368 WILLIAM. H. COLE AND CARLETON F. DEAN 



the explanation for this lay in the incomplete development ol 

 the retina. Laurens ('16) stated that ''the expansion of the 

 melanophores is due primarily to the direct stimulation of the 

 pigment cells themselves, and not to the inhibitory action of the 

 nervous system." In the case of the frog tadpole, which does 

 not respond at all to photic stimulation in the very young stages, 

 (Laurens, '14), but later shows a photokinetic reaction and 

 finally a phototropic one, there must be some relation between 

 these reactions and the development of the nervous system. 

 The only alternative is that the melanophores in the skin of the 

 tadpole are in control of this reaction and until they are far 

 enough developed along the line of direct irritability, their re- 

 actions to light do not appear. Laurens has not reported any- 

 thing regarding the melanophores of the young frog tadpoles 

 and it is not known whether or not they pass through the same 

 stages as those of Amblystoma larvae. If they do, we would 

 expect a response to photic stimulation as soon as the melano- 

 phores become sensitive to direct stimulation. Since these 

 young stages do not respond, we must conclude either, that the 

 melanophores are not yet fully developed, or that a nervous 

 mechanism controls the response, but it is not yet fully de- 

 veloped. We are inclined to believe that the latter condition 

 exists in the frog tadpole. 



Similar to the conditions in other Amphibians (Parker, '03 

 and '05; Reese, '06; Pearse, '10; Laurens, '14, and others), we 

 found that the skin of the tadpole is sensitive to light, being the 

 important receptor in the photokinetic reaction. The tail is the 

 most sensitive region, shown both by the reaction time being 

 shorter than for any other region, and by a more vigorous 

 response. 



The reactions of those animals subjected to strong light or 

 total darkness for a long time previous to experimentation 

 showed that such adaptation has very little effect on the re- 

 sponse to photic stimulation. We had expected that dark 

 adapted animals would show a much greater per cent of photo- 

 kinesis, but such was not the case. The melanophores of these 

 animals were fully expanded, and since this condition did not 



