SEX-RATIOS IX GAMBUSIA HOLBROOKI. 



193 



ance with a definite structural plan. The gonopod is formed by 

 the modification of the third, fourth, and fifth anal-fin rays in 

 the species whose anal fin possesses from six to ten rays. The 

 general plan of the gonopod, with few exceptions, is as follows. 

 The first and second ray are undivided and very short; the third 

 ray is very much elongated; the fourth and fifth, also, are con- 

 siderably elongated ; while the sixth to the tenth rays are normal, 

 bifurcate, and segmented. The third to fifth finrays, also, pos- 

 sess modified ossicles. (See Geiser, '23^.) Genera and species, 

 while following this general foundation-plan, vary widely in 

 the details of structure of the gonopod, as Langer ('13) has so 

 beautifully shown. The primitive form of the gonopod occurs 

 in Petalosoma. The apex of the gonopod may have a spoon- 

 shaped process, the " prepuce " to aid in the transferrence of the 

 sperm-balls to the genital papilla of the female during copulation. 

 Such is the case in Petalosoma, Pascilia, and Molliensis. Other 

 modifications serving the same purpose are found in Phalloceros, 

 in the form of hooks, and in Phallotorynus in the form of a 

 curious structure resembling a garden trowel. 



The writer's observations on the finer structure and progressive 

 differentiation of the gonopod itself are reserved for future 

 papers. 



Hildebrand ('17) has called attention to the fact, also, that 

 in young Gambusia the anal fins of the males and females are 

 similar, and that the gonopod develops gradually, and at no 

 definite age or length of the fish. When the young are only 

 13 mm. long and less than three months old the gonopod is some- 

 times developed: on the other hand, the fish may be 17 mm. long 

 and five months or even a year old, and it is still not developed. 

 A lot of 43 young of which he writes, born in May, 1914, were 

 examined on October 15 of that year (when the smallest was 

 17 mm. long) and it was thought that they all were females, as 

 no gonopod had developed. On June 3, 1915, however, six of 

 the surviving fish were easily recognizable as males. The mor- 

 tality was not stated, although he remarks elsewhere in his paper 

 that the death-rate during early life in aquaria is inordinately 

 high. From his results he concluded that the " modification of 

 the anal fin into an intromittent organ may take place when the 



