400 Marton L. Durbin 
upon which the operation was performed is always distinct. ‘The 
difference between the regenerated and old parts of the tail lies 
in the facts that the regenerated part is unpigmented; that it is 
thinner in proportion than the old part, and that after operation 
the large blood-vessels near the cut turns blackish while those in 
the new tissue are colorless or reddish with their blood contents. 
The diameter of the notochord in the regenerated part is always 
distinctly less than in the old part, but as the old notochord 1s 
often pushed out a short distance (0.5— Imm.) into the new tail 
the constriction on the notochord is not an infallible point from 
9 o °o 
Ne w h $ 
Rate per day in millimeters 
y 
: is 10 4 18 
Days since operation a te 
Fig. 2 Curve of rates of regeneration for Experiment II. For explanation, see Fig. 1. 
which to measure the length of the regeneratedtail. “The measure- 
ments of length of the regenerated part of the tail were made 
along the notochord, from a point on the line of operation over 
the notochord to the tip of tail. 
METHOD 
The technique employed has been very similar in the four 
experiments performed. ‘Tadpoles collected from a pond were 
put into individual glass berry dishes containing about 150 cc. 
of tap water and some Spirogyra. ‘There were two series of ani- 
mals used in each experiment, except III, an uninjured or con- 
trol series and an operated series. All of the animals used in an 
experiment were as much alike as possible. The two series were 
made to correspond, individual for individual, in regard to com- 
