PREDETERMINATION OF SEX 307 



rings, appear and in this condition they are carried onto the 

 polar spindle. 



In the bearberry aphid, the stem-mother produces many male 

 embryos as she gets older. Their presence in the mother serves 

 as an index of the condition of her ovary at this time. An ex- 

 amination of her ovarian eggs at this time failed to show in the 

 chromosomes any process suggestive of synapsis, although no 

 doubt the steps preparatory to the elimination of the two sex 

 chromosome must be taking place at this time. As only two 

 chromosomes are involved it is c^uite likely that even if they 

 went through a contraction phase independently of the rest of 

 the chromosomes (if such were possible) it would be difficult to 

 recognize such a process. The negative evidence has no special 

 value and is mentioned here only to show that the stages were 

 examined for evidence of synapsis. 



In the phylloxerans the ovary of the stem-mother continues 

 throughout her life to produce a series of eggs that develop by 

 parthenogenesis. All stages in the development of the eggs can 

 be found in almost any female. There is never in any of them 

 the slightest evidence of a contraction phase, and, since the eggs 

 show exactly the same conditions as do the parthenogenetic eggs 

 of the bearberry aphid, it will not be necessary to repeat here 

 what has already been said. The ovary of a young female that 

 will later reproduce by parthenogenesis is represented in dia- 

 gram 4. 



The migrant generation of the phylloxerans also produces eggs 

 that will develop by parthenogenesis. In P. caryaecauUs all of 

 the eggs develop at nearly the same time, so that the conditions 

 for the study of parthenogenetic stages are not so favorable as 

 in P. fallax, where the wingle>ss 'migrants' produce one egg at a 

 time over a considerable period. In neither species have I seen 

 any evidence of contraction, nor have I seen any other evidence 

 of conjugation of the sex chromosomes, with the exception already 

 noticed where two chromosomes of double size were found in 

 two eggs that would give rise to male embryos later. Here, 

 however, the nucleus was ripe and ready to take part in the for- 

 mation of the polar spindle. The observation only shows that 



THE JOURXAI. OP EXPERntEXT.VL ZOOLOGY, VOL. 19, NO. .3 



