292 LIFE AND DEATH. 



surface is accessible, the incorporation of similar 

 particles is possible only by external juxtaposition, 

 and the edifice increases only because a new layer of 

 stones has been added to those which were there 

 before. On the contrary, the body of an animal is a 

 mass essentially penetrable. The cellular elements 

 that compose it have more or less rounded and 

 flexible forms. Their contact is by no means perfect. 

 They have neither the stiffness nor the precision 

 of adjustment that the crystalline particles have. 

 Liquids and gases can insinuate themselves from 

 without and circulate within the meshes of this loose 

 construction. Assimilation can therefore take place 

 throughout its whole depth, and the edifice increases 

 because each stone is itself increasing. 



The Secondary and Commonplace Character of the 

 Process of Intussusception. — The apparent opposition 

 of these two processes is doubtless diminished if we 

 compare the simple mineral individual with the 

 elementary living unit, the crystalline particle with 

 the protoplasmic mass of a cell. Without carrying 

 analysis so far as this, it is yet easy to see that appo- 

 sition and intussusception are mechanical means 

 that living beings employ at one and the same time 

 and combine according to their necessities. The hard 

 parts of the internal and external skeleton increase 

 both by interposition and superposition, at once. It 

 is by the last method that bones increase in diameter, 

 and the shells of molluscs, the scales of reptiles and 

 fishes, and the testae of many radiate animals are 

 formed. In these organs, as in crystals, life and 

 nutrition occur at the surface. 



Apposition and intussusception are then secondary, 

 mechanical arrangements having relation to the 



