398 Marion L. Durbin 
at which the differentiation of the materials in the new tail takes 
place was also investigated. It was found that immediately fol- 
lowing the removal of a part of the tail the rate of increase in 
length is slow, but rises rapidly to a maximum which it reaches 
on the sixth to tenth day. From this point there is a rapid 
decrease for about three to five days and then a progressively 
slower decrease until the process is complete. “Phe “curve 
expressing these rate changes is shown in Figs. 1, 2, and 4. It 
agrees in general features with the curve of the ordinary growth 
of rabbits and guinea-pigs as it is described by Minot.’ “These 
animals, soon after birth, but not immediately, attain a maximum 
rate of growth from which the decrease is rapid at first and 
gradual later. In connection with the factors controlling the 
process of regenerations the changes in rate show that the full 
force of the tendency to regenerate is attained within a short 
period after the operation, but that the retarding factors appear 
early and increase in intensity rapidly at first and then more 
slowly until they have completely overcome the positive growth 
tendencies. 
MATERIAL 
Tadpoles of Rana clamitans (Latrielle) were used for this work 
because: (1) They are abundant and easily obtained in large 
numbers. (2) They are capable of accommodating themselves 
to laboratory conditions, a fact shown by the low per cent of mor- 
tality.in series maintained under observations for from one to 
two months. (3) They resist in a very satisfactory way the 
shock of an operation. (4) The tail offers a convenient appendage 
for operation and observations, as more than half its length can 
be removed without seriously injuring the animal; at the same 
time it has a high regenerative potential so long as the individual has 
not begun to grow its forelegs.* (5) It is always easy to distinguish 
the regenerated part of the tail from the old part, that is the line 
3 Minot, C. S. ’08: Age, Growth and Death. 
4Kammerer, Paul, ’o5: Ueber die Abhangigkeit des Regenerationsvermégens des Amphibienlar- 
ven von Alter, Entwicklungsstadium und spezifischen Grosse. Archiv fiir Entwicklungsmechanik, 
Bd. 19, Heft 2. 
