SEX-DIFFERENTIATION IN XIPHOPHORUS HELLERI. 6l 



tion as well. The formation of the lumen of the posterior part of 

 the duct is the same in both sexes. The anterior portion, however, 

 presents some slight differences. In the male the lumen is formed 

 by the extension of the intra-testicular sperm duct into the poste- 

 rior primordium. The ends of the two sperm ducts meet slightly 

 back of the testis and fuse to form a single duct. Not only is the 

 origin and method o.f formation of the male and female ducts simi- 

 lar, but the young sperm duct and the young oviduct are identical 

 in structure. Specialization takes place only later in development. 



VI. LATE SEX -DIFFERENTIATION OF THE FEMALE. 



The smallest normal mature female found in my material is 

 26.7 mm. in total length. The largest is 80.0 mm. long, which can 

 be considered a full-grown specimen. The average form index is 

 5.34 and the fin ratio is 1.28. 



Externally the sexually mature female can be recognized by the 

 appearance of a dark spot on each side in the region of the pelvic 

 fins. This is not due to pigmentation of the dermis, but of the 

 peritoneal lining of the body cavity above and posterior to the 

 ovary. In males and young females the peritoneal lining is silvery 

 white or slightly pigmented. As the female approaches sexual 

 maturity the pigment increases immensely and can be seen through 

 the translucent body wall as a black spot. The name " Trachtig- 

 keitsfleck " has been applied to denote this characteristic, but since 

 it is maturity instead of " Trachtigkeit " that causes its appearance, 

 a more appropriate name would be puberty spot. The size of the 

 spot varies directly with the size of the ovary and the period of 

 gestation. It is very conspicuous at the time of the birth of the 

 young and this probably explains the origin of the German name. 



A female with a puberty spot always contains mature ova. 

 There may be from one to nearly one hundred in number, depend- 

 ing upon the age of the fish. They are of a brilliant amber color, 

 measuring on the average 1.6 mm. in diameter. The ova lie in the 

 cortex of the ovary and often occupy the entire space between 

 ovarian lumen and ovarian wall (Fig. 27). Each is contained in 

 a follicle consisting of a single layer of cuboidal cells. The fol- 

 licles are surrounded by a very slightly developed theca folliculi 

 which emerges indistinctly into the surrounding stroma. Imma- 



