

SEX-DIFFERENTIATION IN XIPHOPHORUS HELLERI. 65 



morphosis is the thickening of the third ray ; in fact, it should be 

 said rays, as all of them are paired and lie side by side (Figs. 44 

 and 45). As the thickening of this ray continues, the third, fourth, 

 and fifth rays elongate until approximately twice the length of the 

 original fin is reached (Figs. 48 and 49). Next the tips of the 

 'last-named rays form knob-like projections which are to be trans- 

 formed into hooks in later development (Fig. 47). As a general 

 rule, spermatogenesis begins at this stage of the transformation of 

 the anal fin, although variations are not uncommon. The first, 

 second, and sixth to tenth rays are subject to no special changes 

 and remain rudimentary. The third ray, which has increased 

 immensely in thickness medially, forms an S-like structure apically 

 and the distal arm projects out on the ventral margin of the fin 

 to form a copulatory hook. This hook is reinforced by the tip of 

 one of the members of the fourth rays, which has turned ventrally, 

 and we shall call it the ventral part of the fourth ray. The same 

 kind of a hook is formed on the opposite margin of the fin by the 

 union of the two members of the fifth ray (Fig. 46). Further- 

 more, the third and the dorsal part of the fourth ray form two 

 rows of symmetrically placed " teeth " which project backwards 

 and outwards, forming a hollow external groove in the former 

 which leads toward the tip of the gonopod. There are approxi- 

 mately 8 " teeth " on each side of the third ray and 9 in the fourth. 

 The two members of the fifth ray in the secondary growth regions, 

 anterior to the hook, have fused and broadened latero-dorsally to 

 form a concave groove on the dorsal margin of the gonopod. All 

 the apical modifications of the rays are supposed to be for copu- 

 lation and transmission of the spermatophores from the male to 

 the female. At the time the anal fin is metamorphosed into a 

 gonopod the spermatophores are formed and ready for discharge. 



Besides the gonopod the beautiful sword of the helleri male and 

 the pelvic fins are secondary sex characters and must be considered 

 here. Both of them, however, start their transformation some- 

 what later than the anal fins. 



The sword shows the first signs of development shortly before 



spermatogenesis begins. It is formed from the ventral lobe of 



the tail fin of the m^e fish. The ten ventral rays of the caudal 



fin are involved; most of the elongation falls upon the sixth to 



5 



