THE RECAPITULATION THEORY 309 



The immediate effect of a large amount of food 

 yolk is to mechanically retard the processes of 

 development; the ultimate result is to greatly 

 shorten the time occupied by development. This 

 apparent paradox is readily explained. A small 

 egg, such as that of Amphioxus, starts its develop- 

 ment rapidly, and in about eighteen hours gives 

 rise to a free swimming larva, capable of indepen- 

 dent existence, with a digestive cavity and nervous 

 system already formed ; while a large egg like that 

 of the hen, hampered by the great mass of food 

 yolk by which it is distended, has in the same 

 time made but very slight progress. From this 

 time however other considerations begin to tell. 

 Amphioxus has been able to make this rapid start 

 owing to its relative freedom from food yolk. This 

 freedom now becomes a retarding influence, for the 

 larva, containing within itself but a very scanty 

 supply of nutriment, must devote much of its 

 energies to hunting for and to digesting its food, 

 and hence its further development will proceed more 

 slowly. 



The chick embryo, on the other hand, has an 

 abundant supply of food in the egg itself; it has 

 no occasion to spend time searching for food, but 

 can devote its whole energies to the further stages 

 of its development. Hence, except in the earliest 

 stages, the chick develops more rapidly than 

 Amphioxus, and attains its adult form in a much 

 shorter time. 



The tendency of abundant food yolk to lead to 

 shortening or abbreviation of the ancestral history, 



