(>4() Mast, Orientation in Euglena with some Remarks on Tropisms. 



In swimming Euglena deflects continuously toward the sur- 

 face near which the eye-spot is located, but at the same time it 

 rotates on the longitudinal axis. This results in a spiral course the 

 axis of which is fairly straight regardless of the degree of deflec- 

 tion. A slight deflection merely results in a narrow spiral course 

 while a greater deflection results in a wider course. The general 

 direction of the course is usually changed by an increase in deflec- 

 tion on one side of the spiral and a decrease on the opposite side. 

 Only slight changes can occur by changes in deflection on but one 

 side of the spiral. It is manifestly impossible to obtain a 

 change in the general direction of motion without either 

 a decrease or an increase in deflection. This is an impor- 

 tant point of which we shall make use later. 



If, when a given Euglena is proceeding toward a source of 

 light, the direction of its rays, without any alteration in the inten- 

 sity, is suddenly so changed that it becomes perpendicular to the 

 axis of the spiral, the deflection is usually increased on the side 

 of the spiral facing the light and decreased on the opposite side. 

 Ordinarily the increase in deflection is greater than the decrease 

 and the spiral becomes momentarily somewhat wider. Whether 

 or not the decrease in deflection is ever precisely the same as the 

 increase so that there is no change in the diameter of the spiral, 

 as Bancroft maintains, I am unable to say. However that may 

 be, this change in the deflection results in a gradual turning in 

 the axis of the spiral until it is directed toward the source of 

 light and the Euglena is oriented. 



One further point should be noted here. An unoriented Euglena 

 as it rotates on its long axis is successively illuminated from all 

 sides. In one position in the spiral the surface containing the eye- 

 spot faces the light, in another position this surface is shaded and 

 the opposite surface becomes illuminated. Since this creature, 

 though relatively translucent, contains structures (the eye spot in 

 particular) which are relatively opaque, it is evident that shadows 

 are produced which, as it rotates, travel over different parts of 

 the body, and this results in changes in the intensity of light in 

 all parts. The amount of these changes in illumination decreases 

 as orientation proceeds, and disappears entirely when the axis of 

 the spiral comes to be directed toward the light no matter what 

 its diameter may be. 



Concerning the process of orientation as described, there is, 

 as previously stated, no contention. The point at issue concerns 

 the cause of the changes in deflection resulting in orientation. 

 Both schools hold that owing to the shadows mentioned above, 

 the sensitive tissue in Euglena receives different quantities of light 



