Sex Determination in Phylloxerans and Aphids 299 
It is not my wish to enter here into a detailed account of the 
spermatogenesis in aphids, since Stevens has already gone over 
the ground, but I should like to state a few of the results of my 
own observations that will serve to form the basig of comparison 
between the spermatogenesis of the aphids and that of the phyllox- 
erans. In one not unimportant point I get a somewhat different 
impression of the facts from that given by Stevens in her last 
paper, namely, that the process, by which the division of the lagging 
chromosome takes place, while variable, is more regular than 
Stevens’.account might lead one to infer. That irregularities 
occur, that whole cysts may “go wrong,’ as in other insects, is not to 
be denied; but in the great majority of cases the lagging chromo- 
some moves into the large cell from which the two spermatozoa 
are ultimately formed (after the second division). 
I have chosen of the several species of aphids examined three 
species from the Willow, for description. Presumably the first 
of these is the same species studied by Stevens who did not identify 
the species she used. Mine were obtained abundantly at Woods 
Hole, Mass.; those of Stevens, at Harpswell, Maine. 
In Aphis salicola the spermatogonia contain 5 chromosomes 
beyond any doubt (Fig. XX, A-C). These can best be 
counted at a stage just before the equatorial plate is produced. 
Three nearly equal chromosomes appear in the equatorial 
plates of the first spermatocyte (Fig. XX, D-F). At the first 
spermatocyte division two of the chromosomes divide, the third is 
drawn out (Fig. XX, G-M). Later stages in the division are 
shown in Fig. XX, M—X. Here, as in the phylloxerans, the lag- 
ging chromosome is not withdrawn into the larger cell until the 
very last moment, when in fact the nuclear sac has begun to appear 
around the chromosomes. ‘The final stages in the process seem to 
involve a flowing or contraction of the chromosome, whose end in 
the large cell gradually increases in size as the chromosome shortens 
In the final stages the lagging chromosome is often surrounded by 
only a thin film of protoplasm, so that the two cells—the larger 
and the smaller—appear to be connected by a very delicate bridge. 
This bridge is found at times when the end of the lagging chromo- 
some, still within the small cell, is slightly enlatged. The bridge 
