36 CELLULAR 



m a variety of ways, and that the subject de- 

 serves more careful investigation. 



Another problem which has been attacked by 

 the use of these large cells is whether proto- 

 plasm is permeable to ions. This question, an- 

 swered in different ways by opposing schools, 

 has become a center of controversy. Each side 

 has assembled an imposing array of facts on 

 every point of importance but the evidence re- 

 mains conflicting and the interpretation doubt- 

 ful. It is evident that the most satisfactory way 

 of attacking the problem is to study the pene- 

 tration of substances which are partly ionized 

 and to determine by direct analysis whether the 

 concentration of such a substance inside the cell 

 corresponds to the ionized or to the non-ionized 

 part of the substance in the external solution. 



A suitable material for this purpose is hydro- 

 gen sulphide, which penetrates readily, is easily 

 measured, and is only slightly toxic at low con- 

 centrations. 



The experiments show that when a living cell 

 of Vdonia is allowed to take up hydrogen sul- 

 phide until the sap comes into equilibrium with 



