272 GENETICS 



of the zygote depends entirely upon the method of 

 maturation of the ovum, the retention or expulsion of 

 the ^-chromosome being the deciding factor in the de- 

 termination of sex. If in any way maturation can be 

 controlled by factors exerting an influence either from 

 within the egg itself or external to it, then sex ratios 

 may be altered from the normal 50 : 50. This has 

 been done by Seller in the case of moths by controlling 

 the temperature of the developing ova at the critical 

 time in the process of maturation. 



The control of maturation offers a possible explana- 

 tion of such sex ratios as have been obtained by Riddle 

 in his forced breeding experiments on doves, where 

 females are produced in the latter part of the breeding 

 season from large eggs and males in the early part 

 of the season from small eggs. 



3. SEXUAL CYCLES 

 A. APHIDS AND PHYLLOXEBANS 



Most enlightening observations on the determina- 

 tion of sex by means of influencing maturation, have 

 been made upon aphids and phylloxerans by Morgan 

 and by Von Baehr. It is well known that in the case 

 of Aphis fertilized eggs always produce females. Under 

 favorable conditions both males and females are pro- 

 duced parthenogenetically, the males, however, always 

 arising from smaller eggs than the females. 



It has been observed too that in these smaller eggs 

 (Fig. 84) an entire ^-chromosome is extruded in the 

 giving off of the one polar cell, leaving in the egg 



