THE ORIGIN OF MAN 57 



velop on land they must be different from those 

 that hatch in the water. In the latter the em- 

 bryos have functional gills for the use in the 

 water, but in the reptiles a wholly different organ 

 was developed to provide the growing young with 

 the necessary oxygen. Reptile eggs are large 

 and contain a great deal of yolk on which the 

 embryo lives. At one pole of the egg lies the 

 fertilized germinal vesicle which develops into 

 the embryo. There is formed during its earliest 

 growth at either end of the elongate embryo a 

 two-layered, crescent-shaped fold, called the am- 

 niotic fold, which arches over the embryo and 

 finally unites to cover it with a protective hood. 

 This latter is the amnion and between it and the 

 embryo there is a shallow cavity containing a 

 watery amniotic fluid, bathing and protecting the 

 outer surface of the embryo. While the amnion 

 is being formed a sac grows out from the hinder 

 end of the embryo. This sac, which is of the 

 greatest importance to the animal, is never seen 



