MOTILE REACTIONS 1 9 



reiidily-staiiiing plasm, which corresponds to a " eeutrosoiue " or 

 centre of plasniic forces (see below, pp. 11"), 121, 1-11); it lias 

 lu'en termed a " lilepliaroplast." ^ 



.\.gain, the cytoplasm may have differentiated in it delinite 

 streaks of specially contractile character ; sucli streaks within its 

 substance are called " myonemes " ; they are, in tact, muscular 

 fihrih. A " muscle-cell," in the Higher Animals, is one whose 

 protoplasm is almost entirely so modified, with the exception of a 

 small portion of granular cytoplasm investing the nucleus, and 

 having mainly a nutritive function. 



Definite muscular fibrils in action shorten, and at the same 

 time become tliicker. It seems probable tliat they contain elon- 

 gated vacuoles, and that the contents of these vary, so that 

 when they have an increased osmotic equivalent, the vacuoles 

 absorb water, enlarge, and tend to become more spherical, i.e. shorter 

 and thicker, and so the fibril shortens as a whole. The relaxation 

 would be due to the diffusion outwards of the solution of the 

 (tsmotically active substances which induced expansion."^ 



The Motile Reactions of the l*rotozoa ^ require study from 

 another point of view: they are either (1) "spontaneous" or 

 "arbitrary," as we may say, or (2) responsive to some stimulus. 

 The latter kind we will take first, as they are characteristic of 

 all free cells. The stimuli that induce movements of a responsive 

 character are as follow'S : — (i.) mechanical : such as agitation and 

 contact ; (ii.) force of GRAVITY, or centrifugal force ; (iii.) 

 CURRENTS in the water; (iv.) radiant energy (light); (v.) 

 changes in the temperature of the medium ; (vi.) electric 

 cuRitENTS through the medium ; (vii.) the presence of chemical 

 substances in the medium. 



These, or some of them, may induce one of three different 

 results, or a combination thereof: (1) a single movement or an 

 arrest of motion ; (2) the assumption of a definite position ; (3) 

 movement of a definite character or direction. 



' A similar body lies at the centre to which the axial filaments of the radiating 

 liseudo[)odia of the Heliozoa converge, and might be termed by parity a 

 " podoplast" ; but " centrosome " is a convenient general term to include all such 

 bodies. It is clearly of nuclear origin in Trypanosoma (Fig. 39, p. 120). 



^ See for development of this view AV. M'Dougall in Juurn. Anat. Physiol. 

 xxxi. 1897, pp. 410, 539. I put it forward in the first draft of this essay in 1894. 



^ The best general account is to be found in Davenport, Experimental 

 Murphology, 1897. 



