LIFE CYCLE OF HYDATINA SENTA 301 
hatched line here shown is only 21.2. How much of this decreased 
vigor may be correlated with long duration of the resting egg 
stage, and how much with inbreeding, can not be ascertained 
from the experiment. 
Period at which the nature of a female is determined 
At what time in the life of an individual is it determined 
whether she will bea male- ora female-producer? Is it determined 
at a definite period,.or is it a gradual process covering a long 
interval? These questions were answered variously by the early 
students of Hydatina. Nussbaum (’97) believed that a young 
female if treated properly could be made to produce either males 
or females, at the will of the experimenter. A young female 
that was starved, became according to his view, a male-producer. 
Maupas (791), on the other hand, concluded that the nature of 
the female was determined in the egg from which she hatched, 
a conclusion which he expressed by saying that the sex of the off- 
spring is determined in the body of the grandmother. Though 
Woltereck (11) apparently accepts Maupas’s view, partly because 
a female produces only males or only females, partly perhaps 
because of some provisional statements of my own (Shull, 710), 
statements which were confirmed in a paper subsequent to Wol- 
tereck’s (Shull, ’11 b), hitherto there has been no satisfactory 
proof of either proposition. 
It has now been found possible to determine the time, or at 
least one time, at which the character of a female is decided, in a 
manner that seems to me conclusive. Both fertilized eggs and 
parthenogenetic eggs afford evidence on this point. 
EVIDENCE FROM FERTILIZED Eaas. It has long been known 
that fertilized eggs of Hydatina produce only females. I find 
no statement, however, as to whether these females are always 
female-producers, or whether they may be of both kinds. They 
could not be all male-producers, or the species would have per- 
ished long ago. 
In my breeding experiments I have reared to maturity 469 
females from resting eggs, and every one has been a female-pro- 
