ii4 ANIMAL BIOLOGY. [Parti. 



aids it in its search for food, she does not prepare for it food- 

 stuff by her own vital processes. The rabbit embryo is provided 

 with but little food-yolk. It is dependent on the mother from 

 the first, not only for warmth, but also for nutriment. It is 

 developed within the maternal organism, with which it is con- 

 nected in such a way that diffusion of nutrient fluid can take 

 place from the one to the other. And when it is born it is for 

 a while dependent on the mother, not only for protection and 

 aid, but also for food-milk, which is elaborated by her own vital 

 processes. 



In all three types the essential nature of the process of 

 fertilisation is the same. In all, development results from, and 

 is dependent upon, the union of ovum and spermatozoon. And, 

 in all, this union gives rise to a process of cell-division or segmen- 

 tation. But the character of the segmentation differs. In the 

 simple alecithal ovum, in which there is no food-yolk, or in which 

 the little that there is is distributed uniformly, the segmentation 

 is complete, or holdblattic, and regular. In the frog's ovum, in 

 which the yolk is arranged in polar fashion (telolecUhal), the 

 segmentation, though still holoblastic, is unequal. In the fowl's 

 ovum, where the yolk is also arranged in polar fashion (teloled- 

 thal), but is in enormous quantity, the segmentation is partial 

 (meroblastic) and irregular. In the rabbit's ovum there is very 

 little food-yolk, and that arranged uniformly (alecithal), and the 

 segmentation is complete, or holoblastic, but slightly unequal. 



At the outset we made use of a supposititious case of simple 

 embryonic development for purposes of illustration. The sup- 

 posed organism possessed an alecithal ovum, which underwent 

 holoblastic segmentation in a quite regular fashion. A hollow 

 blastosphere was formed which underwent simple invagination, 

 by which the segmentation cavity was obliterated. Thus a two- 

 layered gastrula was produced, the inner layer being hypoblast, 

 the outer layer epiblast. The gastrula-cavity was the archen- 

 teron ; the opening of the gastrula-cup the blastopore. Meso- 

 blast was formed between the two primitive layers ; and the 

 body-cavity, or ccelom, arose by cleavage of the mesoblast. 

 Along the dorsal surface of the embryo the medullary axis was 

 differentiated from the epiblast. We may now notice that, with 

 certain modifications, the little Lancelet (Amphioxus) presents us 



