STRUCTURE AND DEVELOPMENT OF VERTEBRATE OVARV. 417 



mented off from the nest as a special cell — a permanent ovum 

 (figs. 13, 14, 15, 16). Not all the nuclei in a nest undergo the 

 whole of the above changes ; certain of them, on the contrary, 

 stop short in their development, atrophy, and become employed 

 as a kind of pabulum for the remainder. Thus it happens that 

 out of a large nest perhaps only two or three permanent ova 

 become developed. 



{b.) In the second mode of development of ova the nuclei 

 and protoplasm undergo the same changes as in the first mode ; 

 but the ova either remain isolated and never form part of a nest, 

 or form part of a nest in which no fusion of the protoplasm 

 takes place, and all the primitive ova develop into permanent ova. 

 Both the above modes of the formation continue through a great 

 part of life. 



(5.) The follicle, — The cells of the germinal epithelium arrange 

 themselves as a layer around each ovum, almost immediately 

 after its separation from a nest, and so constitute a follicle. 

 They are at first flat, but soon become more columnar. In 

 ScylHum they remain for a long time uniform, but in large eggs 

 they become arranged in two or three layers, while at the same 

 time some of them become large and flask-shaped, and others 

 small and oval (fig. 29). The flask-shaped cells have probably an 

 important function in the nutrition of the egg, and are arranged 

 in a fairly regular order amongst the smaller cells. Before the 

 egg is quite ripe both kinds of follicle cells undergo retrogressive 

 changes (PL XVIII, fig. 23). 



In Eaja a great irregularity in the follicle cells is obsr viable 

 at an early stage, but as the ovum grows larger the cells gradually 

 assume a regular arrangement more or less similar to that in 

 Scyllium (PI. XIX, fig. 30—33). 



(6.) The egg rdemhraiies. — Two membranes are probably always 

 present in Elasmobranchs during some period of their growth. 

 The first formed and outer of these arises in some instances before 

 the formation of the follicular epithelium, and would seem to 

 be of the nature of a vitelline membrane. The inner one is the 

 zona radiata with a typical radiately striated structure. It is 

 formed from the vitellus at a much later period than the proper 

 vitelline membrane. It is more developed in Scyllium than in 

 Eaja, but atrophies early in both genera. By the time the 

 ovum is nearly ripe both membranes are very much reduced, and 

 when the egg (in Scyllium and Pristiurus) is laid, no trace of 

 any membrane is visible. 



(7.) The vitellus. — The vitellus is at first faintly granular, but at 

 a later period exhibits a very distinct (protoplasmic) network of 

 fibres, which is still present after the ovum has been laid. 



The yolk arises, in the manner described by Gegenbaur, in ova 



