186 S. O. MAST 



rate of change in illumination. They are in this respect like 

 the responses in Euglena known as ' shock reactions ' — ' Schreck- 

 bewegung. ' 



9. All of the photic reactions are probably due to changes in 

 the illumination on the nerve-endings at the inner surface of 

 the pigment cup of the eye. Movements of the light or of the 

 tadpole that produce changes in illumination of the optic nerve- 

 endings owing to the shadow cast by the pigment cup, are followed 

 by the same reactions as like changes in the illumination of the 

 entire field. 



10. Orientation is the result of one or more shock reactions 

 caused by the alternate shading and illumination of the optic 

 nerve-endings, owing to rotation on the longitudinal axis. In 

 negative specimens the former causes the tail to bend toward the 

 ocular, the latter toward the abocular side; in positive speci- 

 mens precisely the opposite. The one set of responses directs 

 the organism from, the other towards the source of light. After 

 the tadpoles are oriented, the retina is continuously approximately 

 equally illuminated, the shock reactions cease, and they continue 

 on the course established. They remain oriented not because 

 they are held on their course by continuous stimulation, as is so 

 frequently maintained, but because they tend to continue on a 

 direct course when they are not stimulated. 



11. Orientation in this organism is in no way dependent upon 

 a balanced effect of stimuli acting continuously on symmetri- 

 cally located photoreceptors in accord with the DeCandolle- 

 Verworn theory of orientation accepted by Loeb and others. 



