INTRODUCTION. 



The most important difference between the de- 

 velopment of Mammalia and Aves depends upon the 

 amount and distribution of the food-yolk in the ovum. 

 In birds, as we have seen (Ch. i.), the ovum is large and 

 the greater part of it so heavily charged with food-yolk 

 that it is unable to segment. The segmentation is con- 

 fined to one small portion, the germinal disc, the pro- 

 toplasm of which is less burdened with food-yolk than 

 that of the remainder of the ovum. Such partial seg- 

 mentation is known as merohlastic. 



In Mammals, on the other hand, the ovum is small \ 

 and contains but a slight amount of food-yolk ; the little 

 there is being distributed uniformly throughout. In con- 

 sequence of this the whole ovum is able to segment ; the 

 segmentation therefore belongs to the holohlastic type. 

 This fundamental difference in the constitution of the 

 ovum of Birds and Mammals is accompanied not only by 

 differences in the segmentation but also by impoitant 

 differences, as we shall see, in the stages of development 

 which immediately follow segmentation. Finally, in 



^ The human ovarian ovum is -^^-g to i|^ of an inch in diameter. 



202 



