252 BRADLEY M. PATTEN 



of an intensity which does not produce a reaction, it persists 

 until fortuitous movement takes the stimulated organisms 

 thither with resulting aggregation in the socalled ''region of 

 optimum intensity." 



It may be assumed that in this phenomenon the animal is so 

 attuned to light that at some degree of illumination internal and 

 external conditions are in equilibrium and 'stimulation' ceases. 

 The intensity at which a particular form is unstimulated appar- 

 ently depends on some internal factor bound up with the ultimate 

 make-up of the organism. It may not be any more uniform than 

 the amount of potassium or any other chemical present in the 

 protoplasm of various animals. 



In many organisms, the kinetic response to light is the only 

 one present. The directed reaction, however, is never present 

 without the kinetic response. It is not a primary phenomenon; 

 it is the kinetic response with something added to it that comes 

 from the association of the response with a specific organic 

 structure, a structure that may localize the access of light and the 

 material most sensitive to it, or that may localize the reaction, 

 As a result, movements are restricted to locomotion in a more or 

 less definite direction. It is not, therefore, in the fundamental 

 reaction to light that the difference between photokinesis and 

 phototaxis lies, but in the presence in the latter of factors which 

 localize the action of the light or which limit the direction of the 

 response. 



b. Orientation in the blowfly larva. The orientation of the 

 blowfly larva was taken up in the hope that a careful analysis 

 of its typical directed response would yield some evidence as 

 to the factors involved in phototaxis. Mast ('11) has devoted 

 considerable space to the consideration of orientation in this 

 form. His experiments are carefully carried out and accurately 

 recorded. I have made free use of his results in this discussion. 



The process of orientation under a single horizontal beam of 

 light is the simplest aspect of the problem. But before this can 

 be approached, the details of normal locomotion must be con- 

 sidered (Mast '11, p. 176): 



