EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 



73 



from the embryo as the umbihcal vessels. The embryo receives its nutriment and 

 oxygen, and gets rid of waste products through the walls of the villi. The region 

 where the attachment of the chorionic villi to the uterine wall persists during fetal 

 life is known as the placenta. It will be described later with the decidual mem- 

 branes of the uterus. We saw how the allantois of Ungulates had assumed the 



Inner cell mass 



Inner cell mass 



Trophectoderm 



Embryonic ectoderm 



Maternal tjlood vessels 



Entoderm 



Trophodcrm 



____^_^^^^ I Cytolrophohlasl 



Embryonic ectoderm Entoderm 



Fig. 75.— Sections showing the formation of the amnion in bat embryos (after Van Beneden). 



X about 160. 



nutritive functions performed by the yolk sac in birds, with a consequent degene- 

 ration of the ungulate yolk sac. In man and Unguiculates the functions of the 

 allantois are transferred to the chorion, and the allantois, in turn, becomes a rudi- 

 mentary structure. 



The Amnion.— This is formed precociously in Unguiculates and in a manner 

 quite different from its mode of origin in Ungulates and birds. It is assumed that 



