10 GENERAL AND PHYSICO-CHEMICAL. 



of living cells to dye-stuffs, as well as the special ease in which certain 

 substances, which are not soluble in water or only slightly so, but are readily 

 soluble in fats or fat-like bodies, pass into animal and plant protoplasms 

 has led OVERTON to the conclusion that the protoplasmic limiting layer 

 behaves like a substance layer having the solvent properties similar to the 

 fatty oils. According to OVERTON the protoplasmic layer is probably 

 impregnated with lipoids, i.e., bodies more or less similar to the fats 

 in regard to their solubilities and their solvent power upon certain sub- 

 stances. The lipoids do not form a chemically definable class of bodies. 

 Certain of them are still of an unknown constitution while others are 

 known, especially the lecithins (the phosphatides as a group) and the 

 cholesterin are to be especially mentioned on account of their great 

 importance. 



The assumption that an accumulation of lipoids occurs, as a special 

 limiting layer, in the cells is not sufficiently founded and not generally 

 true at least for the animal cells. Still this assumption is not absolutely 

 necessary for a comprehension of the action of lipoids in the above 

 sense. Objections have been raised by a few investigators against 

 OVERTON'S theory, which has found general acceptance. 1 Thus it fails 

 to explain all cases, although this was suggested by OVERTON himself, 

 for instance according to COHNHEIM, it does not explain the absorption 

 processes in the intestinal canal, and according to MOORE and ROAF it 

 cannot explain certain properties of the cells, namely the varied composi- 

 tion of the electrolytes within and outside of the cells, and the selective 

 taking up of certain soluble substances such as food products, drugs, 

 toxins and antitoxins by the cells. The investigations of the last men- 

 tioned experimenters are based essentially upon investigations of the 

 behavior of mineral substances, and they show that the above theory 

 offers certain difficulties in explaining the exceedingly important exchange 

 of mineral substances between the cells and the external fluid. Also the 

 fact that the cells are readily permeable for water is explained with diffi- 

 culty by OVERTON'S theory. 



J. TRAUBE 2 especially has put forth objections to OVERTON'S theory. 

 According to him, the passage of a substance from a watery solution 

 into the cells, is in the first place due to its so-called solution tenacity in the 

 watery solution. This solution tenacity is according to TRAUBE the attrac- 

 tion between the solvent and the solute; and is not identical with the 

 osmotic pressure, but is measured by the surface tension of the solution. 



1 See O. Cohnheim, Die Physiologic der Verdauung u. Ernahrung (1908). J. Loeb 

 in Oppenheimer's Handbuch der. Biochem. Bd. 2, 105. T. B. Robertson Journ. of 

 biol. Chem., 4 (1908). B. Moore and H. Roaf, Biochem. Journ., 3 (1908). 



'Pfliiger's Archiv., 105, 541 (1904); 123, 419 (1908); 132, 511 (1910); 140, 109 

 (1911). 



