1 82 S. W. GEISER. 



Materials. The Gambusia used in this study were from the 

 P^stock of the experimental litters (q. v. in Section III. of 

 this paper). Thus the material for both cytological and breed- 

 ing studies was from the same source. The stock had a very 

 great preponderance of females, the males being outnumbered 

 by the females nearly eight to one. 



Methods. Preliminary tests with Flemming's solution, and 

 with many modifications of Bouin's fluid, particularly Allen's 

 ('18) modification, demonstrated the fact that the best fixation 

 of prophase stages in the material was obtained by the use of 

 Bouin's fluid, modified by the addition of one per cent, of 

 Merck's c. p. urea crystals. This was the fixing agent employed 

 in all subsequent work on the cytology of Gambusia. With the 

 best of care, however, it was not possible with any fixing solu- 

 tion to get good separation of the chromosomes in the spermato- 

 cyte metaphase plates, even though the earlier phases in the 

 same section were beautifully fixed. This difficulty in working 

 with teleost testes is one that has troubled a number of workers. 

 Turner ('19) in his work on the perch, e.g., was unable to ob- 

 tain spread equatorial plates of chromosomes. 



Both section and smear preparations were made. Sections 

 were cut five and seven micra thick. All preparations were 

 stained by Dodd's ('10) long process, with iron-hematoxylin. 

 Occasionally counter-stains were used. 



The General Anatomy of the Testis. In the adult fish the 

 two testes are fused, forming a single gland lying just below the 

 posterior portion of the swim-bladder, and anterior to the gon- 

 opod (Text-fig, i). The posterior portion of the intestine 

 lies below and anteriorally to the combined testes (hereafter 

 called the "testis"). The large muscle controlling the move- 

 ment of the gonopod lies just behind the testis. 



The testis is whitish in color. It lacks the heavily pigmented 

 investing membrane so characteristic of the ovary. It is sus- 

 pended from the swim-bladder and the vertebral column by a 

 very thin mesorchium. The surface of the testis when examined 

 in a fresh condition shows a more or less favose network of 

 cell-borders under the thin investing membrane. These nettings 

 are caused by the arrangement of the cysts in the testis. 



