PHOTOKINETIC REACTIONS OF FROG ^TADPOLES 367 



After a few seconds of exposure even to weak light they would 

 orient themselves positively and remain in that position for 

 long periods. When a beam of high intensity was used, vigor- 

 ous swimming brought the animal towards the source of light, 

 the head being pressed against the side of the jar. 



During the first half of the trials ^\dth animals 5 and 8 they 

 seemed photokinetic but later phototropic, their reaction time 

 for the former kind of response being much longer than that 

 for animals 13 or 14. None of the trials with animals 5, 8, 13 

 and 14 were included in the summary of the photokinetic 

 animals, because their positive orientation was very evident. 



DISCUSSION 



From these observations it seems clear that frog tadpoles 

 with a length of at least 40 mm. or more, are sensitive to Ught, 

 showing a photokinetic response at all stages, and a positively 

 phototropic one towards the end of the larval stages. This 

 suggests the idea that the nervous mechanism controlhng the 

 characteristic phototropic reaction of the adult frog may not be 

 fully developed until metamorphosis is completed. In the 

 mid-larval stages, the animals show a photokinetic reaction, or 

 in other words, the rate of smmming increases with an increase 

 of light intensity, while in the later larval stages the reaction 

 becomes a positively phototropic one, which is the only one ob- 

 served in the adult. This condition gives evidence contrary to 

 the statement of Pearse ('10), that 'Hhe photokinetic quality is 

 apparently little developed in frogs and toads, though they are 

 strongly phototropic. Generally speaking there seems to be 

 no correlation between the photokinesis and the phototropism of 

 Amphibians." Our evidence indicates that there is a relation 

 between the two kinds of reactions the phototropic one being a 

 further development of the photokinetic one. 



Laurens ('15) found a similar condition in the response of the 

 melanophores of Amblystoma larvae. These do not react to 

 photic stimulation until the animals are at least 16 mm. long. 

 Babak ('10) noticed that Axolotl larvae, 17 mm. long and less, 

 do not respond to light as the older ones do, and thought that 



