SEX RATIO IN GAMBUSIA. 397 



proper material and because of the difficulty involved in deter- 

 mining the sexes in young fish. Sexing immature fish, however, 

 was found easier than the writer had anticipated, and a brief de- 

 scription is offered of the technique, pursued. 



Sexually mature fish at first were dissected with the view of 

 learning definitely the exact position of the gonads and to study 

 the general appearance of these organs in fish in which the sex 

 was definitely known. It was found that with scissors the tail 

 and a part of the back of the fish might be removed by a single 

 clip and by making this cut through the base of the anal and slop- 

 ing it forward at an angle of about 45 degrees the viscera, after 

 removing the peritoneum, was exposed while remaining intact. 

 The sexual organs, lying dorsally of the visceral mass, may then 

 be examined in position or removed for examination. It will be 

 noticed at once that the ovary has a black membranous covering, 

 whereas, the testes in preserved specimens are pale in color. The 

 testes, although lying very close together and described as " fused " 

 by Geiser (1924), nevertheless, show distinctly a median depres- 

 sion and on this line they may be separated with a sharp needle 

 into two nearly equal parts. The ovary, on the other hand, is 

 definitely fused, it has no median line of depression and cannot be 

 separated into equal parts without causing rough, unequal breaks. 

 The more prominent projection into the abdominal cavity of the 

 interhaemal spines in the male is another difference that is usable. 

 These characters can be used in sexing immature fish from 15 mm. 

 and upward in length, as they are evident under a binocular 

 microscope. 



In sexing fish that are less than 15 mm. long it is necessary to 

 be more careful in removing the minute gonads and for differ- 

 entiation they must be placed on a slide. As it is necessary to use 

 transmitted light, the contents have to be spread. This may be ac- 

 complished by placing a cover glass over the glands and tapping it 

 lightly. It is desirable at first to mount and study the gonads of 

 somewhat larger fish, that is, of fish large enough to admit of sex 

 determination by the method described in the preceding paragraph. 

 This is desirable, because confusion may arise from the fact that 

 the sperm cysts might be regarded as eggs. It will be noticed, 

 however, under proper magnification that the cysts are more 



