526 GENERAL PHYSIOLOGY 



gelatine, the cell begins gradually to liquefy the gelatine in its 

 vicinity, i.e., to bring the solid substance into solution, and from 

 the accumulation of liquid thus arising and surrounding the 

 bacterium, the dissolved nutrient substances are able to diffuse 

 into the cell. In cells possessing a naked protoplasmic surface 

 and no cell- wall extracellular digestion is naturally not required, 

 because the food-stuffs, even when they cannot diffuse, are able to 

 come into chemical relation with the surface of the protoplasm 

 directly. 



These considerations allow us to form a general idea of the 

 mechanics of the process by which resorption and secretion in the 

 living cell are able to go on continually and automatically. The 

 great quantity of energy that is involved in the two can also be 

 understood, if the chemism of the living cell be taken into suffi- 

 cient consideration ; for, if very strong chemical affinities for food- 

 stuffs exist in a cell, and if a very active transformation of sub- 

 stance takes place, it is then very easily conceivable that the 

 great amount of chemical energy can lead to the performance of 

 very considerable work. Special cases are still puzzling ; but the 

 solution of these cases belongs elsewhere. 



While, as may be supposed, the exchange of dissolved substances 

 between cell and medium is based upon the same principle in all 

 cases, that of solid substances is performed in very different ways in 

 different cases. The only thing common to all these latter is the fact 

 that the exchange is mediated solely by active movements of the 

 cell in question, but in different cases these movements are 

 influenced in very different ways by the food. The ingestion and 

 output of solid substances is not wide-spread and occurs, indeed, 

 only in naked protoplasmic masses, such as RJiizopoda and 

 leucocytes, and in Infusoria in so far as they possess a special mouth- 

 opening. In many Infusoria and especially in those that lead a 

 sessile life, such as Stentor and Vorticella, the ingestion of food 

 appears to be left solely to chance, which occasionally leads small 

 free-swimming food-particles, such as Alga-ce\\s, swarm-spores, 

 Bacteria, etc., into the region of the lively whirlpool that is pro- 

 duced by the circlet of cilia upon the peristome. This whirlpool, 

 while capable of being influenced in direction by changes in the 

 action of the cilia, is so regulated at the time of food-ingestion 

 that it leads directly into the mouth-opening of the cell-body. 

 Free-swimming Infusoria and most naked protoplasmic masses 

 seek solid food. Either they are attracted from a distance by 

 chemical stimuli which go out from the masses of food by the 

 diffusion of certain substances, or they are led to take up the food by 

 mechanical stimulation through direct contact with the food-masses. 

 In the former case the ingestion of food is the result of a positive 

 chemotaxis; since the cell moves toward the source of the chemical 

 stimulus, and. its protoplasm comes into very close connection 



