METABOLISM AND THE DISTRIBUTION OF MATERIAL 239 



and alkaline reacting substances may lie side by side in colloidal 

 (amphoteric) media for a long time without neutralizing each other 

 (R. E. LIESEGANG). In the case of phagocytes, that is, in living cells, 

 the existence of acid areas in alkaline protoplasm has been shown by 

 staining with neutral red (E. METSCHNIKOFF). We thus see that in 

 different portions of the organism, the most various crystalloids are 

 present, and may functionate specifically without being accompanied 

 by any exchange or mixture; this only occurs when a crystalloid sub- 

 stance accumulates and becomes osmotically active. Swelling and 

 shrinking may also be of importance for the circulation of crystal- 

 loids, since dissolved substances are soaked up with the water of 

 swelling or are expressed during shrinking. 



If these crystalloids are at the same time electrolytes, they may 

 increase or diminish the swelling according to their nature (acid or 

 salt) ; and in this way, either aid or impede the entrance of crystal- 

 loids. 



Circulation of Colloids. 



Compared with crystalloids, the osmotic pressures in the case of 

 colloids are extremely small. To be sure, we know (see p. 55) that 

 proteins may diffuse through gels, so that they also are independ- 

 ently motile. Of great significance is the discovery of H. ISCOVESCO 

 to the effect that colloid diffusion is dependent on the electric charge. 

 In general, however, the colloids, as opposed to the crystalloids, 

 furnish the stable element of the organism. 



The Influence of Membranes Upon the Interchange of Substances. 



The physico-chemical conditions for the interchange of substances 

 through cell membranes was for a long time completely ruled by the 

 theory of OVERTON, which is somewhat as follows: Protoplasm is 

 surrounded by a fatty lipoid membrane; an exchange of substances 

 can only occur if the given substance is soluble in such a membrane. 

 OVERTON'S theory has not proven universally applicable; it is ever 

 becoming better recognized that the problem will probably be solved 

 when we cease to look entirely to the osmotic conditions and mem- 

 branes for the factors governing the interchange of substance. The 

 fact that both cell content and cell membrane consist of colloids 

 capable of swelling must be taken into consideration. 



The earliest fundamental investigations of the physical inter- 

 change of matter in individual cells were made on plant cells. I 

 refer particularly to the investigations of W. PFEFFER and H. DE 

 VRIES. In plants, especially, we find that the cell content is very fre- 



