MAMMALIAN SPERMATOGENESIS — OPOSSUM 35 



The fusion which masked the true spermatogonia! chromosome 

 number in Jordan's preparations is doubtless responsible for his 

 failure to find 11 chromosomes in the first spermatocytes. I 

 found it very difficult to make counts in equatorial plate views 

 of the first spermatocyte spindles until after I had practically 

 learned to recognize each chromosome by its size and shape. To 

 make convincing drawings of such views is very difficult. In side 

 views of the spindles, however, under favorable conditions (early 

 phases of the spindle in cells fully differentiated), it was a simple 

 matter to make accurate counts. In his second spermatocyte 

 cells, Jordan reported 'double reduction' which does not occur 

 at all in the best of my material. 



Altogether, it seems clear that Jordan failed to find 22 chromo- 

 somes in the male opossum, because there had been a fusion of 

 some of the chromosome elements in his material. This fusion 

 has been universally met with by all investigators on vertebrate 

 germ cells up to, perhaps, the last five years, and it seems very 

 probable that the chromosome numbers for nearly all vertebrates, 

 as they have been reported before this period, are to be accepted 

 with some reservation. The truth is, the old methods of pres- 

 ervation were simply inadequate to handle the vertebrate 

 chromosomes, and it would be grossly unfair to the pioneer 

 workers in this field to regard their work as carelessly or inac- 

 urately done. 



The reason Hartman found 12 to be the haploid number of 

 chromosomes is due, as it turns out, to a precocious splitting of 

 one of the tetrads in the second polar spindle of the egg. Figures 

 13 and 18 of the present paper will show that the X-chromosome 

 splits so early in second spermatocytic division that, if one did 

 not know the origin and fate of the halves, he would interpret 

 them as being two separate chromosomes. And yet, as figures 

 14, 16, and 19 show, each pole of the cell receives one of these 

 dumb-bell-shaped halves. Before I had an opportunity of 

 examining Doctor Hartman's preparations, I had concluded that 

 probably this is what happened in the eggs where he found 12 

 chromosomes. In other words, that in such cases Hartman's 

 preparations would show 10 tetrads plus two diads. 



