DEDIFFERENTIATION IN PEROPHORA 669 
simply dependent on nutritional relations. Child has naturally 
stressed this important point (Child, 1919). However, the 
nutritional aspect is also important, and does as a matter of 
fact determine many relations of dominance and subordination 
of parts in organisms ; and it is to some of the implications of 
this aspect that I wish to draw attention. 
If one reaction or associated set of reactions is proceeding 
faster than another, in the same system, it will occur to 
a correspondingly greater extent; cf. Mellor, ‘ Chemical 
Statics and Dynamics’, p. 70: ‘In any system of parallel 
chemical reactions which consume the same substrate and 
are proceeding simultaneously in a mixture, the extent to 
which each reaction will occur is proportional to its velocity.’ 
This means that if two sets of reactions are going on in an 
organism at an equal rate and that subsequently one of them 
is stimulated to a 10 per cent. increase, then the end-products 
of those reactions (the amounts of two different types of tissues, 
let us say) will, if the available food remains constant, change 
from the proportion 1000:1000 to 1048:952. A similar 
result will occur if the other reaction’s intensity is correspond- 
ingly lowered. This is important in explaining many changes 
resulting from a change in environment acting upon the tissues 
which respond to the change at different rates (see Child, 1916 ; 
Robertson and Ray, 1920; Lillie and Knowlton, 1902, &e.).* 
One of the best examples is the relation of head-size to 
body-size in a regenerating piece of Planaria. Apparently 
the temperature-coefficient of the processes of the head-region 
is greater than those of the body, for the relative development 
of head increases with temperature. It is also decreased by 
increase in concentration of narcotics. 
There is, however, another aspect of the question which it 
is rather more difficult to understand. That is the fact that if 
in an organism two sets of reactions are going on at different 
1 This will, of course, only occur up to a certain limit. A condition of 
hyper-activity may be induced, as for instance by excess of thyroid-secre- 
tion, or by excess of nervous stimulation in certain forms of neurasthenia, 
which results in a wasting of the tissues concerned, 
NO. 260 x Y 
