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A^. M. Stevens 



first spindle, it divides in the first division, as in Anasa and other 

 Hemiptera heteroptera (Wilson '05, '06) and in Photinus (Stevens 

 '08). In the latter case the division products of the odd chromo- 

 some make connection with only one pole of the second spindle. 



In the aphids the unpaired heterochromosome is connected 

 with fibers from both poles of the first spindle and, in most cases, 

 until a very late anaphase or telophase, appears to be about to 

 divide equally. Both cell and heterochromosome begin to do 

 one thing, and finally do something quite different. One can 

 hardly fail to be impressed with the idea that we have here a case 

 where the karyokinetic apparatus is imperfectly adjusted to the 

 demands made upon it by the cell as a whole or by the chromo- 

 somes, but that the end is finally attained in spite of this imperfect 

 working of the spindle. This peculiarity in the first maturation 

 division, it would seem, must have come in with a change from 

 purely sexual reproduction to parthenogenetic, involving the sup- 

 pression of the male-producing spermatozoa. 



The apparent equal division of the lagging chromosome in 

 anaphases of the first spermatocyte division together with the 

 facts that there is no condensed heterochromosome in growth 

 stages of the spermatocytes and that all of the chromosomes of the 

 second spermatocytes certainly divide longitudinally ('05, PI. IV, 

 Fig. 41; '06, PI. II, Fig. 50; PI. IV, Figs. 108 and 109) led to the 

 conclusion that there was no heterochromosome in the aphids. 

 The late shifting over of the whole lagging chromosome and a 

 variable amount of cytoplasm into one of each pair of second sper- 

 matocytes, together with the significance of the smaller degener- 

 ating cells were overlooked until the preparations were reexamined 

 in the light of Morgan's results on Phylloxera. This experience 

 with the germ cells of the aphids indicates the probability that a 

 sex-determining differentiation of chromosomes in the male germ 

 cells may exist in other cases where it has not yet been detected 

 because of a large number of small chromosomes, or of some 

 unexpected peculiarity in the behavior of the heterochromo- 

 somes. 



I desire to express here my indebtedness to Prof. T. H. Mor- 

 gan for urging a reinvestigation of my aphid material, and also 



