IOO J- M. ESSENBERG. 



at its anterior end. The testes were well formed and contained 

 abundant spermatophores. 



For convenience of description the process of sex- reversal in 

 Xiphophorus helleri may be divided into three, more or less, 

 definite stages. The first ranges from a normal female to the 

 enlargement of the anal fin, the second includes the formation 

 of the gonopod, and the third is concerned with the development 

 of the "sword" in the tail fin. 



II. FIRST STAGE IN SEX-REVERSAL. 



The first indication of sex-reversal in these fishes is the absence 

 of the regular monthly delivery of young. To be sure, there 

 are other factors which inhibit reproduction such as lack of 

 fertilization, certain temperatures, light, and composition^ of the 

 water, but these are exceptions and can be eliminated in normal 

 and well-balanced aquaria. With the failure of reproduction 

 the observer's attention is focused on other landmarks in the 

 history of sex-reversal, the most important of which is a char- 

 acteristic black area known as the puberty spot. 



The puberty spot is found in all sexually mature females and 

 is located just above the pelvic fin. Its size varies with the age 

 of the fish or rather with the size of the ovary. In young virgin 

 females it is less than one fourth of an inch in diameter, whereas 

 in females of three years of age it approaches half an inch in 

 diameter. The puberty spot is caused by the black pigment 

 visible through the body wall which accumulates in the peri- 

 toneum adjacent to the ovary. With onset of sex-reversal the 

 black pigment in the peritoneum gradually fades and in the 

 course of six to eight weeks the puberty spot has entirely dis- 

 appeared. 



Sex colorations in Xiphophorus helleri exhibit very little, if any, 

 dimorphism. During sex-reversal the bright red and black 

 pigments of the fins and lateral line dim to such an extent that 

 the fish becomes, so to speak, "invisible." This appearance of 

 the animal is retained until the gonad of the opposite sex has 

 begun to grow and is capable of producing a certain amount of 

 the male sex hormone. 



The internal changes which take place during this stage are 



