GENERAL PHYSIOLOGY 173 



that the extreme tip of the advancing pseiidopodiiim is the most 

 actively metabolic part. 



On the basis of some such physical interpretation of amoeboid 

 movement, the problem of harmonizing pseiidopodium formation 

 with the activities of flagella, cilia and ciliary aggregates, does not 

 appear as hopeless as it does upon the surface tension hypothesis. 

 Elasticity of Amoeba ectoplasm and of endoplasmic solidification 

 (stereome) in Foraminifera, elasticity and contractility of axial 

 filaments in Heliozoa or in axial fibrils (kinetic) of flagellates and 

 ciliates, may ultimately be harmonized on the basis of some physical 

 explanation of this nature. 



Whether repeated shocks leading to changes in the nature of 

 protoplasmic response or to changes in direction of movement should 

 be interpreted on the basis of "memory" and "learning" or in some 

 other way is largely a matter of personal idiosyncrasy on the part 

 of the observer. Numerous observers have described processes of 

 food "selection" by Amoeba (e. g., Gibbs and Dellinger, 1908; 

 Schaeffer, 1917 and elsewhere, jMetalnikoff, 1910, et. al.). INIast and 

 Pusch (1924) interpret an obser\'ed change in the protrusion of 

 pseudopodia of Amoeba proieiis in respect to a beam of light as 

 evidence of something analogous to "learning" in higher animals, 

 etc. "Learning" involves "memory," and such terms connote 

 processes of an entirely different nature which we associate with the 

 highest types of animals. It is conceivable that fatigue, to use the 

 term in its broad sense implying total or partial exhaustion of pro- 

 toplasmic constituents necessary for a reaction, and therefore a 

 purely physical matter, is adequate for explanation without calling 

 upon any obscure pan-psychic analogy. Similarly with Kepner 

 and Taliaferro's (1913) evidence of "purpose" in methods of food- 

 getting by Amoeba protons. 



Many of the reactions of Protozoa are bound up with the coor- 

 dinating mechanism of the cell through which the organism acts as 

 a unit. The specific response of an organism to a stimulus is the 

 result of its particular j^rotojilasmic architecture expressed through 

 its coordinating mechanism and motile organs. This has been 

 elaborately worked out by Jennings (1904 to 1909) in connection 

 with the "motor response" of many different kinds of Protozoa. 



The discussions and controversies over the matter of directive 

 stimuli or tropisms in Protozoa have evidently been due in large 

 part to a lack of a common understanding of the definition. If by 

 "tropism" is meant the orientation of an organism in respect to the 

 path of a stimulus, then tropisms, as Jennings was the first to point 

 out, play little part in the activities of the Protozoa. If, however, 

 by "tropism" is meant "the direct motor response of an animal to 

 an external stimulus" (Washburn, 1908), then tropisms play a most 

 important part in such acti^■ities. The two definitions are not 



