14 THEOPHILUS S. PAINTER 



course that the diploid or somatic chromosome number for the 

 female opossum is 24. ^ 



The discrepancy between Jordan's and Hartman's counts 

 amounted to 6 chromosomes — a number far too large to be 

 explained on the basis of differences in the sex-chromosome 

 complex. 



The author undertook the present work with a view of deter- 

 mining, if possible, the cause for the difference in the chromosome 

 counts of the two investigators whose work has just been cited. 

 The problem, however, is not simply a matter of chromosome 

 numbers, but one which has a broad and very important bearing 

 upon the whole subject of spermatogenesis and sex-determination 

 in the mammals. For the discrepancy in the works upon the 

 opossum is typical of the confusion in the literature over the 

 number of chromosomes for many other mammals; in fact, there 

 is less difference in the counts for the opossum (6 chromosomes) 

 than for such forms as the pig (where 18 and 40 have been given 

 as the diploid numbers), or for the male of the human species, 

 which has been variously reported to have 22, 24, and 47 chro- 

 mosomes.^ It is clear that, as long as the total number of chro- 

 mosomes possessed by an animal is a matter of doubt, we cannot 

 safely accept any conclusions regarding the sex-chromosomes of 

 that form. 



In a paper on the spermatogenesis of lizards, the author 

 ('21a) has shown that the presence of a bipartite body at 

 one end of a spindle in maturation divisions does not neces- 

 sarily mean that it is a sex-chromosome as has been so often 

 assumed in vertebrate spermatogenesis. It is frequently either 

 a whole tetrad or half of a tetrad which has divided, the other 

 half remaining in the equatorial plane of the spindle. In the 

 present paper (p. 30) a number of such false sex-chromosomes 



- Hill ('17), working on the South American opossum, Didelphys aurita, 

 estimated that there were 12 (haploid) chromosomes in the egg of this species. 



5 Wodsedalek ('13), gives 18 as the diploid chromosome number for the pig, 

 while Hance ('17) shows that there are 40 chromosomes in both the germinal 

 and somatic cells. Among the recent counts for the male of Homo sapiens we 

 have the following: Guyer ('10), 22; Montgomery ('12), 23 or 24; von Winiwarter 

 ('12), 47; Wieman ('17), 24. 



