MAMMALIAN SPERMATOGENESIS — OPOSSUM 29 



DISCUSSION 



1. Sex-determination in the opossum 



In the foregoing pages evidence has been presented which 

 seems to demonstrate conclusively that sex-determination in the 

 opossum is of the X-Y type for the male, and not the X-0 type, 

 as reported by Jordan. The facts adduced to prove this con- 

 clusion are briefly as follows: In the spermatogonia there are 22 

 chromosomes (figs. 1 to 6). These can all be paired up except 

 the two smallest which have no synaptic mates; these are the 

 X-Y chromosomes. The X-component is a small, somewhat 

 elongated rod, the Y-element is a single rounded chromosome. 

 In the growth period of the first spermatocytes we find a nucleo- 

 lus which is probably the X-Y components lying side by side 

 (text fig. 2, D). In the first spermatocyte spindles there are 11 

 chromosomes, showing that all 22 spermatogonial chromosomes 

 have mated (figs. 7 to 11). Of these eleven chromosomes, ten 

 are conspicuous tetrads, while the eleventh is made up of three 

 parts arranged linearly. The figures, however, show that the 

 tripartite chromosome is really the X and Y elements joined end 

 to end. When division occurs the three elements are separated 

 so that the two egg-shaped parts (the X-components) go to one 

 pole, while the single rod-like Y goes to the other. In the second- 

 ary spermatocytes we find in part of the cells the X-element 

 alone, and in other cells the Y-chromosome occurs by itself. In 

 both cases the sex-chromosome divides equationally, so that all 

 spermatids receive 11 chromosomes. Half of the spermatids 

 receive 10 autosomes plus the X-chromosome, and half receive 

 10 autosomes plus the Y-chromosome. As an additional bit of 

 evidence substantiating the facts outlined above, are the dividing 

 cells of opossum embryos, in which we either have the X-Y con- 

 ditions (males) or the 2-X condition (females). Sex determina- 

 tion for the opossum follows the scheme given below : 



Sperm contain: Eggs contain: 



10 autosomes + X, plus 10 autosomes + X, = 20 autosomes + 2X (female) 

 10 autosomes + Y, plus 10 autosomes + X, = 20 autosomes + XY (male) 



