150 GENERAL PHYSIOLOGY 



stance that some shells possess small sand-grains, and others 

 considerably larger ones is likewise to be referred in part to the 

 character of the material at their disposal, in part, however, to 

 other external conditions, such as the narrowness of the opening 

 of the capsule, which does not allow the protoplasmic body to draw 

 through large sand-grains. It accordingly appears that in most 

 cases the construction of the capsule by Diffiugice involves no real 

 selection of material, and thus far no case has become known 

 where such a selection has really been established with certainty. 

 There is, therefore, no justification in drawing a parallel, as is 

 often done, between the ingestion of structural material in the 

 building of the Difflugia-ca,-psuie and the act of food-selection by 

 the living cell. 



B. THE TRANSFORMATION OF INGESTED SUBSTANCES 



The process of construction of living substance out of the in- 

 gested food-stuffs can be designated best by generalising, as is 

 frequently done, a conception of the botanists and employing the 

 word assimilation. By assimilation in the narrow sense has 

 been understood for a long time in botany the synthetic formation 

 in plants of the first visible organic material, starch, out of the 

 ingested inorganic compounds. But it is advantageous to extend 

 the conception and employ it also for the construction of higher 

 organic compounds, especially the proteids, and, indeed, not only 

 in plants, but also in animals. By assimilation, therefore, is under- 

 stood the sum of the processes that lead to the eonstruction of living 

 substance to the maxirmiin of its most complex constitution, the syn- 

 thesis of proteids. Construction, or assimilation, can then be 

 contrasted with destruction, or dissimilation. 



1. Extracellular and Intracellular Digestion 



" Corpora non agunt nisi soluta." This old dictum plays in the 

 life of the cell a very great role. In order that the ingested food- 

 stuffs may work chemically and be of use for the construction of 

 living substance, they must be in a dissolved condition ; since, 

 however, the food taken in by the organism is in part solid food, 

 it must first be transformed into soluble form, and this process is 

 termed digestion. It has been seen that only a few cells have the 

 power of taking in solid food ; in these there occurs so-called intracel- 

 lular digestion, the transformation of the solid food into soluble com- 

 pounds taking place in the interior of the cell. The great majority 

 of cells, however, cannot take in solid food ; in them, therefore, 

 the transformation of the solid into the soluble form must take 

 place outside of the cell, in order that ingestion may be possible ; 



