56 MENDELISM AND 



rise to males segment in the usual way, and all the 

 cells both of soma and gametocytes contain only 

 N chromosomes. In the maturation divisions re- 

 duction does not occur, N chromosomes passing to 

 one gamete, none to the other, and the latter perishes 

 so that the sperms all contain N chromosomes. 

 When fertilisation occurs the zygote therefore 

 contains 2N chromosomes and becomes female. 

 Here then we have no segregation of Fxf in the 

 ova. The difference of sex merely corresponds 

 to duplex and simplex conditions of nucleus, but it 

 is curious that the simplex condition in the gametes 

 occurs in both ova and sperms. 



In Daphnia and Rotifers the facts are different. 

 Parthenogenesis occurs when food supply is plenti- 

 ful and temperature high. In this case reduction 

 of the chromosomes does not occur at all, the eggs 

 develop with 2N chromosomes and all develop into 

 females. Under unfavourable conditions reduction 

 or meiosis occurs, and two kinds of eggs larger and 

 smaller are formed, both with N chromosomes. 

 The larger only develops when fertilised and give 

 rise to females with 2N chromosomes. The smaller 

 eggs develop without fertilisation, by partheno- 

 genesis, and become males. Here then we have 

 three kinds of gametes, large eggs, small eggs, and 

 sperms, each with the same number of chromosomes. 

 It is not the mere number then which makes the 

 difference, but we find a segregation in the ova 

 into what may for convenience be called female 

 ova and male ova. 



In Aphidae or plant lice a third condition is found. 

 Here again parthenogenesis continues for generation 

 after generation so long as conditions are favourable, 



