168 AMCEBOID MOVEMENTS. [BOOK i. 



So long as we narrow our idea of contractility to what we see 

 in a muscular fibre, and understand by contraction a movement of 

 particles in relation to a definite axis, necessarily followed by a 

 reversal of the movement in the form of relaxation, we shall find 

 a difficulty in speaking of the substance of the amoeba or of the 

 white blood corpuscle as being contractile. If however we conceive 

 of contractility as being essentially the power of shifting the 

 position of particles in any direction, without change of bulk (the 

 shifting being due to intrinsic molecular changes about which we 

 know little save that chemical decompositions are concerned in 

 the matter), we may speak of the substance of the amoeba and 

 white blood corpuscle as being contractile, and of muscular con- 

 traction as being a specialized kind of contraction. 



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. 



