THE HEREDITY OF SEX 57 



i.e. in summer, and the eggs are in the same condition 

 as in Daphnia, etc., that is to say, reduction does not 

 occur, and the number of chromosomes is 2N. 

 Under unfavourable conditions males are developed 

 as well as females by parthenogenesis, but the males 

 arise from eggs which undergo partial reduction 

 of chromosomes, only one or two being separated 

 instead of half the whole number. The number 

 then in an egg which develops into a male is 2N 1, 

 while other eggs undergo complete reduction and 

 then have N chromosomes. The latter, however, 

 do not develop until they have been fertilised. 

 In the males, when mature, reduction takes place 

 in the gametes, so that two kinds of sperms are 

 formed, those with N chromosomes and those with 

 N 1 chromosomes. The latter degenerate and 

 die, the former fertilise the ova, and the fertilised 

 ova develop only into females. The chief difference 

 in this case then is that the reduction in the male 

 to the N or simplex condition takes place in two 

 stages, one in the parthenogenetic ovum, one in the 

 gametes of the mature male. In Hymenoptera 

 and in Daphnia, etc., the whole reduction takes 

 place in the parthenogenetic ovum, and in the 

 mature male, though reduction divisions occur, no 

 separation of chromosomes takes place: at the 

 first division one cell is formed with N chromosomes 

 and one with none, and the latter perishes. 



In many insects and other Arthropods which 

 are not parthenogenetic the male has been found to 

 possess fewer chromosomes than the female. The 

 female forms, as in the above cases of partheno- 

 genesis, only gametes of one kind each with N 

 chromosomes, but the male forms gametes of two 



