CHAPTER XX. DEVELOPMENT OF THE RABBIT. 



1. The ovum of the rabbit is just about the same 

 size as that of Amphioxus, and is practically alecithal. 

 We might therefore not unreasonably expect to find 

 segmentation, invagination, and formation of the germinal 

 layers in the simplest way, and an embryo set free from 

 the egg at about the gastrula stage. Yet instead of this 

 we find that after the first stages the general development 

 resembles in all essentials that of the chick, a yolk-sac 

 (without any yolk), an amnion and an allantois being 

 formed; while the embryo is retained, not merely within 

 the egg, but within the oviduct, up to a stage of develop- 

 ment quite equal to that of the chick when hatched. 

 Evidently to make this long development from an alecithal 

 egg possible, there must be some other source of food 

 replacing the absent yolk. The yolkless condition of the 

 rabbit's ovum is not primitive, as is that of Amphioxus; 

 it is a secondary modification from a yolk-laden condition 

 like that of the bird. In this fact we have a clue to the 

 chief peculiarities in the rabbit's development. 



2. The Graafian Follicle. The ovarian ovum differs 

 from that of our previous types in the much more complex 

 character of the follicle. The gradual development of this 

 follicle is indicated in fig. 138. The cells from which both 

 ovum and follicle are developed are, to begin with, a 

 specialized portion of the general ccelomic (peritoneal) 

 epithelium (germinal epithelium). Little groups of these 

 cells sink into the underlying connective tissue, and soon 

 we have an ovum surrounded by a single layer of follicular 

 cells. So far, all our Vertebrate types are practically alike. 

 But now, in the rabbit, the follicular cells multiply, and 

 presently a split appears in them, and grows into a large 

 cavity, filled with liquid. The cells on the outer side of 

 this cavity constitute the "granulosa," those around the 



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