II> J. F. MCCLEXDOX. 



Many of Overton's critics do not distinguish between lipoids 

 proper and a host of ether-soluble substances which are also 

 ca'lled lipoids, and of the data which they present we will con- 

 sider only that on lipoids proper. Ruhland 1 found that certain 

 dyes stain plant cells but are not soluble in solutions of cholesterin 

 (and vice versa). Robertson 2 observed that methyl green freed 

 from methyl violet was insoluble in a nearly saturated solution 

 of lecithin in benzol, whereas it stained living cells. Hober 3 

 obtained Ruhland's results, when using certain animal cells, 

 but found that certain nephric tubule cells absorb all dyes that 

 are not suspension colloids. 



Faure-Fremiet, Mayer and Schaeffer 1 state that pure choles- 

 terin does not stain with any dyes (contrary to Overton), mala- 

 chite green (considered lipoid-insoluble by Ruhland and Hober) 

 stains lecithin, and Bismarck brown (considered lipoid-insoluble 

 by Ruhland) stains lecithin, cholesterin-oleate and cerebrin. 

 A mere trace of free fatty acid greatly affects the behavior of 

 lipoids toward stains. 



Mathews 5 considers the absorption of dyes by cells as a chemical 

 process. Since basic dyes combine with albumin in alkaline 

 solution, lipoids in the membrane are not necessary for the ab- 

 sorption of such dyes. 



Traube objected to Overton's hypothesis on the ground that 

 Overton's plasmolytic series is the same as found by Brown, who 

 used the membrane of the barley grain, 6 and the same as the 

 series of the attraction pressures of the substances in water. 

 But Traube admits in his later papers that the chemical character 

 of the membrane affects osmosis. 



We may conclude that, although the plasma membrane of 

 some cells may be lipoid in character, this lias not been proven, 

 but, in general, it is more permeable the more the diffusing sub- 

 stance lowers the surface tension of water. 



1 Juhrb. wiss. Bol., 1908, XLVI., i, and Ber. DeiUsch. hot. Gesellsch., 1909, 

 XXVI., 112. 



2 Jour. Bio. Chem., 1908, LV., I. 



3 Biochem. Zeit., 1909, XX., 55. 



4 Arch, d' Anal. Mic., 1910, XII.. 19. 



* Jour. Pharmacol. and Exp. Tlier., 1910, II., 201. 



6 But this is not true of all seed coats. Atkins, Sci. Proc. Roy. Dublin Soc., XII., 

 n. s.. No. 4, p. 35, observed that the membranes of the bean seed are freely pcrme- 

 at4l, semipermeable plasma membranes arising only after germination. 



