138 AMCEBOID MOVEMENTS. [BOOK I. 



In this respect, in that there is no change of bulk, but only a 

 shifting of particles in their relative position to each other, the 

 amoeboid movement resembles a muscular contraction ; but in 

 other respects the two kinds of movement seem different, and 

 the question arises, have we the right to speak of the substance 

 which can only execute amoeboid movements as being contractile ? 

 We may, if we admit that contractility is at bottom simply the 

 power of shifting the relative position of particles, and that 

 muscular contraction is a specialized form of contraction, the 

 shifting of particles is specialized in the sense that it has always 

 .a definite relation to the long axis of the fibre. 



The protoplasm of the amoeba or of a white corpuscle is, as we 

 have said, of a consistency which we for want of better terms call 

 semi-solid or semi-fluid. Consequently when no internal changes 

 are prompting its particles to move in this or that direction, the 

 influences of the surroundings will tend to give the body, as they 

 will other fluid or semi-fluid drops, a spherical form. Hence the 

 natural form of the white corpuscle is more or less spherical. If 

 under the influence of some stimulus internal or external, some 

 of the particles are stirred to shift their place, amoeboid move- 

 ments follow, and the spherical form is lost. If however all the 

 particles were stirred to move with equal energy, they would 

 neutralize each other's action, no protrusion or retraction would 

 take place at any point of the surface and the body would remain 

 a sphere. Hence in extreme stimulation, in what in the muscle 

 corresponds to complete tetanus, the form of the body is the same 

 as in rest; and the tetanized sphere would not be appreciably 

 smaller than the sphere at rest, for that would imply change of 

 bulk, but this as we have seen does not take place. This result 

 shews strikingly the difference between the general contractility 

 of the amoeba, and the special contractility of the muscle. 



