THE MECHANISM OF LIFE 523 



granules, chlorophyll-bodies, etc., of protoplasm share, at least 

 temporarily, in the metabolism, do we obtain an approximate idea 

 of the complexity of the cell-metabolism and of the endlessly 

 multiform relations in which nucleus and protoplasm are united. 



A far-reaching correlation between the individual elements of 

 the cell, especially between the nucleus and the protoplasm, 

 follows from these close metabolic connections. The one is 

 conditioned by the other. One is dependent upon the substances 

 that the other produces. Thus the profound changes are explained 

 that the life of the cell experiences when the individual links of 

 the great metabolic chain are changed, whether spontaneously in 

 the course of development or as a result of the action of external 

 stimuli. Every change of one biotonic link brings about a change of 

 many others and, if for any reason one drops out, the metabolic 

 chain is broken, and necrobiosis, which finally ends in death, begins. 



b. The Mechanics of the Ingestion and Output of Substances 



There now remains the question of the mechanics of this 

 involved process of cell-metabolism. Since the metabolic relations 

 between the nucleus and the protoplasm, like those between 

 the whole cell and the medium, are based upon the ingestion and 

 output of substances, this question may be simply comprised in 

 the problem of the mechanics of these processes on the part of 

 the cell. 



It is advantageous to consider separately the cells that receive 

 and give off substances in solution only, and those that receive and 

 give off solid substances also. 



For a long time the processes of exchange of dissolved substances 

 between the cell and the surrounding medium, both resorption and 

 secretion, were regarded as conforming directly to the laws of 

 filtration and diffusion. But recently attention has been directed 

 to various facts which prove that in most cases filtration plays no 

 rdle at all in these processes, and also that diffusion or osmosis 

 alone is not sufficient to explain them. Especially from the later 

 observations of Heidenhain ('94) it is known that the vital process 

 in the cell itself plays the most important role in the exchange ; 

 diffusion alone is unable to explain, e.g., the propelling power 

 with which the secretion is extruded in many cases from gland- 

 cells, or the considerable energy with which certain food-stuffs are 

 taken up by the intestinal epithelium-cells. Hence, in explaining 

 the mechanics of resorption and secretion the two factors, diffusion 

 and the chemism of the cell, must be taken into consideration. 



By diffusion or osmosis is understood, as is well known, the fact 

 that two different gases or liquids which are miscible will mix 

 with one another spontaneously into a homogeneous mass, when 

 they are brought into contact. As a rule, the word diffusion is 



