REGULATORY PROCESSES IN ORGANISMS 23 
the cells of the region 6 adjoining the part removed, be chiefly 
affected correlatively by the removal of a or if they react much 
more rapidly than others further away, the process of formation of 
a will be a ‘regeneration.’ But if parts further away from a are 
also affected to a considerable extent by the change and are cap- 
able of reacting as rapidly or almost as rapidly as b, then they may 
also take part in the process of replacing a, which then takes on 
more or less completely the character of a ‘redifferentiation.’ In 
some cases we can determine experimentally whether a part shall 
be formed chiefly by regeneration of redifferentiation. In Planaria, 
for example, the amount of regeneration, as opposed to rediffer- 
entiation, in the formation of a new posterior end increases with 
increasing distance of the cut surface from the old posterior end. 
The farther the level of the cut from the old posterior end, the 
more completely is the development of the new part confined to 
cells near the cut, and vice versa. The cells near the cut are those 
which are most affected by the removal of the part a. Even when 
this part does not develop anew, they react by healing the wound. 
That is to say, they change their reactions mostrapidly of all cells, 
they lose their old specification, they become capable of a more 
rapid metabolism (Child, ’11b) and being subjected to the correla- 
tive factors of the parts c d——~—n, they begin to develop into some- 
thing more or less like a in advance of other parts. The processes 
in these cells establish certain correlative factors which determine 
that the cells farther away from the cut shall remain as they are or 
take other forms of reaction. 
But if we decrease the rate of metabolism in Planaria by extreme 
starvation or by the use of anesthetics, then parts which under the 
posterior or proximal to it and controls their development directly or indirectly. 
Briefly stated, the regulatory formation of a dominant part is a reconstitution re- 
sulting primarily from isolation, while for the formation of a subordinate part 
correlation with other parts of the original system is necessary. For example, a 
piece of the tubularian stem may reconstitute itself into a hydranth without any 
other parts (Child, ’07a, b, ¢), or a pieceof Planaria into a head without other parts 
but a tubularian stem or stolon or a planarian tail is never formed except in con- 
nection with a more distal or anteriorregionof the original organism. The ques- 
tion of the dominance and subordination of parts and its significance will be dis- 
cussed more fully elsewhere. 
