Physiology of Regeneration 609 
fact they can easily be overlooked on superficial inspection. But 
the same regenerating tails at the end of 35 to 45 days will have 
already attained a large size, and worms from which but few 
segments were removed can hardly be distinguished from normal 
ones. . During those 20 to 30 days very few segments were added, 
but what really did take place was a considerable increase in size 
of each of the regenerated segments. 
In 1908 I kept a record of measurements of the regenerating 
tails made at regular intervals. The tails were always measured 
in an extended condition. ‘The data of the average length of the 
tails at various periods of their regeneration are presented in the 
right-hand columns of Tables III and IV.? Glancing over these 
data it can be seen at once that for a time they form a progressive 
series, 1.e.,the increments for successive, equal periods are grow- 
ing larger. In diagram, Figs. 3 and 4, these data are represented 
graphically by the horizontally striated columns. The horizontal 
units indicate the number of days (four days to each column or 
period), while the height of the columns represent the increment 
in mm. for the given periods. One-hundredth of a millimeter of 
the actual length of the regenerating tail is represented by 0.1 
of an inch in the diagram. ‘The horizontally striated columns are 
superimposed upon the obliquely striated ones in such a Way 
that the average increment in the number of segments (obliquely 
striated columns) and the average increment in the length of 
regenerating tails (horizontally striated columns) for the same 
periods of time coincide with one another. It will be readily seen 
that while the former present a retrogressive series, the latter 
present (at least in some portions of the total period) a progressive 
series. Thus it appears as if we were dealing here with two 
phenomena, antagonistic to each other in the sense that while one 
is going on with great and sometimes, increasing intensity, the 
other is gradually diminishing. These two phenomena are: for- 
mation of segments from the proliferated tissue, and growth or 
increase in size of those segments. 
2 The average lengths of tails on the sixth day after the operation (August 16) are not given in these 
tables. They were 0.30 mm. for tails regenerating from anterior, and 0.22 mm. for those regenerat- 
ing from the more posterior level. 
