CONDUCTIVITY AND PERMEABILITY 227 



certain dyes penetrated the*^ outer surface much more 

 readily than the inner.*^ The experiments of the writer 

 show a difference in the two surfaces and emphasize the 

 conception that the permeability of the protoplasm is not 

 alike in all its parts.** 



The real question is whether a special layer exists at 

 the outermost surface of the cell which admits some sub- 

 stances, but not others. Furthermore, is it possible that 

 substances which penetrate the outermost layer with 

 difficulty can spread freely throughout the cell when they 

 have passed the outer layer? 



It is a well-known fact that. substances, which, like 

 protoplasm, contain a considerable amount of protein 

 readily form films at their surfaces when brought into 

 contact with liquid.*^ By means of the ultra-microscope 

 Gaidukov*" observed a differentiated film at the surface 

 of the cell. Such films or membranes have been shown to 

 exist in some cells by micro-dissection and there are indi- 

 cations that they also exist at the surfaces of vacuoles,*'' 

 and of nuclei.*^ The surface of Amcxba and of some other 

 protozoa is covered with a thin membrane capable of 



"de Vries (1885). 



" The objection might be made that the dye cannot penetrate the 

 inner surface until the protoplasm has become saturated with it, and 

 this might be confused with a difference in permeability. 



" See page 229. 



«Nageli (1855) /, pp. 9, 10; Hanstein (1870); Pfeffer (1900); 

 Kobertson (1908); Hober (1914) p. 65; Harvey (1912, A, B) ; Metcalf 

 (1904); Kuhne (1864); Eamsden (1905); Rosenthal (1901); Schiitt 

 (1904); Shorter (1909); Ehode (1906). For a general summary, see 

 Zangger (1908). 



"Gaidukov (1910); Marinesco (1912); Price (1914). 



"Seifriz (1918); Chambers (1915, 1917, 1920, 1921); Kite (1912); 

 Pfeffer (1900) 1:107; Overton (1895, 1900, 1901, 1902). 

 « Chambers (1921). 



