36 



PROTOPLASM AND THE CELL 



most fundamental processes of all these are digestion, assimilation, 

 and excretion. 



DIGESTION is the organic function of preparing food for assimilation 

 to the tissues or for the liberation of its energy in the tissues (these 

 being the two uses of food). The important fact concerning this process 

 is that digestion in its essentials is apparently the same in all animals, 

 however simple or complex. Protoplasm is in the most general sense a 

 particular sort of substance in all animals, and the foods of all animals 

 have certain characters in common. The means, therefore, by which the 

 food is changed into protoplasm would be naturally only one general 

 process. Thus we find the digestive juices in a simple mollusk to be 

 practically of the same composition as those of man. When an ameba 

 (see page 49) surrounds a minute particle of food and secretes from its 

 homogeneous substance a vacuole of liquid, this liquid digests the food- 

 particle in all probability quite as a like fluid would digest a like food- 

 particle in the human stomach. 



FIG. 9 



Euglena viridis, Ehr. : a and 6, the freely active condition, 6 being one phase of its peculiar 

 contortions; c, d, and e, encysted and dividing conditions. (Stein.) 



* 'ASSIMILATION is the process by which nutritive materials supplied as 

 food, having been digested, are incorporated into the substance of the 

 animal's protoplasm. Here the new materials may be katabolized and 

 furnish energy (heat, power of movement, electricity, light, etc.), or 

 they may become an addition to the protoplasm of the animal either to 

 promote growth (as in early life) or merely to replace the tissue lost in the 

 inevitable wear and tear of use. It is in this process of assimilation that 

 is found the greatest difficulties in explaining the changes which have 

 to be made before a food can become bioplasm. Little is actually 

 known about the chemical changes by which the digested food materials 

 are elaborated into tissue-protoplasm. The reason for this is obvious 

 when it is considered that the building up is a molecular process, deep 

 in the hidden tissues of the body, and that it immediately stops when 

 analysis or direct observation is made possible. In other words, seem- 

 ingly paradoxical, the anabolism of protoplasm occurs only in living 

 matter, and the process cannot be observed, because in order to do so the 

 matter must be killed. 



