528 GENERAL PHYSIOLOGY 



is based is evidently the same as that which controls in general 

 atoms and molecules, namely, affinity. It is surely no less wonder- 

 ful that an atom of phosphorus unites very easily with an atom of 

 oxygen, but not with an atom of platinum, than that an intestinal 

 epithelium-cell takes up fat-droplets but never pigment-granules. 

 And it is no less comprehensible that a Vanipyrella surrounds with 

 its body-protoplasm and digests only Spirogyra threads and nO' 

 other bodies, than that a drop of rancid oil, as Gad ('78) has shown, 

 sends out amoeboid processes to an alkaline liquid and uses the 

 alkali for the manufacture of soap, but is inactive toward an acid 

 liquid. But the behaviour of the Vampyrella and the intestinal 

 epithelium-cell is by no means peculiar, every living cell behaves, 

 similarly. Every tissue-cell in the human body takes up from 

 the common nutrient liquid, the blood, certain substances only, as 

 is evident from the fact that gland-, muscle-, and cartilage-cells, 

 jDroduce wholly different and characteristic substances. In this 

 respect, as Haeckel ('66) has already emphasised, the cell behaves 

 exactly like a crystal, for example a crystal of alum, which out of a- 

 mother liquor containing numerous salts in solution always selects 

 alum molecules only, in order to employ them for its growth, or,, 

 if it has been injured, for its regeneration. Thus the mystical ob- 

 scurity that some investigators have endeavoured to wrap about, 

 the so-called selection of food-stuffs on the part of the individual 

 cell does not really exist. What has been called by the anthropo- 

 morphic term the "selection of food" by the cell is an absoluteljr 

 necessary consequence of the fact that the living substance of every 

 cell possesses its own specific composition and its own characteris- 

 tic metabolism. 



Thus, the phenomena of cell-metabolism may all be referred to 

 chemical and physical principles, as they are found in inorganic 

 nature, and although at present we are unable to trace in individual 

 cases the finer details of the special metabolic processes, we are 

 certain that the whole metabolism comes about in a purely 

 mechanical manner, and that phenomena are never met with that 

 cannot be explained mechanically. There can evidently be no 

 exception to the conclusion that everything that consists of matter 

 must obey the laws of matter. 



2. The Mechanics of Changes of Cell-form 



Although in the present condition of our knowledge of cell- 

 processes, we do not know what special share in the whole meta- 

 bolism is taken by the individual constituents of the cell, with 

 what chemical processes in the history of the biogens the nucleus 

 and the protoplasm with their specific constituents are associated, 

 our discoveries so far regarding the general metabolic relations in 

 the cell are sufficient to enable us to recognise that the phenomena. 



