398 SAMUEL F. HILDEBRAND. 



opaque and granular in appearance than the ova and they do not 

 have a central " nucleus " which is quite characteristic of the eggs. 

 It is possible to sex Gambusia at birth, that is when only about 8 

 mm. long by this method. Consequently, no tedious processes of 

 embedding and sectioning of gonads need be involved. After a 

 little practice one becomes quite proficient in removing the organs, 

 as well as certain of the diagnosis. 



It of course is understood that the specimens, especially the very 

 small ones, must be well preserved or sexing by the process de- 

 scribed becomes difficult and unreliable. The best results were 

 obtained by the use of 80 per cent, alcohol to which 40 per cent, 

 formaldehyde was added in the proportion of about 30 cc. to 

 400 cc. of the alcohol. The strength of the preservative of course 

 may be considerably varied according to the temperature. Dur- 

 ing cool weather 80 per cent, alcohol alone preserves specimens 

 quite as well as the mixture of the two, in the proportions given, 

 during hot weather. 



The rapidity with which immature fish could be sexed, in the 

 manner described, made it possible to examine a comparatively 

 large number of fish, as shown in Table II. Few males have be- 

 come sexually mature at a length of 20 mm. and no appreciable 

 change in the ratio of males to females has taken place in fish of 

 this size. This is brought out in Table II., in which fish, 20 mm. 

 and under in length, are divided into two groups, namely those of 

 15.5 to 20 mm. being placed in one group and those of 15 mm. and 

 under in another. It will be seen that in each group the males 

 and females are very nearly equally numerous. This seems also 

 to show that no thinning out of males takes place before they have 

 become sexually mature. 



The data for the next group consisting of larger fish (20.5 to 

 25 mm.) listed in Table II. are not representative of the sex ratio 

 in unselected fish of that size. It is understood that many fish 

 20.5 to 25 mm. in length are sexually fully mature, the males being 

 recognizable by the modified anal fin and the females, if not 

 actually gravid, nevertheless, frequently have a small elongate 

 blackish spot or line in the position where the prominent black 

 spot is situated when the abdominal walls are distended with eggs 

 and young. Such fish are not included in this group, as it is based 



