REVIEW OF BIOLOGY OF SEX-DETERMINATION 
7 
The reverse of the foregoing mechanism described for insects 
and mammals is to be found in the Lepidoptera and the birds. 
In these groups the presence of 2n + 2X or 2Z (as the sex-chro- 
mosomes are here called) results in a male and 2n + X or Z 
produces a female. These facts are well borne out by breeding 
experiments in both groups. The cytological basis is not so 
strong, for both avian and lepidopteran chromosomes are rather 
difficult to study. 
In the moths definite results have been reached by Seiler and 
also by Doncaster, showing that two types of ova are produced. 
Those which extrude the Z-chromosome in the polar cell will 
when fertilized produce females, those which retain the Z- 
chromosome when fertilized will produce males. 
It is obvious from the diagram, figure 2, that the sex of the 
zygote depends entirely upon the maturation of the ovum, the 
retention or expulsion of the Z-chromosome being the deciding 
factor. If in any way maturation can be controlled by factors 
exerting their influence either from within the egg itself or ex- 
ternal to the egg, then sex ratios may be altered from the normal 
50:50. Seiler has done this in the case of moths by varying the 
temperature. It also offers a possible explanation of such sex 
ratios as have been obtained by Riddle in his forced breeding 
experiments of doves, where females are produced in the latter 
part of the season from larger eggs and males in the early part 
of the season from smaller eggs. 
3. COMPLICATED LIFE CYCLES 
The most enlightening observations on the determination of 
sex through maturation are those in the aphids and phylloxerans 
studied by Morgan and von Baehr. Let us examine the case of 
Aphis saliceti of von Baehr. It is well known in these insects 
that fertilized eggs always produce females. During unfavorable 
conditions both males and females are produced by partheno- 
genesis, the males, however, always arising from smaller eggs. 
It has been shown that in these small eggs (represented at the 
extreme right in figure 3) a whole X-chromosome is extruded in 
