6l6 V. T. NACHMIAS AND J. M. MARSHALL, JR. 



Structure or function. The cell does not at any stage relinquish its control 

 over permeability; there must be, therefore, highly selective exchange 

 mechanisms operating in both directions across the vesicle membrane, or 

 between the primary vesicle and the microvesicles. 



If this general view is correct, it follows that active transport mechan- 

 isms are essential elements within the overall process of pinocytosis, but 

 the two phenomena are not strictly equivalent. The debate between 

 physiologist and morphologist may be resolved by considering pinocytosis 

 (in this instance, of protein) to include the entire sequence from the initial 

 binding reaction to the point of metabolic utilization. Alternatively, it may 

 be desirable to retain the somewhat arbitrary morphological definition of 

 pinocytosis as extending Only to the formation of vesicles, and to speak of 

 digestion and assimilation as subsequent processes. In either case, the 

 mechanisms of trans-membrane exchange, and the role of the microvesicles 

 within the cytoplasm, remain to be investigated. 



Acknowledgments 



The authors are indebted to Miss Marianne Pieren and to Mrs. Diane 

 Evans for technical assistance. 



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