34 THEOPHTLUS S. PAINTER 



4. The chromosome number for the male and the female opossum 



It has been shown in the foregoing pages that the diploid or 

 somatic number of chromosomes for both the male and the female 

 opossum is 22, and not 17, as stated by Jordan, or 24, as concluded 

 by Hartman, How are the results of this paper to be reconciled 

 with the chromosome counts of Jordan and of Hartman? The 

 following suggestions are offered. 



In the case of the males, a careful comparison of Jordan's 

 figures with my own preparations indicates that Jordan had con- 

 siderable fusion of the chromosomes in his slides. The Y-chro- 

 mosome seems entirely lacking from all his figures, and the 

 X-component is present in only one or two cells. The main 

 error in counting, however, was in the chromosome ring. The 

 fact has already been pointed out in the early part of this paper 

 that in spermatogonia one frequently finds cells in which two, 

 sometimes four, chromosomes lie close together, or even partially 

 overlap. Unquestionably, with fixing fluids less adapted to 

 vertebrate chromosomes than the newer methods, these chro- 

 mosomes would fuse or lie so close together that the most com- 

 petent investigator could not distinguish their separate outlines. 

 Here, it is believed, is the explanation for Jordan's failure to find 

 more than 17 chromosomes in the spermatogonia. 



In this connection it is illuminating to consider Hance's ('17) 

 work upon the pig. Wodsedalek ('13) had reported 18 as the 

 spermatogonial number of chromosomes for the pig, while Hance, 

 using cold Flemming solution with urea, found 40 chromosomes 

 in both somatic and germinal cells. Hance made careful com- 

 putations of the mass of chromatin in Wodsedalek's figures, and 

 then, after making similar computations for his own preparation, 

 he compared the results. He found that, in Wodsedalek's figures 

 with 18 chromosomes, there was approximately the same mass 

 of chromatin as in his cells showing 40 chromosomes. While the 

 author has made no attempt to compare his results in this way 

 with those of Jordan, there is little doubt but that the same mass 

 of chromatin would be found in both cases. 



