iz6 PARTHENOGENESIS [CH. 



The manner of the alternation, however, varies greatly. In 

 some species, for example, many of the Hymenopterous 

 Gall-flies (Cynipidae), parthenogenetic and sexual genera- 

 tions succeed each other quite regularly there are two 

 generations in the year and one is always parthenogenetic 

 and the other bisexual. More commonly the alternation is 

 less regular, and several parthenogenetic generations inter- 

 vene between each bisexual generation, as in most Aphids, 

 Daphnids and Rotifers; or again in other members of the 

 same groups there may be no regularity, and one individual 

 female may at one time lay parthenogenetic, at another 

 fertilised, eggs. Finally, in some species parthenogenesis 

 may be "facultative," that is to say, the eggs are capable 

 of being fertilised but develop equally well parthenogenetic- 

 ally; this condition is best known in the Hymenoptera, 

 for example in the Bee and in various members of other 

 Hymenopterous families. In all these cases it is the rule 

 that fertilised eggs produce only females; in most Hymen- 

 optera unfertilised eggs produce only males, and in other 

 groups and in some Hymenoptera, either males and females, 

 according to the nature of the maturation divisions. To 

 this question of sex-determination further reference will 

 be made later. 



It has been seen in the chapter dealing with the matura- 

 tion of the egg that this process normally involves the 

 reduction of the chromosomes to half the somatic number, 

 the full number being restored at fertilisation by the union 

 of the egg and sperm nuclei. Evidently, then, some modi- 

 fication of the process is to be expected in eggs which are 

 destined to develop parthenogenetically, for otherwise these 

 eggs would give rise to individuals having only the reduced 

 number of chromosomes, unless some process of doubling 

 occurred later. And, in fact, it is found that various 



