308 A. FRANKLIN SHULL 
Experiments 6 and 7 together indicate that the nature of a 
female (with respect to the kind of offspring she will produce) 
is determined before the egg from which she hatches is laid, but 
not until within several hours of the time when the egg is laid. 
Or, if this determination is a gradual process, it has proceeded 
so far before the egg is laid that manure solution can not reverse 
it, but has not proceeded so far until within several hours of lay- 
ing, but that manure solution can reverse it. Microscopic exami- 
nation of the living animals, which are so transparent that the 
eggs and odgonia may readily be seen, shows that the last several 
hours of the egg stage, within the parent’s body, includes the 
entire growth period. 
DISCUSSION 
The decrease in the proportion of male-producers with long- 
continued parthenogenesis, which was shown to occur in some 
parthenogenetic lines of Hydatina, is of interest from several - 
points of view. First, may not this decrease account for part of 
the differences observed between parthenogenetic lines in cases 
where the ages of the lines are not known? If one line, started 
immediately from a fertilized egg, be compared with another 
removed by a hundred generations from the fertilized egg, the 
latter line might be expected to show fewer male-producers, 
even if in their early generations both lines -had been equal. 
If differences between parthenogenetic lines may thus be sec- 
ondarily produced, how does this phenomenon affect the results 
of crossing reported in my earlier paper (Shull, ’11 a)? That 
depends on the relative ages of the lines. The Baltimore line 
was started from a female collected in March. The winters 
are sufficiently rigorous in Baltimore, I think, to prevent continued 
reproduction during that season. A female collected in spring, 
therefore, must descend from a fertilized egg that hatched prob- 
ably not earlier than February of the same year. The Balti- 
more line can hardly have been more than a month or two old 
when I obtained it. Regarding the age of the New York line 
there is less certainty. The parent of this line was found in Janu- 
ary in a culture in the laboratory, which had been stocked with 
