PRESIDENTIAL ADDRESS. 13 



is that the embryo hatches at a much later stage of its development, 

 and if the amount of food material is sufficient may even leave the egg 

 in the form of the parent. In such cases the earlier developmental 

 phases are often greatly condensed and abbreviated ; and as the 

 embryo does not lead a free existence, and has no need to exert itself 

 to obtain food, it commonly happens that these stages are passed 

 through in a very modified form, the embryo being as in a four-day 

 chick, in a condition 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 protoplasm 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 number of the eggs, and 

 hence of the offspring ; and it can be well understood that while some 

 species derive advantage in the struggle for existence 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 

 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 

 development rapidly, and in about eighteen hours gives rise to a free 

 swimming larva, capable of independent existence, with 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. Am- 

 phioxus 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. 



