QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 255 



locomotion, extends it toward the source of light. Then it is at once 

 turned from the light to such an extent that it frequently makes a 

 right angle with the posterior end. Later it is swung back but only 



part way But why does the organism turn toward the 



light, if the lateral illumination is very intense? Whenever the larva 

 is stimulated, it turns the anterio • end in a direction opposite to that 

 in which this end is when it receives the stimulus. The tip of ante- 

 rior end is relatively very sensitive; in diffuse light the larvae are 

 stimulated only when this end is extended and fully exposed, but in 

 very intense light, owing to the translucency of the surrounding tissue, 

 it is stimulated no matter in what position the anterior end is; conse- 

 quently if this end is turned from the source of light when the organism 

 is exposed it is at once turned sharply in the opposite direction, i.e., 

 toward the light. 



This explanation of Mast's not only clears up an apparent 

 inconsistency in experimental results, but also establishes a 

 point the significance of which might be further emphasized. 

 When the anterior end of the blowfly larva is stimulated by being 

 turned toward the light, a contraction takes place on a physio- 

 logically definite side — the side on which the muscles are passively 

 stretched." This is very suggestive of the motor reflex of a 

 form like Stentor, where stimulation of the light sensitive region 

 produces a "jerking toward a structurally definite side," the 

 aboral pole. In one case the organism in its locomotion is put 

 in touch with various regions of its environment by the swing- 

 ing from side to side of a bilaterally sensitive anterior region, 

 in the other case by a spiral turning which brings the single sen- 

 sitive region first to one side and then to the other. In the blow- 

 fly larva, when the 'head' is swung to the left, it is partially 

 shaded from a light coming from the right; when a Stentor ro- 

 tates so its oral pole lies toward the left it also is partially shaded 

 from the light coming from the right. The change of effective 

 intensity when the sensitive region is turned from the shaded 

 position toward the light produces a motor reflex which involves 

 a change in the direction of locomotion. The reflex is repeated 

 until a direction of locomotion is attained in which the changes 



^ The tendency of a passively stretched muscle group to contract more readily 

 than muscles already partially contracted is familiar in many reflexes, not only 

 in cases where a nerve net is involved, but often where there are definite afferent 

 and efferent nerves (see Uexklill, '09, chapter, Die Seeigel; Howell, '12, p. 21, etc.). 



