ANIMAL PEDIGREES 221 



the embryo being, as in a four-day chick, in a con- 

 dition in which it is clearly incapable of independent 

 existence. 



The nutrition of the embryo prior to hatching is 

 most usually effected by granules of nutrient matter, 

 known as food yolk, and embedded in the proto- 

 plasm of the egg itself; and it is on the relative 

 abundance of these granules that the size of the 

 egg chiefly depends. 



Large size of eggs implies diminution of their 

 number, and hence in that of the offspring ; 

 and it can be well understood, that while some 

 species derive advantage in the struggle for exist- 

 ence by producing the maximum number of young, 

 to others it is of greater importance that the young 

 on hatching should be of considerable size and 

 strength, and so better able to begin the world on 

 their own account. In other words, some animals 

 may gain by producing a large number of small 

 eggs, others by producing a smaller number of eggs 

 of larger size i.e., provided with more food yolk. 



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 



