AMEBOID MOVEMENT 89 



itself to new investigations and observations is the surface tension 

 theory. The earlier theories under this head were mistaken how- 

 ever in looking to the superficial film of the ameba as the source 

 of energy. But with the increase in knowledge of the chemistry 

 of colloids, the source of the surface energy came to be located 

 in the interfaces between the phases of the colloidal system. As 

 has already been remarked, there is more than sufficient free 

 energy here to account for all the movements observed in proto- 

 plasm; there remains the problem of explaining how the surface 

 energy is transformed into that of movement. As Graham ('61) 

 remarked : "The colloidal is in fact a dynamical state of matter. 

 The colloid possesses energia. It may be looked upon as the prob- 

 able primary source of the force (energy) appearing in the 

 phenomena of vitality." 



Now, viewing streaming wherever it occurs in the protoplasm 

 of animals or plant cells the surface tension theory, as far as 

 observations permit, applies to the various conditions of stream- 

 ing as follows. 



In the first place we shall begin with the assumption that is 

 generally held, that protoplasm is a reversible colloidal solution 

 consisting mainly of proteins, with some carbohydrates, lipoids, 

 etc., on the one hand and water on the other. Its reversibility 

 consists of course in being able to change from a sol to a gel 

 state and the reverse, the water being in the disperse phase in 

 the gel state. The consistency of the protoplasm therefore de- 

 pends upon two factors : upon the amount of water present, and 

 upon the degree of its dispersion; the smaller the droplets the 

 more solid will be the gel because of the increase in surface of 

 the mass. Colloids exhibit the property of contractility in pro- 

 portion as the droplets of water are decreased in size; or, which 

 amounts to the same thing, in proportion as the amount of the 

 surface of the water is increased. It appears as if the source of 

 energy of contractility was the free energy in the surface films 

 of the internal phase of the gel. 



Taking the amebas as a group and applying these principles of 

 colloidal solutions, we find that we can arrange the amebas in a 

 series of four or more grades representing differences of fluidity 

 of the protoplasm. Among the most fluid are Amoeba limicola 



