Unpaired Heterochromosome in Aphids 121 



parthenogenetic female, sexual female and male, of approxi- 

 mately the same age and therefore developing under the same con- 

 ditions of temperature and nutrition. Thus Castle's ('03) theory 

 in regard to the appearance of males among parthenogenetic 

 insects would be realized by a slightly different method of matura- 

 tion. This would bring us one step nearer the conclusion that 

 sex and sexual characters are really represented, in the germ cells 

 of insects at least, by the heterochromosomes. 



As to the fact that the lagging chromosome of the aphids is a 

 heterochromosome intimately connected with the phenomenon of 

 sex determination, the present reinvestigation of the male germ 

 cells, I think, leaves no doubt. The question as to how and when 

 the number of chromosomes of the parthenogenetic female genera- 

 tions is reduced to that of the male individuals will be further 

 investigated as soon as suitable material can be obtained. 



The discovery of Morgan that only female-producing sper- 

 matozoa develop in Phylloxera, and the above corroborating facts 

 for the aphids at once suggest the idea that in the bee and the ant 

 an unpaired male heterochromosome may be left in the male- 

 producing eggs after maturation, and that, as in the phylloxerans 

 and aphids only female-producing (containing a male sex-chromo- 

 some) spermatozoa develop. The complete reduction in number 

 of chromosomes occurring during the maturation of the egg 

 instead of in the spermatocytes might prevent the detection of 

 such a heterochromosome in these forms, but study of other 

 hymenopterous insects such as Nematus, where in some species 

 only females come from the parthenogenetic eggs, in other species 

 only males, and in still others both males and females, may throw 

 light on the problem. 



In all of the other cases familiar to me, where an unpaired 

 heterochromosome is present in the spermatocytes, if it is destined 

 to divide in the first maturation mitosis, it is attached to 

 mantle fibers from only one pole of the spindle. This is true for 

 many Hemiptera homoptera (Stevens '06; Boring '07); for the 

 Orthoptera (McClung and others) and for several of the Coleop- 

 tera (Stevens '06 and '08). On the other hand, when the odd 

 chromosome is attached to mantle fibers from both poles of the 



