In Daphnia and Rotifers the facts are different. Parthenogenesis occurs when food supply is plentiful 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 parthenogenesis, 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, _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 2_N_. 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_N_-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_-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.