278 Experimental Zoology 
difference of rate is found even in a greater degree, and the two 
surfaces are practically of the same size. It also seems probable 
that the difference in rate is not due to any difference in the initial 
stimulus of the operation, for the new growth at the two levels 
is more nearly the same at first than it is later. So much for 
what the difference is not; and in this list we may seem to have 
exhausted nearly all of the physiological possibilities in the ordi- 
nary sense of that term. The results seem comparable in many 
ways with the results of normal growth. A young animal grows 
rapidly at first, later more slowly as the adult form is approached. 
The same thing happens with the regenerated part. In both 
cases some inhibition takes place when a certain form or size 
is attained. Before discussing this point further let us consider 
more in detail some further facts about the earthworm which 
give a better basis for the discussion that will follow. 
When the posterior end is cut off near the tip of the tail, re- 
generation of a new tip goes on with great slowness. When the 
worm is cut in two near the middle of the body, the regeneration 
of the posterior end takes place much more rapidly than in the 
last case. If even more of the posterior part is removed, the 
missing part is regenerated somewhat faster than when cut in the 
middle. On the other hand, when the anterior end of a worm is 
cut off, the results appear to be different, but in reality the differ- 
ence is more apparent than real. When one, two, three, four, 
or five anterior segments are cut off, the same number that was 
removed comes back, as a rule; but when more than five are cut 
off, only five at most come back. In these cases all the new seg- 
ments are laid down at once, and no more are formed later. In| 
the tail region the new terminal part is also laid down for all 
levels at about the same time, but a growing region is formed 
near the end, and from this the new segments are added. It is 
in this terminal growth, characteristic of the posterior end alone, 
that the difference in the rate of growth is found. 
It can be shown, I think, with some probability, that we have to 
deal in these cases with more than a single factor. In the first 
place, the kind of differentiation at each level may determine not 
