214 C. M. CHILD 
usual conditions are formed chiefly by regeneration, e. g., the head, 
may be formed largely by redifferentiation (Child, 10a). Tais 
means simply that the cells near the cut do not react so rapidly 
s under the usual conditions, so that other cells further away have 
ime to change their reactions and take part in the process, while 
ordinarily they would be prevented from doing this by the cor- 
relative factors arising from the activity in the cells near the cut. 
These instances are merely special cases under the general rule 
that the less rapidly the missing part is replaced, the more ex- 
tensive are the changes in the remaining parts, so far as their con- 
stitution permits change. 
Regeneration and redifferentiation in their extreme forms repre- 
sent the extreme terms of a graded series, of which all terms are 
essentially of the same physiological character, 2. e., all consist 
in a change in reaction in consequence of a change in physiological 
correlation. The designations ‘regeneration’ and ‘redifferentia- 
tion’ serve merely as convenient descriptions of the visible phe- 
nomena. 
Where all the cells of the remaining parts are so sensitive to the 
absence of the correlative factors originating in the part removed 
that they cannot maintain themselves after its removal, the old 
structure of all the remaining parts may disappear to a greater or 
less extent, 7. e., a ‘dedifferentiation’ occurs, as, for example in the 
isolated pieces of the branchial region of Clavellina. In the differ- 
ent cells of the mass the metabolic processes become less specified 
and in this respect it approaches the ‘embryonic’ condition, and 
the correlative factors in the mass approach those existing in the 
embryo. But during this process some of the cells have been sub- 
jected to correlative factors more or less similar to those to which 
the part removed was subjected at some stage of development, 
consequently these become in some degree the physiological repre- 
sentatives of that part. In short the system becomes physiologi- 
cally a whole, but in consequence of the rapid dediffereatiation 
of the old parts it corresponds to a whole in a relatively early 
stage of development. From this condition renewed differentia- 
tion as a whole results necessarily from continued metabolism, 
