262 



ZOOLOGY. 



Fig. 138. DIAGRAM OF DEVELOPMENT OF GRAAFIAN 

 FOLLICLE OF RABBIT. 



ovum itself the " discus proligerus " ; but these terms are 



not expressive, and scarcely necessary. 



The vitelline membrane in the rabbit is thick and 



marked by 

 radiating lines 

 (cf. fig. 110). 

 It is therefore 

 commonly re- 

 ferred to as the 



zona radiata. 



3. Matura- 

 tion, Fertiliza- 

 tion, and Seg- 

 mentation. The 



ovum escapes 

 from the follicle 

 and enters the 

 fallopian tube. 

 It gives off two 



(Altered from polar bodies in 



the usual way, 

 and is fertilized high up in the fallopian tube. Its seg- 

 mentation is holoblastie, and fairly equal, with a partial 

 resemblance to that of Amphioxus, which later development 

 shows to be apparent rather than real. The fertilized 

 ovum divides into two, four, and eight cells ; but at this point 

 the segmentation cavity that was beginning to form 

 becomes filled up by one of the cells passing into the centre. 

 The three sister-cells of this one (i.e. the three formed 

 from the same one of the two first-formed cells) also show 

 a tendency to pass into the centre, and as segmentation 

 proceeds the cells derived from them do so more and more, 

 until at last we have a solid mass of cells (morula), con- 

 sisting of two layers an inner layer of cells, all derived 

 from one of the two cells into which the ovum first divided, 

 and an outer layer of rather smaller cells, all derived from 

 the other of the two. The outer layer does not completely 

 surround the inner at one point the inner appears at the 

 surface (fig. 139 [1]). 



The arrows lead to 



successive stages. 

 Marshall.) 



