Amount of Tail Regenerated to Removed in Tadpoles 42= 
of Experiments 2 and 3 were almost submerged in water of a 
known temperature which transmitted a very constant amount of 
heat to the water within the jars. The dishes in which Experi- 
ments 6 and Io were conducted, were placed in a chicken incuba- 
tor; and those of Experiments 5, 7, and 8, in a chicken brooder. 
Both incubator and brooder gave very constant temperatures by 
means of their automatic temperature regulators. 
4. Light Light was taken as a constant in these experiments as 
both the brooder and incubator had glass doors which allowed the 
animals within them the same light as those kept upon tables in the 
room. No animal received direct sunlight. 
5 Age There was no definite way of obtaining the absolute 
age of any of the tadpoles except those used in Experiment 10, 
which were hatched from the eggs in the laboratory. Because of 
this fact, body length and tail length were used as units in the 
determination of the relative ages of the various individuals. 
It was assumed that tadpoles of the same species, collected from the 
same part of a given pond, and of the same body measurements 
were of about the.same age. There must, of course, be some 
exceptions to this calculation of age; but the uniformity of results 
and data show that there was probably little error from this 
source. 
6 Individual Variation The effect of individual variation 
was eliminated as far as possible by the use of large numbers of 
tadpoles. The deviation of any one animal from the mass is 
easily noted from the individual data. 
NATURE OF OPERATION AND MEASUREMENTS 
The method of removing a part of the tail was the same for all 
animals used. A tadpole to be operated upon was placed on a 
smooth block of parrafhine with its tail evenly spread and the body 
covered with a piece of wet cloth. The amount to be removed 
was then measured with dividers and the cut made at right angles 
to the notochord by a single stroke of a razor. Only those animals 
with a clean even cut were used. In determining the amount to be 
removed the measurements were always made from the tip of the 
