230 THE THEORY OF THE GENE 



different process from that which takes place in other 

 insects, but the same mechanism is utilized in a different 

 way to bring about the same end-result. 



There is one further fact of unusual interest in this 

 case. In the Phylloxerans the female that gives rise to the 

 male eggs — she is called the male-egg producer — forms 

 smaller eggs than did her parthenogenetic forbears. The 





k 







a 



b 

 Fig. 134. 



a, The first polar spindle of a" male egg ' ' of Phylloxera in which 

 two chromosomes lag on the spindle and are ultimately thrown out 

 of the egg, leaving five chromosomes in the egg nucleus, b, The 

 first polar spindle of a female egg, in which all six chromosomes 

 divide leaving six chromosomes in the egg nucleus. 



fate of the eggs is, therefore, indicated before the X- 

 chromosomes are eliminated from them. It might appear 

 that, here, sex is determined by the size of the egg, which 

 might mean the amount of cytoplasm contained in it; 

 but the conclusion is an illegitimate inference from the 

 evidence, since the egg becomes a male only after half of 

 its X-chromosomes are eliminated. What would happen 

 if they were retained we do not know — probably the egg 

 would develop into a female. At any rate, we have here an 

 instance of a change that has taken place in the mother 

 that leads to the formation of the smaller egg, which, in 

 turn, reduces the number of its X-chromosomes to pro- 



