THE EGO. 95 



become here cubical or slightly columnar. The ridges soo n 

 be come more prominen t, 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 

 ea ch genital ridge under goes degen erative Changes at an early 

 pe riod ( Figs. 85, 86), an d ultimately becomes the fat body of 

 t he adult ; the posterior two-thirds develop into the rep roduc- 

 ti ve organ, 0% 



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

 ridge become conspicuous by their larger size and more spherical 

 shape ; t.Tipgg ^^ya t.ViP pn'Tiiitivfi nya, or gminMa.st.s, Eound each 

 primitive ovum t he neighbouring cells become arranged so a s to 

 fo rm a capsule or fnllidp, ; t he follicles forming distinct p rojec- 

 tio ns 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 appe ars at the time of the metamo r- 

 phosis! In the femal e, the changes consist essentially in a great 

 increas e in the si ze of the gen ital ridge s, which now become the 

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

 form ation of the permanent nva. or fig-g s. The permanent ova 

 are formed from the primitive ova, but difi'erent 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, ea ch primitiv e 

 ovuxn di vides rapidly to form a nest of ce lls, one of which 

 becomes a permanent ov um, w hile 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 siz e ;, 

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

 vesicle ; and (iii) in contain ing 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. 



