximal remnant of the allantois degenerates. 



Among mammals there is considerable diversity as to the manner of 

 origin of the amnion and chorion. Once established, however, these 



Fig. 89. — Diagram of the fetal structures of a Tnammal. (The broken lines represent 

 mesoderm.) A, amnion; AL, cavity of allantois; B, brain; C, chorion; E, enteron; 

 EX, e.xtra-embryonic coelom; H, heart; NC, notochord; NT, neural tube; P, placental 

 region of allantois and chorion; SM, somatopleure; SP, splanchnopleure; V, chorionic 

 villi; YS, cavity of yolk-sac. 



membranes possess the same relations to the germ layers and to the 

 definitive body of the embryo as in reptiles. The allantois develops, 

 as in reptiles, from the cloacal region of the enteron. An abortive yolk- 

 sac is present but devoid of yolk except in those presumably primitive 

 mammals, Ornithorhynchus and Echidna, whose manner of development 

 is essentially reptilian. 



In most eutherian mammals the allantois expands until it becomes 

 apposed to the inner surface of the chorion and the two membranes undergo 

 more or less extensive fusion of their contiguous mesodermal layers. 



