PREDETERMINATION OF SEX 309 



genesis of several species of aphids in the hope of getting better 

 figures of this stage, but have found none better than those given 

 by von Baehr. In those species where the formation of the 

 spermatozoa is a continuous process in the testis of the adult 

 male it is possible to obtain in the same testis all stages, from the 

 dividing spermatogonial cells to ripe spermatozoa. A contrac- 

 tion figure is a constant occurrence just after the spermatogonial 

 •stages. The presumption is that here, as in the bearberry aphid 

 and in Aphis saliceti, we are dealing with a true synapsis stage, 

 but as I find a contraction figure often present in older cells also, 

 almost up to the time of the spermatocyte stage, I can not feel 

 that the criterion of the contraction figure alone suffices to iden- 

 tify this stage with certainty, especially when in other forms, as 

 in Largus, a second contraction (not conjugation) stage has 

 been recognized. 



HISTORICAL RETROSPECT 



In Feb. 1908, I described the formation of the functional 

 female-producing spermatozoa of the phylloxerans and also the 

 degeneration of the male-producing spermatozoon. The conse- 

 quences of the formation of a single female-producing class of 

 sperm were pointed out. At that time I had also studied many 

 preparations of aphids (my own as well as some of Miss Stevens') 

 and reached the conclusion that in them too the same process 

 occurred. I did not publish this conclusion, because I wished to 

 give Miss Stevens the opportunity to correct her earlier state- 

 ment in regard to the spermatogenesis in the group. This she 

 did in the paper of 1909. W. B. von Baehr had also been study- 

 ing with Boveri the spermatogenesis and oogenesis of aphids and 

 had come independently to the same conclusion that I had 

 reached. He published his preliminary paper in the Zoologischer 

 Anzeiger, October 13, 1908, referring there to his own as similar 

 to my results in the phylloxerans, and the complete paper in 

 1909. 



In Science (Feb. 5, '09) I reported further facts connected with 

 the spermatogenesis and the number of chromosomes in the 

 male- and in the female-producing eggs. In September of the 



