1 66 SEX-DETERMINATION [CH. 



derived from fertilised eggs, produce galls on the hickory- 

 leaf. They lay parthenogenetic eggs in the gall, and from 

 these eggs winged females develop. These are of two kinds ; 

 some lay small eggs from which males hatch, others large 

 eggs which produce females, and since all the individuals in 

 one gall are of the same kind, the stem-mothers must also 

 be of two kinds those with male grandchildren and those 

 with female. The females produced from the winged forms 

 each contain one egg which requires fertilisation and gives 

 rise to the stem-mother again. The life-cycle is represented 

 schematically on the preceding page. 



In the P. caryaecaulis female there are four autosomes, 

 two large ^-chromosomes and two small " ^-chromosomes " 

 which are attached to the ^-chromosomes. In the male 

 there are the four autosomes but only one X and one x, 

 the other Xx having been extruded with the polar body of 

 the egg. In the spermatogenesis of some males MORGAN 

 found that the X and x remain attached to each other, 

 while in others they separate, giving apparently five 

 chromosomes in some and six in others. He suggests that 

 the x which remains attached to the X is different in char- 

 acter from that which becomes separated, so that there 

 would be two kinds of males producing different spermato- 

 zoa. He suggests that one kind of spermatozoon causes the 

 fertilised egg to give rise to a stem-mother which will have 

 female grandchildren, and the other to one that will have 

 male grandchildren the presence of one or the other kind 

 of x determining whether an X-chromosome is or is not 

 extruded with the polar body of the parthenogenetic egg 

 of the winged female. It must be admitted, however, that 

 this explanation is speculative, and that the "predeter- 

 mination of sex" in Phylloxera is still not much less obscure 

 than the similar phenomenon in the Cynipid Gall-flies. 



