302 A. FRANKLIN SHULL 
ducer. They were reared under the same conditions as other par- 
thenogenetically produced females, many of which were male- 
producers. It is safe to conclude, therefore, that at the moment 
of fertilization it is determined not only that the immediate 
offspring shall be female, but that the individuals of the next 
generation shall be females. Whether this may be spoken of as 
sex determination a whole generation in advance, or not, is not 
clear; for no matter whether a male egg develops parthenogeneti- 
cally and produces a male, or is fertilized and yields a female, the 
next generation in the direct line, if there be any, is necessarily 
always female. Aside, however, from the use of the word ‘sex- 
determination’ there can be no doubt that in this case the nature 
of the females of the first generation is determined in the fertilized 
eggs from which they hatch, and before those eggs are laid. There 
is no a priori reason, therefore, for supposing that the nature of 
other females may not also be determined in the parthenogenetic 
eggs from which they hatch, and before those eggs are laid. 
EvIDENCE FROM PARTHENOGENETIC Eaes. The discovery 
that rotifers bred in a fairly strong solution of horse manure may 
be made to yield only female-producers, as pointed out in my 
earlier article (Shull, 710), was employed in the following experi- 
ments. 
Experiment 5. A line of rotifers was reared in spring water. 
When the first members of a new generation were isolated, two 
were reserved for further breeding. One of these, together with 
all her offspring, was kept in spring water to continue the line. 
The other was reared to maturity in spring water, being examined 
every twelve hours, at 9 A.M. and 9 p.m., daily. As soon as she 
was observed to have laid eggs, she was transferred to a new dish 
of spring water, in which she remained during the next twelve 
hours; while the water on the eggs already laid was removed, and ~ 
replaced with filtered, undiluted manure solution. Every twelve 
hours thereafter the female was transferred to fresh spring water, 
while the eggs laid in the preceding twelve-hour period were placed 
in manure solution. The eggs were allowed to hatch in manure 
solution and the young were reared to maturity in the same solu- 
tion. Accordingly, every egg was laid in spring water, and (with 
