VARIOUS THEORIES OF THE ORGANISM 25 



of substances, and that differences in the aggregate condition of 

 different parts are important factors in the process. 



But even if membranes play the important part which the 

 membrane theory assigns to them, there is no general agreement as 

 to the nature of the conditions which determine permeability, 

 semi-permeabiUty, and impermeability. Some maintain that these 

 properties of membranes depend upon their chemical constitution, 

 and that most substances to enter the cell must combine chemically 

 with the substance of the membrane. Others believe that the 

 entrance of substances into the cell is a matter of solubility in the 

 membrane-substance. According to the famihar theory of Overton 

 and Meyer, the chief constituents of the cell membrane are lipoids, 

 and the passage of at least many substances depends on their 

 solubihty in these lipoids. There is, however, considerable evi- 

 dence against this view that lipoids are in all cases the chief or only 

 factors concerned. Still another hypothesis is that the selective 

 capacity of the membrane depends in one way or another upon 

 its colloid condition. It may well be that many different factors 

 are involved in the permeabihty of membranes in living organisms, 

 but it seems certain that whatever the nature of these factors may 

 prove to be, the pecuHarities of the so-called living substance in 

 this respect are very closely connected with its colloid condition. 

 And when we recall the slight diffusibihty of colloids through each 

 other, it becomes evident that the colloid condition of the sub- 

 stratum is an important factor in determining the accumulation 

 and localization of colloids themselves. 



It has been shown that various inorganic colloids, such for 

 example as colloid platinum, resemble to some extent in their action 

 as catalyzers the enzymes or ferments of the organism. All the 

 known organic enzymes are apparently colloids, and while there is 

 still difference of opinion as to the nature of their action, yet the 

 resemblance between them and inorganic catalyzers is at least 

 highly suggestive.' We know that enzymes are absolutely essential 

 factors in the processes of life, and if enzyme action is in any way 

 associated with the colloid condition the significance of this con- 

 dition for organic life will be still further demonstrated. 



' See Bredig, '01; Hober, '11, pp. 553-614. 



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