198 HENRY LAURENS 



the reason that when several are kept together in a single dish 

 they will nip each other's gills, legs and tails, rendering them- 

 selves unfit for experimentation. 



The larvae were tested in groups of ten, and each individual 

 in a set of tests was given in order a single trial, until each one 

 of the ten had been tested once. This was then repeated until 

 each had had ten trials. A single set of tests, therefore, con- 

 sisted of 100 trials. The larvae were always dark-adapted, being 

 placed in the dark-room during the afternoon of the day before 

 the experiments were made, and therefore subjected to a con- 

 dition of darkness for at least fifteen hours before being tested. 



Some little difficulty was experienced at first in placing and 

 orienting the larvae in the dish in which they were tested. But 

 after a little practice, it became comparatively easy by means of 

 a large pipette to place a larva in the middle of the dish and at 

 right angles to the impinging rays of light. During the ten reac- 

 tions that each individual was given in a set of tests, care was 

 taken that both sides of the larvae should be exposed to the light, 

 but no attempt was made to expose the opposite sides in con- 

 secutive trials. A larva after having been oriented properly 

 was allowed five minutes in which to react. If at the end of that 

 time it showed no response, it was replaced by another. 



It was soon found that the frog tadpoles, both normal and 

 blinded, do not respond to stimulation by light. They were 

 tested both in daylight and in artificial light, and in neither do 

 they show any reaction. As Franz holds ('10 and '13) frog tad- 

 poles are non-phototactic. Franz's experiments of placing a few 

 tadpoles in a small dish and floating this on the water in a larger 

 dish containing tadpoles which show no orientation to light was 

 repeated. But in no case did these tadpoles show any orienta- 

 tion to the light, but continued actively swimming about in all 

 directions, colliding with the sides of the dish and with one an- 

 other. A single individual placed in the small dish behaved in 

 the same way. 



The Amblystoma larvae behave very differently. They are 

 sensitive to light and respond to its stimulus by very definite 

 movements. As has been mentioned, the larvae were given five 



