PROTOPLASMIC STRUCTURE 129 



merits the cell was placed in a 10 per cent solution of 

 KNO3 containing some eosin to serve as indicator of 

 permeability; the outer protoplasm dies in one or two 

 hours and becomes colored, but the vacuole remains at 

 first clear and uncolored, showing impermeability of its 

 membrane to the dye. Later the membrane becomes 

 permeable to the dye, presumably as a result of death- 

 changes, and the vacuole contents become colored. 

 Isolated vacuoles show osmotic properties similar to 

 those of the whole protoplast. The observations of 

 Kite and Chambers on artificial vacuoles in sea-urchin 

 eggs illustrate the same phenomenon, the limiting surface- 

 film of the vacuole having apparently the same properties 

 as the surface-film of the entire cell. 



The evidence just cited shows the error of regarding 

 the properties of continuous protoplasm as identical 

 with those of partitioned protoplasm. At boundary 

 surfaces of whatever kind the living substance exhibits 

 special properties, in particular a high resistance to the 

 diffusion of water-soluble substances including ions. 

 On the other hand, the internal protoplasm appears (at 

 least in many cases) to be freely penetrable to diffusing 

 substances of low molecular weight and to ions; hence 

 it possesses a considerable electrical conductivity. This 

 latter conclusion is of great importance for the theory of 

 stimulation and conduction, as will be seen below. 



As additional evidence for the permeability of con- 

 tinuous or unpartitioned protoplasm to water-soluble 

 substances, we may cite the type of distribution shown 

 by many diffusible substances in cells. Many such sub- 

 stances are concentrated in special regions or structures, 

 in which they appear to be held in chemical or other 



