2O2 EDMUND B. WILSON. 



divisions to form the " accessory," which is in reality double, like 

 that of Syromastes, and sometimes separates into its two com- 

 ponents as it moves to the' pole. Morgan therefore concludes 

 that the true numbers of the chromosomes in the two sexes of 

 this species are eight and six respectively, though the female seems 

 to show but six and the male either five or six (according as the 

 two " accessories " are united or separate). 



The point to which I would call attention is the similarity 

 between Phylloxera carycscaulis and Syromastes in respect to the 

 mode of synapsis. In the spermatogenesis of both forms the two 

 unequal " accessories " unite in synapsis (if the process can prop- 

 erly be so called) to form the bivalent ab, consisting of two un- 

 equal components, which passes into the female-producing sper- 

 matozoa. In the maturation of the male-producing egg of 

 Phylloxera, however (and apparently the same must be true of 

 the sexual eggs), a different process takes place, the two larger 

 and the two smaller components uniting to form the bivalents aa 

 and bb, again exactly as there is reason to conclude in the case of 

 the egg of Syromastes. In Phylloxera, as Morgan points out, this 

 involves a redistribution of the four chromosomes, since in the 

 somatic groups they are united to form ab and ab, but recombine 

 at the maturation period to form aa and bb. This remarkable 

 redistribution, I think, loses much of its anomalous character on 

 comparison with the facts in Syromastes, where a and b are al- 

 ways separate in the somatic groups. 



These facts, together with those determined by Payne in Fitchia 

 and other forms (now in press in this journal) and my own 

 earlier ones on Thy ant a, 1 which shows essentially the same con- 

 ditions as in Fitchia, lead me to a somewhat different interpreta- 

 tion of the " accessory " chromosome in Syromastes from that 

 given in my fourth " Study." In that paper I adopted the con- 

 clusion that the two components of the bivalent " accessory " were 

 identical respectively with the large and small ; ' idiochromo- 

 somes " of such forms as Metapodius or Lygceus. I did not then 

 see that all the facts are equally consistent with the view that 

 these two components, taken together, represent the single odd 



1 Reported at the meeting of the American Society of Zoologists in Decem- 

 ber, 1906, but still unpublished. 



