120 N. M. Stevens 



With the exception of those cited for the Harpswell willow 

 aphid (Figs. 33 and 34), I am unable to find any spermatogonia or 

 male somatic cells in my aphid material, where the number of 

 chromosomes can be satisfactorily counted. It is perfectly cer- 

 tain that the earlier parthenogenetic eggs and embryonic cells 

 contain an even number of chromosomes — 2. complete double 

 series of maternal and paternal chromosomes. The question is: 

 Where does the mate of the unpaired heterochromosome {x) of 

 the spermatocytes disappear ? With no direct evidence at hand, 

 my present opinion is that the two heterochromosomes must pair 

 before the maturation of the male-producing eggs and separate 

 in that mitosis while the other chromosomes divide longitudinally. 

 I have never been able to find a polar spindle in male eggs; that 

 is, in such cases as that of the CEnothera aphid where the males 

 and females are produced by different mothers. There is one 

 peculiar case, however, mentioned in my '06 paper, which may 

 be a case in point. In one parthenogenetic individual of the 

 orange milkweed aphid, two eggs in difi^erent embryos had only 

 seven chromosomes (Fig. 52) in the equatorial plate of the matu- 

 ration spindle, while all others had eight (Fig. 53). The two plates 

 were very much alike, each having a large chromosome in the center, 

 evidently corresponding to the two largest in other plates united. 

 Unfortunately I never found any males of this species, although I 

 continued to collect the aphids at short intervals until the plants 

 were killed by frost; but it is possible that, as in the brown rose 

 aphid, only a few scattering males and sexual females appear, 

 while the parthenogenetic female generations go on until destroyed 

 by freezing or starvation. I have found nothing else of this kind 

 in looking over preparations of parthenogenetic individuals col- 

 lected with the sexual generation, but good equatorial plates of 

 polar spindles, cut and stained so that the chromosomes can be 

 satisfactorily counted, are always rare in aphid material. If my 

 surmise as to the maturation of the male-producing aphid eggs 

 should prove to be correct, it would seem probable that these eggs 

 develop into males because a dominant female sex-chromosome 

 has been removed and for that reason only, since the same par- 

 thenogenetic mother aphid may contain embryos of three kinds, 



