MATURATION AND AMPHIMIXIS 437 



Van Beneden, that each germ-cell is originally hermaphrodite, and 

 that the maturation processes imply the removal of male qualities 

 from the ovum and of female qualities from the spermatozoon, 

 has been abandoned ; and the reducing divisions are recognised 

 as securing a constancy in the number of chromosomes character- 

 istic of each species, for without some such preliminary reduction 

 the number would obviously be doubled at each fertilisation. 

 That a reduction does really occur in both plants and animals 

 seems now incontrovertible, but the precise manner of the re- 

 duction seems to differ considerably in different organisms. 



Reduction in Parthenogenetic Ova. — There is an interesting variety 

 of occurrence. 



(a) In ants, bees, and wasps, all the ova (with 2N chromosomes to 

 start with) undergo reduction, the number of chromosomes being 

 halved (N). Some ova are unfertilised and these develop into males, 

 whose cells have therefore half the normal number of chromosomes 

 (N). There is no reduction of the chromosomes in the making of the 

 sperms. Thus when a spermatozoon (with N chromosomes) fertilises 

 a reduced ovum (with N chromosomes) the normal number, 2N, is 

 restored, and the resulting female retains that number. 



(b) In Rotifers and some water-fleas (e.g. Daphnia) parthenogenesis 

 occurs when the nutritive and other conditions are favourable, and 

 only females are produced. The ova do not undergo reduction, but 

 retain the normal number of chromosomes (2N). In unfavourable 

 conditions eggs of two sizes are produced, and both undergo reduction. 

 The small ones, with N chromosomes after reduction, are not fertilised, 

 and develop into males. The large ones, also with N chromosomes 

 after reduction, are fertilised, and develop in the 2N condition into 

 females. 



(c) In Aphides or plant-lice, parthenogenesis is the rule in favour- 

 able conditions ; reduction does not occur, and females are produced. 

 In unfavourable conditions males appear. Some of the eggs undergo 

 ordinary reduction (to N), and being raised to the 2N condition by 

 fertilisation develop into females. Other eggs produced in unfavour- 

 able conditions undergo a partial reduction to 2N — 1, or 2N — 2, 

 are not fertilised, and develop into males. In the formation of the 

 male-cells there are some with N — 1 chromosomes and some with 

 N chromosomes, but the former degenerate and only the latter 

 become effective spermatozoa. 



