SEX-DIFFERENTIATION IN XIPHOPHORUS HELLERI. 5<) 



irregular, depending upon its previous history. The primordial 

 germ cells are almost invariably in nests, which would indicate that 

 cell division has taken place; mitotic figures, however, have not 

 been seen. These nests of germ cells always occupy the periphery 

 of the testis and have no connection with the tubules. 



The sperm duct or the main tube of the testis of the previous 

 stage is now branched and as development proceeds the branches 

 become subdivided (Figs. 32 and 38-39). Each testis may be 

 roughly compared to a bunch of grapes with the sperm duct as the 

 main stem. The tubules thus produced are radially arranged with 

 their apices toward the periphery of the gonads. This tubule for- 

 mation is peculiar to Xiphophonts hclleri and closely related genera 

 and, as far as the writer knows, has not been described before. 



If one of these radial tubules is examined, it will be found that 

 it consists of an inner epithelial layer and an outer homogeneous 

 membrana propria (Fig. 33). This membrane is very thin and 

 contains a very few small nuclei. The epithelial cells are cuboidal 

 to columnar in shape, according to stage of development. In size 

 and staining reaction the cells are intermediate between peritoneal 

 and primordial germ cells. In more advanced stages of develop- 

 ment the tubules show various stages of germ cell metamorphosis 

 (Figs. 33-36). The apex may become separated from the rest of 

 the tubule by a constriction and assume a spherical form ; the tubu- 

 lar lumen has become obliterated by the growth of cells which are 

 now spherical in shape and differ from the primordial germ cells 

 at the periphery of the testis only in position. The next portion 

 of the tubule shows cells in a less advanced stage ; the lumen may 

 still be present, the cells and particularly the nuclei are smaller in 

 size and less regular in shape ; the cells take a darker stain than the 

 apical cells and there is no delimiting membrane from the more 

 basal cells. The proximal portion of the tubule may differ in no 

 essentials from the young tubule except that the cells show an 

 increase in size. Later on the acini separate completely from the 

 tubule, but for a long time the radial arrangement is maintained 



(Figs. 37-38). 



It will be remembered that in the ovary definitive germ cells are 

 formed from two different sources : 



i. The epithelium of the ovarian cavity. 



