6O J. M. ESSENBERG. 



2. Free cells in the ovarian cortex of peritoneal origin. Similar 

 conditions are found in the testis. The first and by far the most 

 common source is the epithelium of the tubules described above. 

 The second source, precisely as in the female, is the free peritoneal 

 cells of the testicle. The method of transformation differs in no 

 essentials from that found in the ovary. Such germ cells move 

 toward the periphery of the gonad, multiply, and eventually become 

 spermatocysts. 



The variability in the development of the testis in this stage can 

 not be overlooked. They occur too often and too regularly to be 

 classified as haphazard abnormalities. The testis of the young fish 

 from 15-20 mm. in total length is a very definite structure. Its 

 definite shape, its regular contour, and its distinct bilaterality are 

 characteristic (Fig. 38). In cases where transformation occurs, 

 however, the gonad varies in shape, is irregular in contour, and, 

 what is more striking, is only partially bilateral. Generally the 

 posterior part of the testis shows more or less complete bilaterality, 

 while the anterior part is in a fused state, thus producing a bifur- 

 cated testis (Figs. 40 and 41). There is no indication whatever 

 that the typical shape of the testis occurring in those specimens 

 that transform early becomes irregular and the bilateral testis be- 

 comes fused as development advances. On the contrary, the typi- 

 cal testis maintains its definite shape during the life of the animal, 

 while the irregularly shaped testis gradually becomes regular as 

 development proceeds, and at the time of spermatogenesis the shape 

 of the testis is more or less definite in all. All of the above facts 

 lead to the inference that the bifurcated testis has resulted from 

 the epithelium of the ovarian cavity after complete disintegration 

 of the ovules. In fact, all stages can be found between the epi- 

 thelium of a degenerated ovary and a testis. 



The formation of the extra-testicular sperm duct belongs to this 

 stage, although some traces of it are already found in early and 

 middle stages of tubule formation. Generally speaking, the male 

 and female ducts are homologous. Both begin from an anterior 

 and a posterior primordium. In both the posterior primordium is 

 more advanced in origin and extent of formation than the anterior 

 primordium. In both the anterior portion of the duct is of peri- 

 toneal origin, and this is in all probability true of the posterior por- 



