668 



EMBRYOLOGY I MAMMALS 



the trophoblast burrow into the wall of the uterus. These are 

 specially numerous over a thickened, horseshoe-shaped patch 

 of trophoblast which surrounds the hinder part of the embryo, 

 in the region (tr'.) in which the placenta (p. 457) will arise. 

 After the establishment of the membranes the embryo is known 

 as a foetus. 



The early stages of the embryonic development of Man are 

 imperfectly known, but the process appears to belong to a type 

 which resembles that of the chick less than the development 

 of the rabbit does. In it the trophoblast over the embryo does not 

 disappear, and the amnion is formed very early, as a cavity in 

 the embryonic ectoderm, which arises as a mass of cells, not as 

 a layer. In the floor of this cavity the embryo is formed. 



THE PLACENTA 



For a while the yolk sac of mammals forms a union with the 

 uterine wall, and through trophoblastic villi is the main organ 



77/77r/77Z///7V/A 



AJ.St. 



am 



Fig. 523. — Longitudinal section of a rabbit embryo on the tenth day. 



am., Axanion ; am.c, amniotic cavity ; hyp"., endoderm of allantois ; y.st., stalk of yolk sac. Other letters 



as in Fig. 522. 



of nutrition and respiration, but in this it is soon replaced by the 

 allantois, which, as in the chick, spreads out under the false 

 amnion or subzonal membrane, and fuses with it. The organ 



