656 JULIAN S. HUXLEY 
resorption due to the migration of cells from the tissues into 
the blood-stream ; when the stolon is little affected, therefore, 
zooid-resorption, or the reverse of (2), occurs. 
In the most general terms we have a system the two parts 
of which are in equilibrium. This equilibriam may alter in 
either of two opposed directions. There is differential activity 
of the two parts; the one which is more active is capable 
of causing the reduction of the other and utilizing it as food. 
But differential activity is correlated with differential sus- 
ceptibility, which results, in certain unfavourable conditions, 
in a reversal of the direction of change; for these mduce 
dedifferentiation of the zooid, and in this condition it is less 
active than the stolon. 
Similar conditions, viz. (1) a balance in an organic system ; 
(2) differential activity of the parts of the system leading 
to physiological dominance of the most active part ; (8) con- 
sequent differential susceptibility of the parts leading to 
a possible reversal of dominance; and (4) the resultant 
reversibility of the reactions of the system—play an important 
part in general physiology. Often they are not easy to investi- 
gate ; but in Perophera we are fortunately provided with an 
organism in which they appear in a striking form, and are 
readily accessible to study. 
Tt should be added that in all but the weakest KCN solu- 
tions a grey tinge, not seen in dedifferentiating individuals 
in sea-water, was observed in the zooids during resorption. 
4. EXPERIMENTS ON REDUCTION IN ANIMALS 
WITHOUT CIRCULATION. 
At Professor Loeb’s suggestion, to whom I here tender my 
thanks, experiments were undertaken to see whether the 
action of the heart in Perophora was stopped by potassium 
chloride, and if so whether zooids without an active circulation 
would show typical reduction. 
The experiment was carried out as follows. A large and 
a small stolon-zooid system were placed together in finger-bowl 
