298 T. H. Morgan 
Six chromosomes are found in polar spindles of P. subelliptica 
much like the female group of P. caryzcaulis (Fig. XIX A-F). 
The preceding facts and figures show that identification of 
species may be materially assisted by a knowledge of the chromo- 
somes. It may turn out that the failure to discover two kinds of 
groups of chromosomes in the eggs of the migrants has a real 
meaning in connection with the presence of only one form 1 in these 
galls. | 
SPERMATOGENESIS IN APHIDS 
Stevens described in 1904, in the first spermatocyte division in 
aphids, a lagging (accessory) chromosome, which, instead of 
passing to one pole, divides equally so that each of the first sperma- 
tocytes receives a half. All of the chromosomes were found to 
divide again equally in the second division to produce, in all, four 
spermatozoa. ‘The result stood in the way of any attempt to bring 
the aphids into line with other insects possessing an accessory and 
made difficult any conclusion regarding the relation of the acces- 
sory to sex production. Stevens overlooked two essential facts in 
the spermatogenesis, namely, the fact that the lagging chromosome 
does not divide, as she had supposed, in the first division; and sec- 
ond, the fact that one of the products of the first spermatocyte 
division is much smaller than the other and subsequently degener- 
ates. 
During the winter of 1907-1908 I discovered in the phylloxerans 
the two points just noted. These facts | communicated to Miss 
Stevens, and she most generously gave me some of her former prep- 
arations of aphids to study. ‘These with others of my own, that 
I had made some years before, SU me that while the failure 
of the lagging chromosome to divide is ‘nore difficult to make out, 
the aphids show in all essential respects the same relations as dothe 
phylloxerans. I urged this conclusion on Miss Stevens, who, after 
renewed investigation of her material, came to the same conclusion. 
Her results were sent to me in May, 1908, for publication in 
the Journal of Experimental Zoology, but owing to delay in pub- 
lication did not appear until January, 1909. Meanwhile von 
Baehr, who had beén working on aphids, published conclusions 
in 1908 similar to those found by Stevens and myself. 
