SEX-DIFFERENTIATION IN XIPHOPHORUS HELLERI. 53 



nucleoli are by no means uncommon, as many as six have been 

 noticed in one young ovary. The direction of migration is not pre- 

 determined. In retrogressive ova its shape is greatly affected. . It 

 may be elongated, crescent-shaped, irregular, or hollow. 



2. Retrogression in Class I. 



The size limits of the fish vary from 16.5 to 29.5 mm. in total 

 length. The form index averages 4.62 and the fin ratio is 1.16. 

 The germ cells all derived from primordial ova in this stage are 

 anywhere in size between primordial germ cells and medium-sized 

 ova. However, the larger-sized ova are affected first. The first 

 perceivable sign of retrogression appears to be a darker staining 

 zone in the cytoplasm surrounding the nucleus. Later this zone 

 gradually disappears, but at the same time the cytoplasm loses its 

 affinity for stain and appears bleached. The follicular wall, which 

 is one layer of cells thick in the normal ova, now becomes dis- 

 organized and it may form two or more layers of cells which may 

 form strands and migrate into the yolk (Fig. 15). The nucleus 

 appears to be affected last. It shrinks, becomes irregular, and 

 disintegrates together with the yolk mass. After the ovum has 

 been entirely absorbed remnants of the follicle remaining for some 

 time witness to the destruction. Some such atrophied follicles may 

 contain several isolated cells which no doubt are the remains of 

 the ingrowing follicular strands. Sooner or later these remnants 

 are also resorbed (Fig. 25). 



The retrogression is further characterized by the fact that the 

 epithelium of the ovarian cavity shows no signs of being unfavor- 

 ably affected. On the contrary it appears very active. It has 

 been noticed, particularly when the ova are greatly reduced by 

 disintegration, that new primordial germ cells originate in the 

 proximity of the epithelium of the ovarian cavity. These could 

 not be traced to cell division, nor could they be traced to pre- 

 existing smaller primordial germ cells; the source of these cells is 

 the epithelium itself. 



It appears probable that any cell in the epithelium is capable of 

 transforming into a germ cell. All stages of such transformation 

 are encountered in the epithelium as will become apparent from 

 Figs. 17-24, The epithelial cells are small with relatively little 



