_Methods of Growth of the Crystal and of the Living Being. Intussusception. Apposition._—In truth, there seems to be a complete opposition between the crystal and the living being as regards their manner of nutrition and growth. In the one case the method is intussusception; in the other it is apposition. The crystalline individual is all surface. Its mass is impenetrable to the nutritive materials. Since only the 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 apposition 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 physical characters of the body—solidity in the crystal, semi-fluidity in the cellular protoplasm. If we compare the inorganic liquid matter with the semi-fluid organized matter, we recognize that the addition of substance takes place in the same manner in each—_i.e._, by interposition. If we add a soluble salt to a fluid, the molecules of the salt separate themselves and interpose themselves between those of the fluid. There is, therefore, nothing especially mysterious or particularly vital about the process of intussusception. Applied to fluid protoplasm, it is merely the diffusion that ordinarily occurs in mixed liquids. CHAPTER VII. GENERATION IN BRUTE BODIES AND LIVING BODIES. SPONTANEOUS GENERATION.