Physiology of Regeneration 601 
The latter basis is undoubtedly the correct one, since it is obvious 
that when a small piece regenerates a relatively larger tail than 
a larger piece does, the cell-activities underlying all the vital 
phenomena of the organism must be greater in the former case 
than in the latter. It is this vital activity, the metabolic processes 
of the individual cells, which, on this view, must determine the 
rate of regeneration. ‘This physiological explanation is rendered 
probable by the very existence of such a thing as different rates 
of regeneration, and though largely speculative as yet, it opens 
up a promising field for further experimental work. 
III PHASES OF POSTERIOR REGENERATION 
From experiments performed during the summer of 1907, I 
arrived at the following conclusions: first, that there is invariably 
a lapse of some time before the new tail begins to regenerate, 
this period being followed by a proliferation of new, undifferen- 
tiated tissue; secondly, that new segments with definite structural 
characters are formed out of this undifferentiated material, and 
that this formation of new segments takes, place at the highest 
rate somewhere between the fourth and the twelfth day after the 
operation; thirdly, that a sudden rise in the rate of regeneration 
alternates with a sudden diminution in the rate of production of 
new segments, the increments becoming continually smaller and 
smaller till finally no more new segments are formed; and lastly, 
that, although fewer new segments are being added, the segments 
already formed are increasing in size. These inferences were 
drawn from a frequent (usually daily) examination, which ex- 
teaded over nearly seven weeks, and was made on worms cut 
in two near the middle of the body. 
This experiment gave the starting point for some fits ther experi- 
ments which I undertook in the summer of 1908. 
The principal problems to be solved were these: First, At what 
stage of the process of regeneration at different levels does the 
difference in the rate of regeneration first appear, and how is this 
difference maintained throughout the later stages? Secondly, 
