132 



GENERAL CONCEPTS 



B-mh-r^jonic 

 a.rea. 



Ectoderm 

 Enloderm 



Amniotic 

 folds 



Amniotic cavity 



D 



Chorion 

 Ante-rior end 



Amnion 

 YolK sac 



Primitive 

 di^e. stive 

 tract 



Posterior end 



Alla.ntois 



Figure 6.1 0. A-E, Steps in the formation of the extraembryonic membranes— amnion, 

 chorion, yolk sac and allantois— in a typical mammal such as a pig. Arrows indicate 

 direction of growth and folding. 



fuses with the chorion to form a compound membrane, equipped with 

 many blood vessels, by means of which the embryo takes in oxygen, gives 

 off carbon dioxide and excretes certain wastes. 



The mammalian allantois is usually small and has no function as a 

 membrane, but supplies blood vessels to the placenta, the organ formed 

 from chorion, allantois and maternal tissue. Finger-like projections, or 

 villi, of the chorion grow into and become embedded in the lining of the 

 uterus. These villi, their blood vessels, and the uterine tissues with which 

 they are in contact, are called the placenta. This organ, in which the 

 fetal blood vessels come in close contact with the maternal blood vessels, 

 provides the developing mammalian fetus with nutrients and oxygen 

 from the maternal blood, and eliminates carbon dioxide and waste 

 products into the mother's blood. The two blood streams do not mix at 



