6 INTRODUCTION. 



egg with all the essential characters of the parent already esta- 

 blished. 



Large size of eggs implies diminution in number of the eggs, 

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

 while some animals 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 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 eight hours gives rise to a free swimming larva, capable 

 of independent existence, with a digestive cavity and nervous 

 system already present ; while a large egg, like that of the hen, 

 hampered by the great mass of food-yolk by which it is dis- 

 tended, 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, having already exhausted the 

 supply of food originally contained within the egg, must devote 

 much of its energies to hunting for, and to digesting, its food ; 

 and hence its further development will proceed 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 in searching for food, but can devote its whole energies to 

 completing its development. Hence, except in the earliest stages, 

 the chick develops far more rapidly than Amphioxus, and attains 

 its adult condition in a much shorter time. 



Mammals, and some other forms, present apparent exceptions 

 to the rule stated above with regard to the influence of food-yolk 

 on the course of development. A rabbit is developed from a 

 very small egg, an egg which is, indeed, even smaller than 

 that of Amphioxus; and yet the young rabbit at the time 



