Jan., 1913.] Pigment Development in Spelerpes Larvae. 
5i 
with dense black pigment; and (3), the dense dull-black color of 
the larvae compared with which the check often appears very light. 
There is no mistaking the “tvrosin type ”, for an inexperienced 
person will always pick them out as the darkest individuals 
in a series. 
Of the 21 experiments which showed an effect, 15 had a tvrosin 
concentration of 0.010%, 1 of 0.0125%, 2 of 0.020%, 1 of 0.040%, 
and 1 of 0.006% (this last being the only one of the entire 41 
experiments which showed an effect at this concentration, and 
which, as noted above, “reverted” after the 28th day). 
The time of treatment averages about 60 days, and in three 
experiments (Nos. 560, 595, 609) which are still running (Dec. 6) 
the larvae were in tvrosin for 72 days and have since been in pure 
water only (no tyrosin) for 123 days. They are still appreciably 
darker than the corresponding checks, and show enough of the 
characteristics of the “tyrosin type” to be readily classified as 
such. During the later period the larvae have at least doubled 
their previous length, but it is impossible to say whether their 
continued darker color is due to a continued more active pigment 
formation or merely to a distribution over a larger area of the 
dense black mass of pigment already present f. 
Experiments with Orcinol. 
Orcinol, as noted above, inhibits the action of tyrosinase upon 
tyrosin in the test tube, and we hoped to be able to inhibit, or at 
least to modify, the course of pigment development by rearing the 
larvae in solutions of orcinol. We found the drug to be quite 
toxic, not so much so of itself as the oxidation products which are 
formed by the action of light upon a solution'of orcinol. However, 
by changing the solutions every day, or every second day, and 
keeping the dishes, together with the controls, in a dimly lighted 
room, we were able, in part, to prevent the toxic action. In this 
manner we have been able to keep larvae in a solution of 0.020% 
concentration for 50 days. 
Altogether 35 experiments were run, including 513 individuals 
(not including checks). Later it seemed advisable to subdivide 
some of the experiments so as to accurately test the effect of 
varying length of immersion in the drug solution. A total of 115 
such removals were made, each one in reality being a separate 
experiment in itself, thus making a grand total of 150 experiments. 
Concentrations of orcinol ranging from 0.0125% to 0.025% were 
employed. 
t As the larvae become older the characteristic spots of the checks 
become less conspicuous and are later lost so that the types become less 
differentiated, and the depth of color is about the only criterion available 
at this stage of development. 
