314 A. FRANKLIN SHULL 
The effect of crossing on the cycle can not be predicted, therefore, 
from the form of the cycle in the lines to be crossed, but only after 
tests are made by experiment. 
Whatever be the nature of the genotypic constitution, the form 
of the cycle in a parthenogenetic line having a given constitution 
is dependent in part upon the environment. It was earlier shown 
that certain chemical substances were capable of reducing the 
proportion of male-producers. From evidence presented in this 
paper, we may now conclude that the effect of these substances 
is felt only during the growth period of the egg. Once the egg 
has reached its full growth, or at least after it has been laid, 
chemical substances which, when applied throughout life, exclude 
male-producers are powerléss to change the nature of the female 
hatching from the egg. In like manner, these substances are 
powerless to affect the nature of a female before the egg from which 
she hatches begins its growth. So far as these chemical substances 
are concerned, the fate of an egg is irrevocably determined in 
its growth period. Since the maturation spindle is formed in these 
eggs before they are laid (Whitney, ’09), it is not impossible — 
that the influence of external agents is limited to the matura- 
tion period. 
This localization of ‘sex-determination’ in the growth period 
is of interest in several connections. First, it shows why the 
starvation experiments of Punnett (’06) and Whitney (07) did 
not result in an increased proportion of male-producers, as did 
the experiments of Nussbaum (’97) and myself (Shull, 710). Even 
if starvation, as carried out by the former two investigators, so 
altered the chemical composition of the water that a change in the 
life cycle might have been expected, nevertheless it was not 
applied at a timewhen it might havebeen effective. Inthe experi- 
ments of Punnett and Whitney, the females were starved only 
during the first few hours after hatching, not when their eggs 
were in their growth period. 
The localization of the period susceptible to external agents 
also goes to disprove my former explanation of the observed fact 
that late daughters of a family yielded fewer male-producers than 
did their sisters of the early part of the family. I assumed that 
