446 



METAZOAN PHYLA 



may be called a typical mammalian embryogeny the inner cell mass 

 becomes separated into two portions. In the upper portion develops an 

 amniotic cavity, while in the lower portion is formed an archenteron. 

 The amniotic cavity is lined with ectoderm, the archenteron with ento- 

 derm, and from the cells between is formed the mesoderm. From the 

 archenteron a yolk sac develops, which, however, does not contain yolk. 

 Now the development of the embryo proceeds in much the same manner 

 as in the development of a meroblastic egg cell. The entoderm together 



P/acenfer 



Chorion 



Amniotic 

 cavity 



Fig. 344. — Diagram of the embryonic membranes and circulation of a mammal. Stage 

 between / and K in Fig. 343. For comparison with Fig. 271. Arteries erosslined, veins 

 black. Arrows show direction of blood flow. {From Wilder, "History of the Human 

 Body," by the courtesy of Henry Holt & Com.pany.) 



with the splanchnic mesoderm forms the wall of the yolk sac, which is 

 connected with the enteron of the embryo by a slender yolk stalk. The 

 ectoderm and somatic mesoderm grow around the wall of the blastodermic 

 vesicle forming an amnion, which unites with the trophoderm to produce 

 a chorion. The chorion of mammals is, therefore, not homologous with 

 that of reptiles and birds (Sec. 411). Branching processes, larger and 

 more complex than those which attached the trophoderm, extend from 

 the chorion into the uterine tissues, which become thick and congested, 

 the two masses together forming the placenta. Between the two layers 

 of the mesoderm is the extra-embryonic coelom. 



By the development of the amniotic cavity and the extra-embryonic 

 coelom a considerable space is produced between the body of the embryo 



