l6o H. HOLTER 



However this may be, it is certain that pinocytosis can be a means of 

 introducing certain substances, including proteins and amino acids, into 

 vesicles that migrate into the cell interior. The next question is then : Are 

 such substances available for the metabolism of the cell ? This question 

 is not only a figure of speech ; we must not forget that many of the sub- 

 stances which have been found to enter by pinocytosis, are high-molecular 

 compounds to which the cell membrane is assumed to be impermeable — 

 the same membrane, in fact, which by the mechanism of invagination 

 becomes the boundary of the pinocytosis vesicles. The properties of this 

 membrane, therefore, and its possible changes inside the cell become the 

 main problem in our understanding of the physiological significance of 

 pinocytosis. 



During the migration of the vesicles from the periphery toward the 

 interior there occurs a rather intense dehydration which is often plainly 

 discernible in time-lapse movies as a shrinkage of the vacuoles. However, 

 just as often this process of shrinkage is counteracted or even overcom- 

 pensated by another frequent occurrence, namely, the fusion of the 

 original small vesicles to larger vacuoles, so that the actual view in the 

 microscope usually presents an array of vacuoles of very difl^erent sizes. 

 In spite of this, the actual occurrence of dehydration causing a concentra- 

 tion of the vacuolar contents was made rather probable by Marshall and 

 myself [13] by means of centrifugation experiments in which we found 

 that after pinocytosis the density of the pinocvtosis vacuoles, identifiable 

 as such by the content of a fluorescent marker, was steadily increasing. 



Recently Roth [26] has published very interesting observations regard- 

 ing the changes occurring in pinocytosis vacuoles after ingestion. He has 

 shown that the vacuolar membrane displays a rather intense form of 

 "internal micropinocytosis" by which, as he assumes, the contents of the 

 primary vacuoles are distributed in the cytoplasm. Roth [26] also points 

 to an important fact, which perhaps had not been sufiiciently considered 

 by previous authors : pinocytosis, and especiallv if repeated in a second step 

 internally, results in an enormous increase of active internal surface available 

 for diffusion. Chapman-Andresen and Nilsson [8] in our laboratory 

 have found that this process of secondary micropinocytosis begins during 

 channel formation (Fig. 3). 



The only morphological change of the vacuolar membrane in amoebae 

 that can be observed in the electron microscope is that the mucous coat 

 that covers the plasmalemma and after invagination forms a lining on the 

 inside of the pinocytosis vacuoles, disappears after some time. Miiller 

 and Rappay [16] have reported that the periodic acid-Schiff reaction which 

 is given both by the plasmalemma and the vacuolar lining, disappears 

 later on in the vacuole. In our laboratory [7] we studied the permeability 

 of pinocytic vacuoles to radioactive glucose and have obtained evidence 



