SEX-DIFFERENTIATION IN XIPHOPHORUS HELLERI. 51 



The shape of the approaching gonads changes. Instead of being 

 broader laterally the}- now deepen dorso-ventrally. Fusion of the 

 two gonads take place anterio-posteriorly, not along the entire sur- 

 face of contact, but on the dorsal and ventral margins only. The 

 space between becomes the ovarian cavity (Figs. 4-5). It will be 

 recalled that the external lining of the gonad is of peritoneal origin, 

 which means that the epithelium of the ovarian cavity is of peri- 

 toneal origin. At first the ovarian cavity is a narrow slit with its 

 larger axis running dorso-ventrally ; very soon, however, this pic- 

 ture changes to a cross-shaped lumen produced by the invagination 

 of the epithelium laterally. This form of the lumen persists for a 

 considerable time in the young ovary, but ultimately becomes very 

 much modified until in the adult ovary no trace of the original form 

 remains (Fig. 27). That portion of the mesentery which comes 

 to lie between the fusing gonads becomes absorbed. The dorsal 

 portion, or mesovarium, attaches the ovary to the body wall dor- 

 sally, while the ventral portion connects the gonad with the rectum 

 ventrally. 



The formation of the oviduct is begun at the time the two gonads 

 are in close proximity. By the time the gonads are completely 

 fused the duct tissue extends from ovary to urogenital sinus with 

 complete lumen at anterior and posterior ends, but none or only a 

 beginning centrally. In fact, the lumen of the oviduct appears first 

 in its posterior terminal, and only later, at the anterior, end. This, 

 of course, would indicate that the duct formation -proceeds from 

 two primordia, one anterior and the other posterior, which is the 

 case in Xiphophorus hclleri. The anterior primordium is the poste- 

 rior end of the ovary and the posterior, is the lining of the body 

 cavity at its most posterior end in the region of the urogenital 

 sinus. Before there is any indication of duct formation anteriorly, 

 the posterior primordium is actively proliferating cells which at 

 first form a solid cord along the median portion of the peritoneal 

 lining of the body cavity on the dorsal edge of the mesentery. 

 This cord extends anteriorly and at the time a similar strand of 

 cells is formed from the anterior primordium a lumen has appeared 

 in the former posteriorly (Figs. 10 n). No liquefaction of cells 

 has been noticed in the formation of the lumen. 



The anterior cord of cells is a direct extension of the posterior 



