588 THE STRUCTURE AND DEVELOPMENT 



cate vesicles filled with a clear fluid, but contain close to one 

 side a granular mass which stains very deeply with colouring 

 reagents. The granular mass becomes somewhat stellate, and 

 finally assumes a reticulate form with one more highly refracting 

 nucleoli at the nodal points of the reticulum. When a nucleus 

 has reached this condition the protoplasm around it has become 

 slightly granular, and with the enclosed nucleus is segmented 

 off from the nest as a special cell a permanent ovum (figs. 13, 

 14, 15, 1 6). 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 imme- 

 diately after its separation from a nest, and so constitute a fol- 

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

 In Scyllium 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 25, fig. 23). 



In Raja a great irregularity in the follicle cells is observable 

 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. 25, figs. 30 33). 



(6) The egg membranes. Two membranes are probably 

 always present in Elasmobranchs during some period of their 



