SEX INHERITANCE 103 



two chromosomes in the polar body of the male- 

 producing egg has been directly demonstrated. 



Third. In this species the somewhat unusual 

 relation of one stem mother giving rise to the line 

 that culminates in the sexual eggs, and of another 

 stem mother giving rise to the line that culminates 

 in the males, can be explained on the assumption 

 that one pair of the sex chromosomes is heterozygous 

 in some factor indicated in the diagram by priming 

 one of the x's. This explanation is in part theoret- 

 ical, although it is based on the actual observation 

 of two kinds of males that differ in respect to the 

 behavior of one of the smaller x's. 



In other species of phylloxerans, and in many 

 aphids, one stem mother may produce both lines, 

 i.e., some of her offspring may ultimately give rise 

 to sexual females and others to males. In such 

 cases, as is illustrated in the next diagram (Fig. 36), 

 there is but one kind of stem mother, and the four 

 sex chromosomes (there are only two sex chromo- 

 somes in the aphids) are alike. Here some environ- 

 mental influence must determine that in certain eggs 

 conjugation of two pairs of chromosomes takes place. 

 Such eggs give rise to males. In other eggs where 

 this does not take place the sexual female will be 

 produced. 



In both P. carya3caulis and in P. fallax, and also in 

 the other forms referred to above, the difference 

 between the behavior of the chromosomes in the 

 stem mother and in the male-producing migrant is 

 dependent on environmental influences. On the 



