64 J- M. ESSENBERG. 



37, spJi). They stain very dark and with iron-hemotoxylin the 

 cross-section of a spermatophore presents a solid dark ring. The 

 tails are fairly long and are twisted altogether in a sort of spiral. 

 Occasionally a helated spermatozoan has failed to get " in line " 

 and remains in the lumen among the tails. The ripe spermato- 

 phores occupy the central portion of the testis and are found in 

 every part of the sperm duct. In this condition they are discharged 

 and reach the female genital tract, where by the action of the 

 ovarian secretion the outer membrane is dissolved and the sperma- 

 tozoa swim freely in the oviduct or ovarian cavity. 



The question naturally arises as to the meaning of the primordial 

 germ cells at the periphery of the testis (Figs. 37-38, pp(jc'). In 

 comparison with the rest of the germ cells, they appear inactive. 

 It can not be said with certainty that their number decreases with 

 age, nor is there any reliable sign that they take part in spermato- 

 genesis. In a few diseased males which were affected by a tumor 

 growth in the tail region the testis was affected to a marked degree. 

 All the spermatophores, mature and immature, were broken up 

 with the spermatozoa free in the lumen. Formation of new acini 

 was going on in all parts of the testis, but the peripheral primordial 

 germ cells showed no more activity than in the normal. This 

 seems to lend evidence that the primordial germ cells do not furnish 

 a source of definitive germ cells. 



It seems to be necessary to emphasize at this place that no retro- 

 gressive development of any sort is encountered in the- normal 

 process of sex-differentiation in the male of Xiphophorus helleri. 



The sperm duct is slightly longer than the oviduct because the 

 testis is more anterior in position. It consists of three layers : an 

 outer muscular, a middle connective tissue, and an inner epithelial. 

 The cells of the epithelium are flagellated. The contour of the 

 epithelium is very regular. Just as in the case of the female, the 

 sperm duct enters into the uro-genital sinus to open just in front 

 of the aperture. In both male arid female the urethra opens into 

 the anterior part of the uro-genital sinus. The uro-genital aperture 

 lies at the base of the gonopod and does not enter into it. 



It will be remembered that the transformation of the anal fin 

 into an intromittent organ or gonopod begins during the late stage 

 of tubule formation. The first noticeable thing in such meta- 



