THE PERMEABILITY OF CELLS 509 



membrane, designated by him the tonoplast. The generaliza- 

 tions made by these three botanists, in which the discovery of 

 semipermeable precipitation membranes by M. Traube ('67) 

 has played a most important part, have been extended and con- 

 firmed by a host of recent experimenters, Overton, Hoeber, 

 Nathanson, Ruhland, Hamburger, R. Lillie, Koeppe, Gryns, 

 Hedin, Asher, J. Loeb, and many others. Although most studies 

 on permeability have been carried out on plant cells, the same 

 essential relations are exhibited by animal cells. 



2. Theories concerning permeability 



The more recent studies, especially those of Overton ('95, '97, 

 '99, '00), have been concerned with the classes of substances 

 which may or may not pass the plasma membrane. This at 

 once raises two important questions. 



1. How does a substance enter? 



2. What is the nature of the cell surface or plasma mem- 

 brane ? . 



In answer to each of these questions several theories have been 

 advanced. Let us consider first the nature of the plasma mem- 

 brane. 



Quincke ('88), in order to account for the power of movement 

 of amoeboid cells as well as certain peculiar osmotic properties, 

 assumed that a thin film of oil was present at the surface. Over- 

 ton has explained the very rapid entrance of ether and fat- 

 soluble substances by assuming that the plasma membrane is 

 composed largely of lipoids like lecithin. This view was later 

 modified to explain the entrance of substances insoluble in lipoids 

 by regarding the cell surface as a mosaic of proteid plus choles- 

 terin (Nathanson, '04, a) or proteid plus lecithin. On the other 

 hand Pfeffer has always insisted that the membrane is chiefly 

 proteid, while Robertson ('08) considers it a form of modified 

 protein comparable to that which remains about droplets of 

 chloroform shaken up with protein solutions and then washed 

 repeatedly in water. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGT, VOL. 10, NO. 4 



