

f II ■ - ' 



omniotic cavity 



uteri na 

 oi-tery 



.umbilical ^y 

 /art«ry f^ 



umbilical 

 vein i)^ 



chorion 



chorionic 

 villi 



Otiootois 



Section of 

 ovary 



Fallopior) 

 tube 



Fig. 21-4. Special membranes are formed in the eggs of vertebrates that have come out onto land, 

 the reptiles, birds, and mammals. The upper sketch shows the condition found in aquatic forms such 

 as fish and amphibians. The middle figure shows the formation of the amnion and allantois to make 

 embryoiogical development on land possible. In these eggs there is sufficient stored food (yolk) to carry 

 the embryo throughout its early development. In the lower figure is shown the condition found in 

 mammals where, in addition to the membranes seen in the earlier forms, the placenta has evolved, 

 making it possible for the embryo to receive nourishment from the mother. 



