14 SAKYO KANDA. 



476). Lyon's calculation was practically the same as Jennings, 

 /'. e., 1/1,040,000 (16, p. xv). Since it is well known that the 

 "threshold differential required for the perception of differences 

 in pressure" in man is "about i/io," both investigators cannot 

 believe that a differential of 1/2,000,000 or 1/1,040,000 is per- 

 ceptible by paramecia. With regard to the difference in pressure 

 between the anterior and the posterior ends, Jennings thinks 

 that the posterior end being only slightly sensitive to a touch to 

 which the "anterior end .reacts violently," "an increase of pres- 

 sure on the posterior end" "would cause no reaction whatever" 

 (17, p. 4/6). Neither Jennings nor Lyon calculated the differ- 

 ence in pressure between the anterior and posterior ends of the 

 organism, but this is easily made as follows: The atmospheric 

 pressure being 10,000 mm. of water, and the average length of 

 the paramecia which the writer experimented on about 0.2 mm., 

 the writer calculates the difference in pressure between the 

 anterior and posterior ends of the organisms to be about 1/50,000 

 of one atmosphere. If "the reaction still occurs when the atmo- 

 spheric pressure is more than doubled," as Jensen showed, "the 

 effective difference in pressure would then be" about 1/100,000 

 of one atmosphere. It is hardly conceivable that the difference 

 of either 1/50,000 or 1/100,000 of one atmosphere is perceptible 

 to paramecia. Moreover, Lyon's experiments have shown that 

 "geotropism is intensified in a centrifuge tube which is open 

 away from the axis and in which, therefore, no increase of 

 pressure above atmospheric is possible" (17, p. 431). On the 

 basis of such various evidence, we ought to abandon the pres- 

 sure theory of geotropism in Paramecium. 



IV. THE RESISTANCE THEORY. 



A third theory of geotropism, proposed by Davenport, is 

 based on the assumption that the animal "experiences greater 

 resistance (friction + weight) in going upwards even to the 

 slightest extent than in going downward (friction -- weight) " 

 (2, p. 122). Lyon calls this the "resistance or weight theory" 

 (17, p. 426). This theory of Davenport was once accepted by 

 Jennings (9, pp. 477-480), though now he rather favors Lyon's 

 theory (10, p. 77) which we shall consider later. 



