212 THE BEHAVIOR OF LOWER ORGANISMS. 



similar manner. In one respect there is, of course, a striking differ- 

 ence between the two cases. Amoeba does not require that the sub- 

 stratum should be different on its two sides in order that there should 

 be movement ; on the contrary, it may move steadily in a certain direc- 

 tion on a uniform surface. The mechanism of the movement might, 

 nevertheless, be the same as in the inorganic drop. Chemically different 

 substances show different degrees of adherence to the same surface. It 

 may be supposed, therefore, that there is a chemical difference between 

 the anterior and posterior regions of Amoeba of such a nature that the 

 anterior region clings to the surface while the posterior region does not. 

 This chemical difference must, of course, be continually renewed, since 

 new parts of the body continually come in contact with the substratum. 



It may, however, be questioned whether the adhesion of Amoeba to 

 the substratum is of the same character as the adhesion of a drop of 

 water to glass ; in other words, whether Amoeba really plays here the 

 part of a fluid, and " wets" the substratum. This was the view taken 

 by Berthold (1886) and, if I understand him correctly, Le Dantec 

 (1895). Apparently opposed to such a view is the fact that Amoeba 

 may creep on the under side of the surface film of water, as I have 

 often observed. This surface film is, of course, fluid ; if in adhesion 

 Amoeba itself also plays the part of a fluid, we should have two fluids 

 in contact, having the same relation of attraction or adhesion that a 

 fluid has for a solid that it " wets" ; that is, the particles of each fluid 

 have a greater attraction for those of the other fluid than for each other. 

 This, it would appear, could result only in the formation of diffusion 

 currents in the two fluids ; the two would mix. This result does not 

 follow, so that it would appear that in adhesion Amoeba does not play 

 simply the part of a fluid which wets the substratum. As we have 

 seen (p. 165), there is evidence that the adhesion takes place through 

 the mediation of a viscid secretion. 



Whatever the nature of the adhesion, we know it exists at one pole 

 of the Amoeba and not at the other. Given such chemical differences 

 between the two poles as would produce this difference in adhesion, 

 then locomotion would follow essentially as we find it to occur in 

 Amoeba Umax or A. verrucosa. No further properties except those 

 common to fluids would be required.* For the determination of the 

 direction and rate of locomotion, the distribution of these chemical 

 differences would be the essential factor. 



Caution is necessary, however, in transferring the results of these and 

 other similar experiments to Amoeba. The resemblances between the 

 movements of the inorganic drops and those of Amoeba show merely 



*It will be noted that this statement is made for simple locomotion, and does 

 not refer to the formation of pseudopodia. 





