HISTORICAL REVIEW 37 



of the extreme positions into the middle position, the phase 

 of expansion. The fiagellum works, therefore, Hke an oar 

 that is moved alternately to the right and to the left at 

 the bow of a boat. It is evident that, while undisturbed and 

 having equal conditions upon all sides, the infusorian body 

 must move forward in a straight line, if the fiagellum beats 

 equally strongly toward the right and toward the left, i.e., 

 if contraction and expansion occur with equal rapidity 

 toward the two sides. But if a contractile stimulus acts 

 upon the flagellate suddenly from one side, and if the long 

 axis of the body is not already turned in the direction of 

 the stimulus with the posterior pole toward its source, such 

 a position is assumed by means of a few strokes of the 

 fiagellum; for with every oblique or transverse position of 

 the long axis the fiagellum is stimulated to contract more 

 strongly upon the side upon which the stimulus falls than 

 upon the opposite side, it makes stronger strokes toward 

 the former than toward the latter side, and the result is 

 that the anterior part of the body is turned away from the 

 source of the stimulus. Exactly the same relations exist 

 here as in a boat moved by a single oar. The bow of the 

 boat also turns toward the opposite side when the boat is 

 propelled more strongly upon one side than the other. 

 The unequal strength of the flagellar stroke in the two 

 directions continues, and the anterior part of the body is 

 turned constantly more away from the source of the 

 stimulus, until the body has placed its long axis in the 

 direction of the incident stimulus. Then both sides of 

 the fiagellum become equally stimulated and the protist 

 swims in a straight line, so long as the stimulus continues. 

 Thus, negative chemotaxis, phototaxis, etc., appear in 

 uniflagellated Bacteria and Flagellata as a necessary result 

 of a unilateral excitation of contraction in the flagellum." 



Orientation in forms possessing two fiagella and in forms 

 possessing numerous cilia is similarly explained. When an 

 organism of this sort is not oriented it is assumed that the 

 flagella or the cilia are more strongly stimulated on one side 



