HEEEDITY AND SEX 



males, and large eggs, which require fer- 

 tilization, and which form females. In this 

 category of cases, as in that of the hymenop- 

 tera, the egg which develops in the 2 N condi- 

 tion, either from failure of reduction to occur 

 in maturation or from fertilization following 

 reduction, forms a female; whereas the egg 

 which develops in the N condition forms a 

 male. 



In a third category of cases there is a quan- 

 titative difference in chromatin between male 

 and female, just as in the foregoing cases, but 

 this does not amount to a whole set of chromo- 

 somes, N, but to only a partial set, one or two 

 chromosomes (see Fig. 49, right column). This 

 category of cases occurs in plant-lice (aphids 

 and phylloxerans) ; evidence of its existence 

 rests chiefly on recent observations made by 

 von Baehr and Morgan. Females are formed 

 by parthenogenesis without reduction, occurring 

 under favorable conditions, just as in the case 

 of rotifers. Females are also formed by fer- 

 tilization following reduction under unfavor- 

 able conditions, just as in rotifers. In both 

 cases the female is 2 N. Males arise only by 

 12 165 



