ix] SUMMARY 139 



polar divisions and the second polar nucleus sinks back and 

 conjugates with the egg nucleus. 



3 a. In many Hymenoptera and in the male-producing 

 eggs of Rotifers there are two polar divisions, with reduction 

 of the chromosomes to the haploid number. These eggs 

 when they develop unfertilised give rise to males, which 

 have in their primitive germ-cells the reduced number of 

 chromosomes, and consequently undergo no chromosome 

 reduction in spermatogenesis, the first spermatocyte divi- 

 sion being suppressed. Eggs of this type are frequently 

 capable of fertilisation, and when fertilised yield females. 



3 b. In a few Hymenoptera and possibly some other 

 insects there are two polar divisions both of which are 

 equational, leaving the egg nucleus with the diploid chromo- 

 some number. These eggs usually produce females. 



4. In many Cynipidae a sexual generation alternates 

 with a parthenogenetic. The parthenogenetic females 

 which produce males lay eggs which behave as in 3 a above ; 

 the eggs of those which produce females have no maturation 

 division. 



5. In several cases under experimental conditions, and 

 in* one instance in nature (Rhabditis aberrans), partheno- 

 genetic development of an egg is induced by the entrance 

 of a spermatozoon, which, however, takes no direct part in 

 development. 



