EARLY HUMAN EMBRYOS AND THEIR MEMBRANES 



75 



bilical 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 membranes of the uterus (p. 237 ff.). 



Inner cell mass 



Entoderm 



Trophedodcrm 



Cnboidal cells 



(of Langhans) 



Embryonic ectoderm Entoderm 



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



X about 1 60. 



We saw how the allantois of Ungulates had assumed the nutritive 

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

 ration of the ungulate yolk sac. In man and most Unguiculates the func- 

 tions of the allantois are transferred to the chorion, and the allantois, in 

 turn, becomes a rudimentary structure. 



The Amnion. This is formed precociously in Unguiculates, and in a 

 manner quite different from its mode of origin in Ungulates and birds. 



