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AN UNPAIRED HETEROCHROMOSOME IN THE 



APHIDS 



N. M. STEVENS 

 With Two Plates 



In two previous papers on the germ cells of aphids ('05 and '06) 

 the spermatocytes have been described as having no hetero- 

 chromosome of any kind. In all of the aphids which have been 

 studied, there is, however, one peculiar lagging chromosome in 

 the first spermatocyte mitosis ('05, PI. IV, Figs. 37 and 38; '06, PI. 

 I, Fig. 12; PL II, Figs. 33, 34, 42, 49; PI. Ill, Figs. 60 and 78; PI. 

 IV, Figs. 100, loi. III, 112). This chromosome appeared to be 

 a bivalent which separated very late, giving second spermatocytes 

 with equal series of chromosomes ('05, PI. IV, Fig. 39; '06, PI. 

 I, Fig. 25). 



The results recently obtained by Morgan ('08) in the study of 

 the germ cells of Phylloxera (sp. .^), which has a similar lagging 

 chromosome, have led to a reinvestigation of the matter in the 

 aphids. In Phylloxera (sp. .^) Morgan finds six chromosomes in 

 somatic cells of female embryos and five in male embryos. The 

 first spermatocytes contain three chromosomes. The lagging 

 chromosome, though it appears about to divide as in the aphids, 

 does not do so, but remains in the larger of the two second sper- 

 matocytes, the cytoplasm dividing very unequally. The smaller 

 cells, containing two chromosomes, degenerate, while the three 

 chromosomes of the larger cells all divide, giving only one kind 

 of spermatozoa; i. e., such as can fertilize female-producing eggs. 

 The Phylloxerans, therefore, fall into the same category with 

 the other Hemiptera homoptera described by Boring ('07), and 

 with the Hemiptera heteroptera, Coleoptera and Orthoptera, 

 which have an unpaired heterochromosome, the only important 



The Journal of Experimental Zoology, vol. vi, no. i. 



