MAMMALIAN SPERMATOGENESIS — OPOSSUM 15 



are shown (text fig. 7). It is not to be doubted but that some of 

 the 'accessory chromosomes' reported by different investigators 

 of mammaUan spermatogenesis are of this false type. 



In addition to the question of chromosome number and of sex- 

 determination in the opossum, a number of points of rather 

 special interest to cytologists are touched upon; chief among these 

 is the phenomena of so-called ' double reduction ' first described 

 by Guyer ('09) and reported for the opossum by Jordan. 



MATERIAL AND METHODS 



The present study is based upon the testes of four opossums 

 and upon the dividing somatic cells of eight embryos. 



In Texas the breeding season for the opossum begins in Janu- 

 ary. The males, upon which this study was made, were oper- 

 ated on as follows: one male on October 8, 1920; one male on 

 November 15, 1920, and two males on January 14, 1921. Mature 

 sperm, dividing spermatogonia, and cells in all stages of matura- 

 tion were found in all the males but the maturation stages were 

 more abundant in the individuals whose testes were preserved in 

 January. 



In the first male, an attempt was made to secure fixation by 

 injecting the fixing fluid into the blood system, as Allen ('19) 

 recommends. Only partial success attended the use of this 

 method. Spermatogonial chromosomes (fig. 1) show up with 

 sharpness, but the first and second spermatocyte divisions are 

 badly masked by chromosome fusion. This may be due to the 

 ether used in anaesthetizing the animal. In the other cases, no 

 ether was used. The males were tied down, the scrotal sacs cut 

 open with a razor and the testis removed. The testis was cut 

 into several pieces with scissors, and then one or more of these 

 pieces dropped into the fixing fluid, and the tubules quickly 

 teased apart, so that complete penetration would be secured. A 

 period of less than thirty seconds elapsed between the time the 

 tubules were receiving blood from the animal and were being 

 bathed in the fixing fluid. 



Two preserving fluids were used. One was modified Bouin's 

 solution, suggested by Allen ('16, '19), warmed to about 38°C. 



THE JODRNAL OF EXPERIMENTAL ZOOLOGY, VOL. 35, NO. 1 



