616 GENETICS: A. R. MIDDLETON 
of an induced circuit wave of contraction the difference in the amounts 
of CO2 produced is somewhat smaller than in the former experiment 
although the rate of pulsation in the activated half-disk is on the aver- 
age 3.143 times as great as that of a half -disk pulsating under the con- 
trol of its sense organs. It seems to be clearly established from this 
type of experiment that there is some other form of metabolic activity 
which is of greater importance as a source of CO2 and which is more 
directly under the influence of the rhopalia than is the activity of the 
muscular system. 
In the comparison of normally pulsating half-disks and those acti- 
vated by a circuit wave of contraction a series of cuts, equal in extent 
to those used in forming the labyrinth in activated specimens, were 
made in the tissues of the half disks with sense organs in order to guard 
against any inaccuracy in the results due to inequality in the extent of 
the laceration of the tissues. The activity of the muscular tissue — rate 
of pulsation — was 3.143 times as great in the half-disk containing the 
circuit wave of contraction as in the normally contracting specimen, 
but after various periods of from 5 to 15 hours, when the titrations were 
made it was found that in every experiment except one the greater 
amount of CO2 had been produced by the half disk on which the sense- 
organs remained. In the single experiment which proved an exception 
to the regular result the excess of CO2 in favor of the activated specimen 
were very slight in comparison to the difference in rate of pulsation 
which for this specimen was 28 per minute for the half -disk with rhopalia 
and 118 per minute for the activated half-disk. 
HERITABLE VARIATIONS AND THE RESULTS OF SELECTION 
IN THE FISSION RATE OF STYLONYCHIA PUSTULATA 
By Austin Ralph Middleton 
ZOOLOGICAL LABORATORY. JOHNS HOPKINS UNIVERSITY 
Presented to the Academy, November 4, 1 9 1 5 
In organisms in which an admixture of two parents occurs at repro- 
duction, the problems of evolutionary change become most complex 
and difficult. Heritable changes appear abundantly, but most of these 
are shown to be the definite working out of the numerical rules of inher- 
itance. Whether any of the heritable changes that occur are of a differ- 
ent character is in dispute. In reproduction from a single parent these 
difficulties disappear; if evolutionary changes occur independent of 
biparental admixture it should be possible to demonstrate this in uni- 
parental reproduction. Yet most recent work agrees that in such uni- 
