LIFE CYCLE OF HYDATINA SENTA 315 
the accumulation of certain chemical substances in the cultures 
as these became old might cause the offspring of the late females 
to be more largely female-producers than the offspring of early 
females. In the light of the discovery that the period suscepti- 
ble at least to certain chemical substances is limited to the growth 
period, my former explanation regarding the later females of a 
family would account for a preponderance of female-producers in 
the last part of the family, but not for a preponderance of females _ 
that produce female-producers. A preponderance of female- 
producers in the last part of the family, as compared with the 
early part, does not occur, as was shown by compiling data from 
349 families (Shull, 710). 
A comparison with the Cladocera with respect to the suscepti- 
ble period will be of interest. The Cladocera do not lend them- 
selves to an inquiry of this kind as readily as do the rotifers, for 
the offspring of a daphnian are not all of one sex. However, 
according to Woltereck (11), Daphnia has two ‘labile’ periods, 
one just before the eggs enter the brood chamber, the other very 
much earlier, in the odgonial stages. It seems not improbable 
that the labile period immediately prior to the entrance of the 
eggs into the brood chamber falls within the growth period, as in 
Hydatina. 
And finally, not the least valuable result of the discovery that 
manure solution is effective only in the growth period of the egg, 
is that a way now seems open to discover the manner in which 
chemical substances affect the life cycle. The question whether 
these substances alter the events in a given cell, or whether they 
merely decide which of two already differentiated classes of cells 
shall develop, bids fair to be answered. If there are two classes 
of cells already differentiated, and manure solution prevents one 
of them from developing; and if eggs may come to the growth 
period before being affected by manure solution; then females 
from a line producing many male-producers, if placed in manure 
solution, should frequently show traces of degenerating eggs, or 
of eggs that do not develop and must be pushed aside to make 
room for cells of the other class. Observations on this point are 
now in progress. 
