THE STRUCTURAL BASIS OF THE BODY 25 



result in the protrusion of pseudopodia and the ingestion of the food 

 particle. 



VITAL PHENOMENA OF CELLS. A. Assimilation. The activity of 

 every living being, whether uni- or multi-cellular, can be regarded as com- 

 pounded of two phases, assimilation and dissimilation. By assimilation we 

 mean the building up of the living substance at the expense of material 

 )btained from the external world. In this process substances are formed of 

 high potential energy, and this energy can be obtained only at the expense 

 either of energy imparted to the system at the moment of assimilation, as, 

 e.g. in the assimilation of carbon from carbon dioxide under the influence 

 of the sun's rays, or of energy contained in the food-stuffs themselves. In all 

 living organisms, except those provided with chlorophyll corpuscles, it is 

 the latter method which is adopted, and a food-stuff therefore connotes some 

 substance which can be taken in by the cell and can serve it as a source 

 of chemical energy. The evolution of energy, which is required for the 

 movements and other vital activities of the cell, is derived from a disintegra- 

 tion or dissimilation of the protoplasm and is generally associated with the 

 process of oxidation. In assimilation, besides the building up of living 

 protoplasm, there may also be a synthesis of more complex from less complex 

 compounds, without their necessary entry into the structure of the living 

 molecule. In the absence of any definite criteria by which we may judge 

 as to the living or non-living condition of parts of the cell, it is a little 

 dangerous to draw any hard-and-fast distinction between these two sets of 

 processes. Assimilation requires the ingestion of food into the organism, and 

 in the second place its digestion, i.e. its solution in the juices of the cells. 

 These two processes are succeeded, through stages which we cannot trace, 

 by an actual growth in the living material. In naked cells ingestion may 

 occur either at any part of the surface, as in the amoeba, or at a specialised 

 portion, so-called ' mouth,' as in many of the infusoria. Digestion is 

 apparently effected in most cases by the production and secretion around the 

 ingested food particle of solutions containing ferments, i.e. agents which have 

 the power of hydrolysing the different foodstuffs and rendering them soluble. 



In the vast majority of living organisms the energy for their activities 

 is derived from the oxidation, ultimately of the foodstuffs, but immediately 

 of molecules attached to the living protoplasm. A necessary condition, 

 therefore, for the life of these cells is the presence of oxygen in the surround- 

 ing medium, from which it is taken up in the molecular form. We may 

 therefore speak of an assimilation of oxygen ; but it is still a matter of 

 dispute whether the oxygen is built up as such in the living molecule (so-called 

 intra-molecular oxygen) to be utilised for the formation of carbon dioxide 

 when a discharge of energy is necessary, or whether it is taken in only at the 

 moment when the combustion of the carbon and hydrogen constituents of the 

 food or protoplasm is necessary for the supply of energy. However this may 

 be, products are formed as a result of this oxidation which are of no further 

 value to the cell and are therefore excreted, i.e. turned out of the cell. The 

 chief of these are the products of oxidation of carbon and hydrogen, namely, 



