THE FINAL ADJUSTMENTS IO3 



be space for a short outline of the most important 

 results of the chemical study of embryos. The most 

 interesting point of view from which to consider the 

 question is that of evolutionary adaptation ; the dif- 

 ferent environments in which animals live make 

 different demands which must be satisfied by 

 animals which live in them. 



The sea, where life first evolved, does not provide 

 many of the substances necessary for a developing 

 egg. These substances are (i) oxygen, (2) water, 

 (3) inorganic salts, (4) carbohydrates, like sugar, 

 which can be burnt to provide energy, (5) fats, also 

 mainly for fuel, (6) proteins, which are the real 

 **flesh-forming" substances. Of these the sea provides 

 the first three. The others have to be supplied to 

 the embryo in the yolk. But the sea abounds in very 

 small living creatures which provide suitable food for 

 quite tiny larvae, so that the amount of yolk need 

 only suffice for the earliest stages of development. 



The freshwater environment does not contain 

 enough salts to be absorbed by the embryo, and it 

 therefore can only be inhabited by creatures which 

 provide sufficient salts in the egg. But the most 

 difficult evolutionary step, from the embryological 

 as from every other point of view, must have been 

 the conquest of dry land. Here there is no useful 

 substance, except oxygen, which can be absorbed 

 from outside, and the embryo has to be provided 

 not only with its organic materials and salts but also 

 with its water. Amphibia, which are the most 

 primitive land animals, have, as a matter of fact, 



