PREDETERMINATION OF SEX 297 



Galls of the same size on the same leaf must have been sub- 

 jected to as nearly uniform conditions as possible, but the counts 

 from the same leaf are not strikingly at least more like each 

 other than they are like other counts from other leaves. 



It is idle perhaps to speculate as to the factors that predeter- 

 mine whether a given egg shall be a large or a small one especially 

 as this may depend, as in the other species, not on the conditions 

 that affect the migrant, but on the conditions that determined 

 the nature of the migrant herself. The recent work of Whitney 

 and of Shull shows that very slight differences in the environment 

 turn the scale in sex predetermination. Differences like those 

 described by them might accompany the differences in food con- 

 ditions of the leaf during the day when starch is being made and 

 during the night when starch is being converted into sugar. If, 

 perchance, such environmental changes affect the stem-mother, 

 the differences as to output shown by the galls, might arise. 



THE SPERMATOGENESIS OF THE BEARBERRY APHID 



Stevens and von Baehr have described very completely the 

 spematogenesis of several species of aphids. I have also studied 

 at different times a number of species but as they gave nothing 

 new I have not published the results, except for one very brief 

 account (Jour. Exp. Zool. '09. pp. 298-305). There is one 

 species, however, which in clearness and simplicity exceeds all 

 others that I have seen. I give here therefore an account of 

 some of the critical stages in its spermatogenesis and oogenesis. 

 The data furnish, moreover, an occasion to make certain com- 

 parisons between the phylloxerans and the aphids. 



The species in question, Phyllaphis coweni Cockerell, forms galls 

 on the bearberry. My material was found north of Q\iissett, 

 Mass., near Woods Hole, where I have collected material for 

 four summers. I have found the galls on the bearberry during 

 June, July and August. Each contains, as a rule, a single stem- 

 mother — rarely two — and her progeny. As the gall gets older 

 the progeny is seen to be made up of larger individuals with 

 wing pads and in addition a series of immature forms, as well as 

 a few males . The large individuals with wing pads contain sexug,! 



