228 THE THEORY OF THE GENE 



suits show that certain characters follow the Y-chromo- 

 some, so-called, in inheritance. The results are not incon- 

 sistent with those recorded in other cases of sex-linked 

 inheritance, provided the X-component of the compound 

 chromosome is absent in the Y. Whether crossing-over 

 occurs in the female of the two species of fish that have 

 been studied is not evident from the published results, 

 partly because the crosses have not been made in a way 

 to bring out this possibility, and partly because the nota- 

 tion used is such as to obscure this possibility. 



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 egg (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 



