104 HOW ANIMALS DEVELOP 



failed in the last respect, and must return to water 

 to breed since their embryos would otherwise dry 

 up. The first true land animals were the reptiles. 

 They and their descendants, the birds, are quite 

 independent of water for breeding, as they have 

 evolved the device of enclosing the egg in an im- 

 permeable membrane which does not allow the 

 moisture to escape, although it allows gases such as 

 oxygen to be absorbed. Some of the intermediate 

 stages in this evolutionary advance have still sur- 

 vived ; for instance, one sort of turtle lays eggs with 

 a rather soft and permeable covering, which can 

 only develop if they are deposited in wet sand ; the 

 turtle scrapes a hole in the sand, lays the eggs, and 

 then urinates over them to moisten them. 



The closed-box, or "cleidoic" egg^ although it 

 solves the drought problem fairly satisfactorily, is 

 only workable if several other requirements are ful- 

 filled. The most important of these concerns the 

 waste products, of which the embryo, like all other 

 animals, has to rid itself. One waste product is 

 carbon-dioxide, which is gaseous and can be allowed 

 to diffuse out through the shell. The other main type 

 of waste is the nitrogenous matter derived from the 

 breakdown of proteins. Water-living embryos turn 

 this into ammonia which diffuses out into the sur- 

 rounding water. But it would not diffuse away so 

 readily from a closed-box egg and would accumulate 

 to such an extent that it would be dangerous. The 

 closed-box animals have therefore had to evolve 

 another method of getting rid of their nitrogenous 



