18 INTRODUCTORY [ch. 



as a fluid*, it is evidently far less mobile than such a fluid (for 

 instance) as water, but is rather Uke treacle in its slow creeping 

 movements as it changes its shape in response to force. Such fluids 

 are said to have a high viscosity, and this viscosity obviously acts 

 in the way of resisting change of form, or in other words of 

 retarding the efl'ects of any disturbing action of force. When the 

 viscous fluid is capable of being drawn out into fine threads, a 

 property in which we know that some Amoebae differ greatly from 

 others, we say that the fluid is also viscid, or exhibits viscidity. 

 Again, not by virtue of our Amoeba being liquid, but at the same 

 time in vastly greater measure than if it were a sohd (though far less 

 rapidly than if it were a gas), a process of molecular diffusion is 

 constantly going on within its substance, by which its particles 

 interchange their places within the mass, while surrounding fluids, 

 gases and soUds in solution diffuse into and out of it. In so far 

 as the outer wall of the cell is different in character from the 

 interior, whether it be a mere pelhcle as in Amoeba or a firm 

 cell-wall as in Protococcus, the diffusion which takes place throtigh 

 this wall is sometimes distinguished under the term osmosis. 



Within the cell, chemical forces are at work, and so also in all 

 probabihty (to judge by analogy) are electrical forces; and the 

 organism reacts also to forces from without, that have their origin 

 in chemical, electrical and thermal influences. The processes of 

 diffusion and of chemical activity within the cell result, by the 

 drawing in of water, salts, and food-material with or without 

 chemical transformation into protoplasm, in growth, and this com- 

 plex phenomenon we shall usually, without discussing its nature 

 and origin, describe and picture as a force. Indeed we shall 

 manifestly be incHned to use the term growth in two senses, just 

 indeed as we do in the case of attraction or gravitation, on the one 

 hand as a process, and on the other as a force. 



In the phenomena of cell-division, in the attractions or repulsions 

 of the parts of the dividing nucleus, and in the " caryokinetic " 

 figures which appear in connection with it, we seem to see in 

 operation forces and the effects of forces which have, to say the 



* One of the first statements which Dujardin made about protoplasm (or, as 

 he called it, sarcode) was that it was not a fluid; and he relied greatly on this fact 

 to shew that it was a living, or an organised, structure. 



