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 2^—1, 



while other eggs undergo complete reduction and 



then have N chromosomes. The latter, however, 



do not develop until they have been fertihsed. 



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—l 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 eacli with N 

 chromosomes, but the male forms gametes of two 



