186 S. W. GEISER. 



many fissions take place before the spermatogonia are produced 

 which give rise to the final spermatogonial cyst. A computa- 

 tion based on the approximate total number of cysts produced 

 during the lifetime of a male Gambusia gave the number of 

 fissions as at least 16. Such a computation is to be accepted 

 with reservations: certain it is, however, that there are man> 

 cell-divisions from the time of the first segregation in the Keim- 

 bahn to the time when the cyst-forming spermatogonia are finally 

 located in the testis. 



The time of segregation of the first sex-cells is not known. 

 My earliest Gambusia embryos have a snout-anus length of 

 1.58-2.05 mm. Definitive sex-cells are then present in the 

 typical location beneath the swim-bladder. At birth the gonads 

 are developed but slightly beyond the condition found in these 

 embryos. The sexes are not readily distinguishable. The gonads 

 at birth have large, clear cells with a very clear nucleoplasm in 

 which lies a prominent plasmosome. From the plasmosome, 

 and apparently taking their origin from it radiate delicate linin 

 fibrils, upon which frequent granules of deeply staining chro- 

 matic material may be seen. The filaments appear to extend 

 to the inner surface of the nuclear wall. Occasionally the sex- 

 cells in the gonad appear especially large and clear, and each cell 

 is invested with a layer of mesoderm cells, but these have been 

 observed in only a few individuals. It is not improbable that 

 such cells are developing oocytes, but the evidence in the present 

 case is not completely convincing. In a few of the individuals 

 cells unmistakeably developing ova are found at birth. These 

 cases are however, rare. In those cells which have not yet 

 differentiated there is usually a prominent plasmosome, with one 

 or more other conspicuous irregular masses of chromatic mate- 

 rial. In the later spermatogonia (Figs. 1-2 ) 6 which are pro- 

 duced from localized peripheral germ-cells of the testis, in the 

 cyst during its formation, /there are usually two prominent 

 karyosomes. The final spermatogonia (Figs. 8-10) are approxi- 

 mately one-eighth the volume of the cell which by dividing 

 formed the cyst. These final spermatogonia possess a very fine 



6 " Figures " refer to the plates. 



