Heredity and Sex 69 



reproduction by fertilized eggs occurs only when external 

 conditions, including food supply, are not good. Under 

 favorable conditions only female offspring are produced. 

 The conclusion has naturally but erroneously been drawn 

 that good nutrition in itself favors the production of females 

 in animals generally, which is not true. The egg produced 

 by Daplniia, or by a rotifer, under optimum conditions 

 does not undergo reduction (Fig. 34, second row). It remains 

 in the 2X condition, forming but a single polar cell. It is 

 therefore unprepared for fertilization, and in fact it is not 

 fertilized. Its sex is like that of the animal which formed 

 it, female. Under unfavorable conditions, however, the 

 eggs of the rotifer and of Daphnia do not begin development 

 until they have undergone maturation. They are also of 

 two sizes (Fig. 34, third row) — small eggs, which develop 

 without fertilization and which form males, and large eggs, 

 which require fertilization, and which form females. In 

 this category of cases, as in that of the hymenoptera, the 

 egg which develops in the 2.Y condition, either from failure 

 of reduction to occur in maturation or from fertilization 

 following reduction, forms a female; whereas the egg which 

 develops in the X condition forms a male. 



In a third category of cases there is a quantitative 

 difference in chromatin between male and female, just as 

 in the foregoing cases, but this does not amount to a whole 

 set of chromosomes, .Y, but to only a partial set, one or 

 two chromosomes (Fig. 34, 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, occuring under favorable 

 conditions, just as in the case of rotifers. Females are also 



