Unpaired Heterochromosome i72 Aphids' II9 



a maple aphid having sixteen chromosomes. Fig. 47 shows a 

 very common appearance of the telophase in this aphid, the 

 chromosomes being massed at one pole and well separated at the 

 other. The massed chromatin is destined to degenerate. An 

 earlier stage (Fig. 46) shows the chromatin massed at both poles, 

 the heterochromosomes (x) dividing, and the cell apparently about 

 to divide equally or nearly so. Fig. 49 shows more inequality 

 in the two cells, but the heterochromosome divided and one-half 

 in each prospective cell. Fig. 48 is quite a different case. Both 

 parts of the heterochromosome (x) are in the nucleus of the 

 larger cell and the chromatin of the smaller cell is densely massed. 

 In Fig. 49 we have another variation: the two parts of the hetero- 

 chromosome have run together and are in the larger cell, but 

 the chromosomes of the smaller cell are not massed together. A 

 careful comparison of first and second spermatocytes in meta- 

 phase brings out the fact that while in the first spermatocyte 

 there is one chromosome considerably larger than the others, in 

 the second spermatocyte equatorial plate there are two larger 

 than the others and nearly equal. One of these is the division 

 product of the large chromosome of the first spermatocyte and the 

 other is the undivided heterochromosome (x) which is second in 

 size in the first spermatocyte (Figs. 50 and 51). 



In the Oenothera aphid, whose male germ cells were described 

 in my first paper on aphids ('05), the failure of the lagging chromo- 

 some to divide in the first spermatocyte is more difficult to demon- 

 strate than in any of the other species. On a slide where there 

 are dozens of telophases like Fig. 54, only one case could be 

 found where the two parts of the heterochromosome were in one 

 nucleus, leaving only four chromosomes in the other (Fig. 55). 

 The lagging chromosome is the second in size and in several 

 prophases of the second mitosis it appears double as shown in 

 Fig. 56, though not so conspicuously so as in the green rose and 

 star cucumber aphids (Figs. 4, 11 and 12). Smaller cells con- 

 taining only four chromosomes and lacking this double chromo- 

 some can also be found (Fig. 57). In another preparation, 

 which has unfortunately been lost, anaphases of the smaller 

 second spermatocytes were seen, and such may also be distin- 

 guished among the degenerating spermatids. 



