446 Max Mapes Ellis 
the state of development of the tadpole. Taking the term “state 
of development” used by Kammerer, to mean the relative em- 
bryonic age of the tadpole, the larger the tadpole, that is, the 
greater its bodylength, the less embryonic the condition of the tail. 
Applying this to regeneration, take for example, the tail of a 
tadpole that has had approximately ten millimeters removed from 
it. By the very presence of injury, which has removed part of 
the tail, the cells on the exposed surface are given a chance to 
divide; as the pressure and tension at that point have been reduced. 
If the conditions of temperature, food, etc., are optimum, and the 
injury itself has not overwhelmed the animal (that is, such injuries 
as are fatal) these cells exposed will divide because of a lack of 
equilibrium of forces. They will also continue to divide until the 
extra-cellular forces are equal to the force producing cell-division. 
When this point has been reached, they will cease dividing, and 
differentiate. The data collected by Durbin on this point show 
two distinct regions in the period of regeneration as are to be 
expected from the above statements. (1) A region of rapid cell- 
division and no differentiation, which extends over the first part 
of the regeneration period; (2) a region of differentiation with 
little cell-division immediately following the first. “The question 
comes at once, as to the per centum regenerated before cell-divi- 
sion ceases. ‘This is dependent upon the state of development of 
the animal; for the amount of force producing cell-division varies 
with the relative embryonic age of the cells. This theory of 
regeneration is confirmed by the second fact mentioned at the first 
of this discussion, namely, that the final amount regenerated in 
series under the same conditions, by tadpoles of the same size, 
was always the same per cent of the amount removed, regard- 
less of the level of injury. Consider, for example, two series of 
tadpoles of the same size and under the same conditions, one with 
10. mm. removed, and the other with 5 mm. removed. ‘The cells 
exposed have the same division potential, consequently, the same 
ability to overcome the extra-cellular forces inhibiting cell-division; 
and because of this fact, they will continue to divide until the same 
force is exerted against them. It is evident that the regeneration 
of two and one-half millimeters by both series would not produce 
