ZOOLOGY: C. ZELENY 
489 
tively 0.22, 0.25 and 0.26 mm. The corresponding values at four days 
are 0.66, 0.75 and 1.00; at six days 1.36, 1.40 and 1.36; at eight days 
2.18, 2.68 and 2.68; at ten days 3.55, 3.82 and 4.20; at fourteen days 
5.34, 6.12 and 6.08. The advantage is in favor of the second and third 
regenerations as opposed to the first and of the third as opposed to the 
second. Individual comparisons at each of the different times show 
similar results. 
The evidence as a whole shows clearly that when the age factor is 
eliminated there is no decrease in rate of regeneration with successive 
removal. On the contrary the second regeneration is more rapid than 
the first up to the period of maximum rate. The second regeneration 
however passes its maximum sooner than does the first and after the 
tenth day the latter therefore catches up in total amount regenerated. 
There is no striking difference between the second and the third regen- 
erations but in each comparison the third has a slight advantage. 
When successive regenerations within single individuals are compared 
there is a progressive decrease in rate and it is obvious that this is due 
to age. 
The possibility has suggested itself that the second regeneration starts 
out at a more rapid rate than the first because the cells at the cut sur- 
face were undergoing regenerative changes at the time of the new 
operation and can therefore start the process much faster than can the 
old cells at the first surface of regeneration. Following a first removal 
there is a considerable degree of reorganization of the cells at the 
cut surface, accompanied by active migration. During this period which 
in Rana clamitans lasts two or three days there is little or no mitotic 
cell division. Then follows a division period which reaches its maxi- 
mum at seven to ten days. Its decline is associated with the oncoming 
of tissue differentiation. 
A special study has been made of the relative rates of second regenera- 
tion from old cells following a cut inside of the first removal level and 
from new cells following a cut outside of the first level. This compari- 
son shows only a very slight difference in favor of the new cells and this 
is largely confined to the early stages, the period of cell migration. 
The period of increase in rate is the period of active cell multiplica- 
tion and the decline in rate is associated with cell differentiation. The 
second regeneration therefore reaches the period of differentiation 
slightly in advance of the first regeneration. 
Apart from the slowing due to age there is no indication of a limita- 
tion of the amount of new material that may be produced by regenera- 
tion. The actual limitation comes not from the using up of regenera- 
