256 EMBRYOLOGY 



is toxic to the tissues of the embryo. In later development the excretory 

 product is uric acid, which is very insoluble. Very little water is necessary to 

 store uric acid. These compounds are eliminated through the kidney system 

 of the embryo and are stored in the cavity of the allantois. In the cavity itself, 

 then, at the end of development relatively large amounts of uric acid in the 

 form of a solid are left in the shell after the chick hatches. 



So much for growth in the amphibian and chick embryos. In both these 

 forms the egg stores enough yolk to carry the embryo to a stage of hatching. 

 But the mammalian egg is very small and contains little or no yolk. How 

 does the growing mammalian embryo obtain its oxygen and food, and how 

 does it excrete carbon dioxide and nitrogenous wastes ? 



