158 II. HOLTER 



Regardless of these differences in mechanism and dimensions, the 

 morphologically essential feature is the same ; a certain area of the surface 

 membrane of the cell encloses a droplet of the surrounding medium, 

 separates from the surface and migrates into the cell. 



Another important feature of pinocytosis is that it does not occur in 

 all media and all the time but is induced by the presence of certain sub- 

 stances in the medium. So far, the induction of pinocytosis has been 

 studied most extensively in amoebae [13, 9, 5, 6] with regard to proteins, 

 amino acids and certain salts. Typical non-inducers are, for instance, the 



Fig. I. Pinocytosis channels in Amoeba proteiis, induced by i ■5"() solution of 

 bovine plasma albumin. (After Chapman-Andresen and Prescott [9].) 



carbohydrates. Induction depends on the size of the molecule, pH and 

 other factors connected with electric charge. In amoebae there seems to 

 be a certain not very dramatic molecular specificity; but for mammalian 

 cells at least one case of highly specific induction of pinocytosis has been 

 reported, namelv the action of insulin on HeLa cells [2^] and adipose tissue 



The findings regarding pinocytosis induction support a hypothesis 

 pronounced by Bennett [3] according to which one of the essential features 

 of pinocytosis consists in the adsorption of molecules or ions to the cell 

 surface membrane, followed by an invagination of the surface at the loaded 

 area, a sliding of the membrane into the invagination. The first experi- 

 mental support of this hypothesis came from papers by Brandt [4] and 



