FORMATION OF THE FIRST SPERMATOCYTE CHROMOSOMES 569 



two of them are univalent and that theu' behavior in the matu- 

 ration divisions is irregular. 



As previously stated, Stevens finds 12 chromosomes in the 

 first spermatocyte division and usually 12 in the second, although 

 11 and 13 are sometimes present. She accounts for 13 by as- 

 suming that the small idiochromosome may sometimes divide 

 precociously. She sees no way of explaining the 11-group 

 except by assuming that both idiochromosomes occasionally 

 pass to one pole undivided in the first spermatocyte division. 

 Later, I shall attempt to show that in some individuals the 

 13-group in the second division may be caused by the precocious 

 division of one small chromosome. 



Specimen 29-3. This is very similar to 29-2. Two spermato- 

 gonia! counts show 24 chromosomes each (fig. 4, A and B). 

 In the first division (fig. 4, H and L), I find 17 cells showing 

 clearly 12 chromosomes and nine showing 13. Side views of 

 metaphase plates again show 2 univalents in the 13-chromosome 

 group (fig. 4, E, F, G) . Anaphases also show the lagging chromo- 

 somes in some of the cells. Figure 4, M and N are late ana- 

 phases in which the cell plate has begun to form. In M, there is 

 no doubt that one univalent has divided while the other is pass- 

 ing to one pole undivided. In N, one chromosome is passing 

 to one pole undivided while the other is very much drawn out 

 and it seems doubtful whether it will reach either pole. As in 

 29-2, the irregular behavior of these univalent chromosomes 

 causes variations in the number of chromosomes in the second 

 spermatocytes. The counts here gave 63 cells with 12 chro- 

 mosomes and 15 with 13. It seems rather strange that no cells 

 with 11 were present as these might be expected from what 

 was found in 29-2. Four other specimens show the same irregu- 

 larities as the two above. They did not contain as many divid- 

 ing cells, however. Specimen 27-1, in the first division, showed 

 one cell with 12 chromosomes, two with 13 and three with 14; 

 in the second division two cells with 12 chromosomes and two 

 with 13. Specimen 27-2, gave in the first division three cells 

 with 12 chromosomes; in the second division 26 cells with 12 

 chromosomes and four with 13. In specimen 27-3, I found 15 



