THEORIES OF CHEMOTAXIS AND PHAGOCYTOSIS 219 



stimuli, and containing similarly a nucleus and many granules. 

 The differentiation of the protoplasm of the ameba into a clear 

 outer ectosarc and an inner granular endosarc is perhaps an 

 important difference, but so far as the two forms of cells have 

 been studied, the effect of this difference in structure does not 

 seem to have been considered. That the unicellular protozoa, 

 devoid of any central nervous system, and without any apparent 

 co-ordinating mechanism, seem able to move about in a purpose- 

 ful way, going toward food supplies and away from injurious 

 agencies, toward or away from light, heat, and chemicals, has 

 long attracted the interest of physiologists, particularly as in 

 these single-celled organisms we may look for the simplest con- 

 ditions of existence and the most elementary life processes. It 

 seems absurd to imagine that a paramceeium goes toward a 

 dilute acid because it " likes it," that an ameba rejects a piece of 

 glass because it " does not taste good," as we explain similar 

 manifestations in higher forms ; furthermore, it has been shown 

 by Verworn that minute enucleated fragments of protozoan cells 

 react to stimuli just as does the entire cell, and, therefore, it 

 seems that the only possible explanation of movements in proto- 

 zoa must be a direct reaction of the stimulated part to the 

 stimulus. The nature of the stimulus and the nature of the 

 stimulated substance must determine the nature of the resulting 

 reaction, and most of the observations so far made suggest that 

 these reactions can be explained according to the known laws 

 of the physics of fluids. An ameba, or a leucocyte, may be 

 looked upon as a drop of a colloidal solution, surrounded by a 

 delicate surface layer which is more or less readily permeable to 

 solvents and to substances in solution, and suspended in a fluid 

 of quite different composition. 



Surface Tension. Such a drop of fluid suspended in another dif- 

 ferent fluid obeys well-known laws of physics. The particles of each 

 fluid are all under the influence of a very considerable force, called the 

 cohesion pressure, which tends to draw them together closely. Within 

 the drop each particle is subjected to this force alike from all sides, so 

 that the forces neutralize one another, and each particle is as free as if 

 there were no cohesion pressure. But the particles on the surface are 

 subjected to unequal pressure, for that of the fluid outside the drop is 

 different from that inside, and so the pressure on the surface particles is 

 equal to the difference of the cohesion pressure of the two fluids ; this 

 constitutes the surface tension. It is this tension that pulls in upon the 

 surface continually, causing it to tend always to reduce the free surface 

 to a minimum, which condition exists perfectly in the sphere. The 

 amount of cohesion affinity is very different in different fluids, and there- 

 fore some have a high surface tension and some a low. When a sub- 

 stance dissolves in another the surface tension is a resultant of the sur- 



