THE EGG. 95 



become here cubical or slightly columnar. The ridges soon 

 become more prominent, especially at their anterior ends, their 

 growth being due, partly to increase of the epithelial cells by 

 repeated division, the epithelial layer becoming several cells 

 thick ; and partly to ingrowth of an axial core of connective 

 tissue, from the basal membrane of the peritoneum, along which 

 blood-vessels gain access to the ridge. The anterior third of 

 each genital ridge undergoes degenerative changes at an early 

 period (Figs. 85, 86), and ultimately becomes the fat body of 

 the adult ; the posterior two-thirds develop into the reproduc- 

 tive organ, OR. 



At an early stage, certain of the epithelial cells of the genital 

 ridge become conspicuous by their larger size and more spherical 

 shape ; these are the primitive ova or gonoblasts. Round each 

 primitive ovum the neighbouring cells become arranged so as to 

 form a capsule or follicle ; the follicles forming distinct projec- 

 tions on the surface of the genital ridge. New primitive ova 

 are formed from the surface epithelium, and also by division of 

 those already present ; they, also, soon become inclosed in 

 follicles formed by the neighbouring cells. 



Sexual differentiation appears at the time of the metamor- 

 phosis. In the female, the changes consist essentially in a great 

 increase in the size of the genital ridges, which now become the 

 ovaries, and in the number of the contained follicles ; and in the 

 formation of the permanent ova or eggs. The permanent ova 

 are formed from the primitive ova, but different accounts have 

 been given of the details of the process, and it is possible that 

 they are not the same in all cases. As a rule, each primitive 

 ovum divides rapidly to form a nest of cells, one of which 

 becomes a permanent ovum, while the rest form part of the 

 follicle which surrounds it, and serves for its protection and 

 nutrition. In other cases it is stated that a primitive ovum 

 may become directly converted into a permanent ovum. 



The permanent ovum, in whatever manner it is formed, 

 differs from the primitive ovum : — (i) in its much greater size ; 

 (ii) in possessing a very large vesicular nucleus, or germinal 

 vesicle ; and (iii) in containing a number of yolk-granules, im- 

 bedded in the protoplasm of its cell-body. 



The egg nucleus, or germinal vesicle, is a spherical capsule, 

 with a diameter of from one-third to half that of the ovum itself. 



