228 THE THEORY OF THE GENE 



in the Y- and three in the X-chromosomes of Lebistes, 

 that have, as yet, shown no crossing-over. He suggests 

 that this is either because they are close to the male- 

 determining gene in the Y-chromosome, or else that they 

 are identical with the male-determining gene. Five other 

 genes show crossing-over between the X and Y, and one 

 gene is autosomal. He represents the male-determining 

 gene as single and dominant and leaves open the question 

 of the nature of the allelomorph in the X-chromosome, 

 labeling it zero. 



Degeneration of Male-Producing Sperm. 



In two closely related families of bugs, the Phylloxe- 

 rans and Aphids, belonging to the XX-XO type, the male- 

 producing class of sperms (no X) degenerate (Fig. 133). 

 This leaves only the female-producing sperms (X). The 

 sexual Qgg (XX), after extrusion of two polar bodies, is 

 left with one X-chromosome. Fertilized by the X-sperm, 

 these eggs produce only females (XX). These females are 

 called stem mothers. They are parthenogenetic and be- 

 come the starting point of a succession of other partheno- 

 genetic females. After a time, some of these females may 

 produce male offspring, others producing sexual females. 

 The latter are diploid, like their mothers, but in them the 

 chromosomes conjugate and their number becomes re- 

 duced to half. The former individuals that produce males 

 do so by a process that will be described in the next 

 section. 



The Elimination of One X-Chromosome from a 

 Diploid Egg to Produce a Male. 



In the Phylloxerans, as stated above, a certain kind of 

 female appears near the end of the parthenogenetic cycle 

 whose eggs are a little smaller than those of the earlier 

 females. Just before maturation of the smaller eggs the 



