OF VITAL MOTION. 21 



Whatever may be the cause of the appearance of 

 the primary germs or nuclei in the organic fluids, we 

 know that their molecular constitution is closely allied 

 to the substance which forms the coats of cells and 

 vessels, and also that heat is necessary to their future 

 development. In these bodies, therefore, we may 

 argue that expansion is inevitable under the operation 

 of the last-named agent, and thus it is possible to 

 conceive that the enlargement of growth may be no 

 mysterious property but only the natural result of 

 the operation of heat upon the substance. Even the 

 formation of the cavity, by which the originally solid 

 nucleus is converted into a hollow vesicle, may also 

 be a part of the same process, for under the operation 

 of an expansile force the particles of these minute 

 fabrics will tend outwards from the centre. 



When the cell is once formed we may readily under- 

 stand that heat may be the cause of further develop- 

 ment, for the material of which it is constituted will 

 be acted upon like the coats of the laticiferous vessel. 

 The process of rupturing which takes place in many 

 of the free cells of organic fluids may also be the 

 natural consequence of unrestrained expansion, and 

 this result would probably be constant if in many 

 cases the process were not kept within bounds by 

 collateral pressure, as is the case in the cellular 

 elements of solid tissues. 



In the coats of simple cells there do not appear to 



