IRRITABILITY CONTRA CTILITY. 



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attributed to these. It is true the molecules actually move, 

 but the living transparent material in which they are situated 

 moves first, and the molecules are carried by the currents 

 into the extended portion. The movements cannot, there- 

 fore, be ordinary molecular movements. It has been said 

 that the movements may result from diffusion, but no form 

 of diffusion that is known, or other movement with which 

 we are acquainted, at all resembles the movements de- 

 scribed. Some observers have attributed the movement to 

 a difference in density of different parts of the mass, but 

 who has been able to produce such movements by pre- 

 paring fluids of different density and causing them to mix ? 

 But further, even if this had been done, before the non- 

 living matter could be fairly compared with the living 

 matter, it would be requisite that the supposed fluids of 

 different density should make themselves and somehow ba 

 caused to retain differences in density. 



Nor is it any explanation of the movements to attribute 

 them to inherent " irritability," unless it can be shown what 

 the " irritability " really means. Some dismiss the matter 

 by saying that the movements depend upon the "property " 

 of contractility, but the movements of bioplasm are totally 

 distinct from contractility, as manifested by any form of 

 muscular tissue ; since, as has been already remarked, they 

 take place in every direction, and every movement differs 

 from the rest ; while in muscular contraction there is a con- 

 stant repetition of changes taking place alternately in 

 directions at right angles to one another. Hence, if the 

 movements in question are to be referred to contractility, it 

 will be necessary to assume two very different kinds of con- 

 tractile property, the contractile property which determines 



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