THE OVUM. 47 



atrophy of some of them is to ensure the adequate nutrition of 

 a certain number of them. 



In the second and rarer mode of development of permanent 

 ova from primitive germinal cells, the nuclei and protoplasm undergo 

 the same changes as in the first mode, but the cells either remain 

 isolated, and never form part of a nest, or form part of a nest in 

 which no fusion of protoplasm takes place, and in which all the cells 

 develope into permanent ova. 



The isolated ova and nests are situated, during the whole of 

 the above changes, amongst the general undifferentiated cells of 

 the germinal epithelium, but as soon as a permanent ovum becomes 

 formed the cells adjoining it arrange themselves around it as a 

 special layer, and so give rise to the epithelium of the follicle 

 (fig. 19, o). The growths of stroma into the germinal epithelium 

 appear shortly after the formation of the earlier follicles. 



Mammalia. The development of the ovary in Mammalia differs 

 mainly from that just described in that the formation of primitive 

 germinal cells from the indifferent cells of the germinal epithelium 

 takes place at a relatively much later period. 



The stroma grows into the germinal epithelium while it is 

 still formed of rounded indifferent cells, and divides it into trabeculae 

 as described above. At a later period a number of the cells in 

 the deeper layer of the epithelium, as well as certain cells in the 

 superficial part, become primitive germinal cells, while the remainder 

 of the cells become smaller and are destined to form the follicle 

 cells. 



The most conspicuous primitive germinal cells are situated 

 in the superficial layer of epithelium ; and the primitive germinal cells 

 in the deeper layers of the germinal epithelium are not nearly so 

 marked as in most Craniata, so that it is difficult in some cases to be 

 sure of their destination till their nucleus commences to undergo 



O 



its characteristic metamorphosis. 



The change of the primitive ova into permanent ova takes 

 place in the same manner in Mammals as in Elasmobranchii, except 

 that the fusion of the primitive ova into polynuclear masses is 

 much rarer. The formation of the at first quite simple follicles 

 takes place while the ova are still aggregated in large masses ; 

 and the first follicles are formed in the innermost part of the 

 germinal epithelium. Soon after their formation the follicles become 

 isolated by connective-tissue growths. 



Post-embryonic development of the ova. 



The ova of the Vertebrata differ greatly in size and structure. 

 The differences in size depend upon the quantity of the food-yolk. 

 In the Amphioxus and Mammalia, in which the ova are smallest, 

 the comparatively insignificant amount of food-yolk is distributed 

 uniformly through the ovum. A larger quantity of it is present 



