542 ROSS G. HARRISON 



The latter have been studied by Verworn^^ in Rhizopods and 

 by Piitter^" in Flagellates and Ciliates. These observers have 

 shown that different parts of the same organism respond differ- 

 ently to contact stimuli. In Protozoa, where the movement is 

 sufficiently rapid to be directly observed, the location within 

 the cell of both the sensitive region and the responding mechanism 

 can often be made out, whereas it has as yet been found im- 

 possible to do so in the case of the slowly moving tissue cells. 

 This is a difficulty that applies ahke to the study of the reactions 

 to all kinds of stimuli and renders uncertain for the present 

 any attempt at exact description of the process. At the same 

 time the case of Chilomonas, for instance, in which according 

 to Putter stimulation of one of the flagella by sohds calls forth a 

 positive response, while similar stimulation of the other a negative 

 one, shows that we have in this kind of sensitiveness an important 

 directive factor. By analogy in the case of tissue cells, if dif- 

 ferent parts of the same cell are unequally sensitive, it is likely 

 that contact with an even surface would induce movement in 

 a definite direction. Barring some such regional difference in 

 the irritability of the cell, the kind of reaction called forth by con- 

 tact with a solid object could only be a clinging to that object or a 

 recoil from it. The former or positive reaction is that shown by 

 the embryonic cells here studied. Without any further stimulus 

 it is conceivable that the reaction to such contact might result 

 in the change of shape of the cell, either its flattening upon the 

 surface of a glass cover-slip or its elongation upon the fibers of 

 the spider web (fig. 12). Its locomotion, however, must depend 

 either upon some other stmiulus or upon some local internal 

 differences in the cell. 



As to the propriety of calling the reaction to solids a tropism, 

 the position here taken differs from that of Loeb who holds 

 that it is no real tropism., in as much as lines of force do not exist. ^^ 

 It is true that in the case of tactile stimuli the source of the 



^^ AUegmeine Physiologie, 5th Edition, p. 519; Bewegung der lebendigen 

 Substanz, Jena, 1892. 



^° Arch, f . Anat. u. Physiol, physiol. Abt. Supplementband, 1900. 

 5' Dynamics of living matter. New York, p. 156. 



