36 



THEOPHILXJS S. PAINTER 



Later Doctor Hartman was able to find for me one of his 

 clearest cases of twelve chromosomes in the second polar spindle, 

 and placed the slide at my disposal for study. In text figure 

 8, A I have drawn the chromosomes of the egg illustrated by 

 Hartman in plate 14, figures 3 and 4, of his paper. There can be 

 absolutely no question of the accuracy of Hartman's count, as 

 there are 12 chromosomes present in this egg. However, on 

 closer examination, it will be seen that 10 of these chromosomes 

 are tetrads, while two of them are bivalent or are diads. (This 

 condition was indicated in Hartman's figures but the magnifica- 

 tion of the cell was not sufficient to make the fact clear.) Judging 



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Text fig. 8 Showing the chromosomes in the second polar body spindle of 

 two eggs. A. Twelve chromosomes are present in this spindle, of these ten are 

 tetrads and two are diads. This spindle was figured by Hartman ('19) in figure 3 

 and 4, of plate 14, of his paper. B. Eleven tetrads are present in this spindle. 

 (One of the tetrads is drawn to one side to show its morphology.) 



from my experience with the X-chromosome in the second sper- 

 matocytic division, these two diads are halves of what should 

 be the X-chromosome tetrad. 



As a further check on this conclusion, I selected from Hartman's 

 slides an egg showing 11 chromosomes. This egg is illustrated 

 in text figure 8, B. A glance at the figure will show that all 11 

 chromosomes are tetrads, and that no diads are present. 



It is clear, therefore, that the female opossum has 11 chromo- 

 somes (tetrads) in the reduced number, and it would follow from 

 this that the diploid number is 22, as I have found to be the case 

 in the somatic cells of female embryos. 



