REACTIONS OF AMPHIBIAN LARVAE TO LIGHT 207 



this stage, then, all the larvae have already acquired the sensi- 

 tiveness to light. It seemed worth while to attempt to find out 

 at about what stage in the development of the larvae this sensi- 

 tiveness first made its appearance. Accordingly ten young nor- 

 mal and ten young blinded larvae were selected, and their re- 

 sponses to light tested. They were placed at a distance of 25 

 cm. from the lamp where the light had an intensity of about 768 

 candle-meters. A series of readings of a thermometer placed in 

 the water in the testing dish showed that the increase in the tem- 

 perature of the water after the lamp had been burning for a pe- 

 riod of six hours, was less than a quarter of a degree centigrade. 

 The temperature of the room, it may be mentioned, usually rose 

 about 1.5°C. during such a period. 



These larvae when first selected were between 7 and 8 mm. 

 long, responding to mechanical stimulation by rapid swimming 

 movements. At this stage they were, however, insensitive to 

 light, remaining absolutely motionless, at right angles to the 

 rays of light. It was not until the larvae had reached a length 

 of about 11 to 12 mm. that they showed any response. 



At this stage the gills are well-developed and tufted, the bal- 

 ancers are fully developed, and the fore-limb buds are between 

 0.5 and 1 millimeter long. The larvae at this stage still lie on 

 their sides for the most part. The ability to respond to the 

 stimulus of light appeared rather suddenly in all. Six of the 

 normal and five of the blinded larvae gave responses one morning, 

 the rest were indifferent. In the afternoon of the same day they 

 all responded to the light, giving a reaction within five minutes. 

 The afternoon before they had all been insensitive to light. The 

 average reaction time was longer than in the case of older larvae, 

 being here about two hundred and twenty seconds. The larvae 

 at this early stage all responded positively, and by means of very 

 quick swimming movements. Sometimes a very quick move- 

 ment which placed the long axis of the body in the direction of 

 the rays was the total response. But usually they swam vigor- 

 ously in the direction of the light stopping only when they came 

 in contact with the side of the glass dish. Again, in a few cases 

 some of them would swim vigorously for 6 or 7 cm. and then drop 



