I9O S. W. GEISER. 



ratio of adults. The cytological evidence, while not satisfactory, 

 points to an apparently nearly equivalent division of the chromo- 

 somes. The presence of heterochromosomes is indicated. The 

 cytological condition of related forms would seem to indicate 

 the production of equal numbers of male and female-determining 

 gametes. What the proportions of the sexes in experimental 

 litters with known life-conditions would be, remains to be as- 

 certained. 



(/) Summary on Spermat agenesis. The testis of adult 

 Gambusia is fo> med by the median fusion of two original paired 

 testes. This ft: don takes place within the first two months of 

 independent life. Traces of the double character of the adult 

 testis are shown in the usual division of the longitudinal testicular 

 canal, and in differential maturation along the median plane 

 (line of fusion). The testes in summer are somewhat larger 

 than in winter. The relative proportions vary markedly with 

 diverse individuals. The fluctuation in volume is not so great 

 as in other teleosts. 



The testis lacks any strongly-developed connective tissue core. 

 There are no spermatic tubules. The cysts of spermatozoa are 

 the result of the continued fission of germ-cells which at the 

 season of sexual activity migrate to the periphery of the gonad. 



The gonads at birth are undifferentiated, but sex is ascertain- 

 able within four weeks after birth, food and other conditions 

 being optimum. The period of active sexual life is the spring 

 and summer months. Active spermatogenesis occurs only at 

 this time. 



There are no special features of spermatogenesis in Gambusia. 

 The cysts are produced by 10-12 fissions of an original spermato- 

 goium. The final spermatogonia pass through the usual pro- 

 phase-stages. Each cyst contains approximately 1500-3250 final 

 spermatogonia, all in the same stage of development. During 

 spermatogenesis, this relation continues. 



It appears from counts of spermatogonial metaphase plates 

 and from diakinesis stages that the diploid number of chromo- 

 somes in Gambusia Jwlbrooki is 35 or 36. There is no evidence 

 in the spermatogenetic process of an unusual method of chromatin 



