HEEEDITY 



column) is represented by such simple aquatic 

 organisms as rotifers and small Crustacea, like 

 Daphnia. In these parthenogenesis occurs ex- 

 clusively, when the food supply is very abun- 

 dant and conditions otherwise favorable, whereas 

 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 Daphnia, or by a 

 rotifer, under optimum conditions does not un- 

 dergo reduction (see Fig. 49, second row). It 

 remains in the 2 N 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, how- 

 ever, the eggs of the rotifer and of Daphnia 

 do not begin development until they have un- 

 dergone maturation. They are also of two 

 sizes (Fig. 49, third row), — small eggs, which 

 develop without fertilization and which form 



164 



