ix] MATURATION IN GALL-FLIES 137 



only male-producing or only female-producing partheno- 

 genetic daughters. 



The cytological behaviour of the eggs is not less remark- 

 able. The fertilised eggs of the sexual generation undergo 

 a double polar division of a peculiar type, leaving the 

 reduced (haploid) number of 10 chromosomes in the egg 

 nucleus. The peculiarity of the polar divisions lies in the 

 fact that the nucleus divides in the first division without 

 the formation of an equatorial plate; as the nuclear mem- 

 brane disappears the chromosomes seem rather to sort them- 

 selves out at opposite poles of the nucleus. The egg nucleus 

 with 10 chromosomes then conjugates with the sperm nucleus 

 also containing 10, giving rise to a zygote with 20. This 

 develops into the female of the parthenogenetic generation. 

 The eggs of different females of this generation behave dif- 

 ferently from one another. Those of a male-producing female 

 undergo a double polar division of the same peculiar type 

 as is found in the sexual eggs, the chromosomes are reduced 

 from 20 to 10, and the egg develops with the reduced num- 

 ber into a male, as in other Hymenoptera. In the female- 

 producing eggs the nucleus comes to the surface as if about 

 to divide in the usual manner, but then sinks back into the 

 egg and begins immediately to undergo the segmentation 

 divisions. These eggs, having undergone no maturation 

 division, contain the diploid number of chromosomes, and 

 develop into the sexual females. No cytological explanation 

 has yet been found of the fact that some sexual females have 

 only male-producing daughters, others only female-pro- 

 ducing. 



(5) Parthenogenesis consequent on the entrance of a 

 spermatozoon " false fertilisation." 



It has been found experimentally in several cases that if 

 a spermatozoon of one species enters the egg of a species 



