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The fascinating work that Marshall reviews in this monograph has introduced the reader to some of the most recent advances in the study of pinocytosis. It is my purpose to report some of the studies that have been carried out a t the Carlsberg Laboratory, and also to relate some results obtained in other laboratories. Although several of these results have been gained by means of organisms other than amoebae, it was necessary to include them because they are of the greatest significance for the general problem of pinocytosis.T feel that I cannot begin my paper in a more fitting way than by paying respect to the pioneers in the field, Warren Lewis, S. 0. Mast, and W. L. Doyle, who twenty-five years ago discovered and analyzed the process of pinocytosis in mammalian tissue culture cells and in amoebae. The more I see of pinocytosis, the more I admire the accuracy 01 these scientists' early observations, obtained without phase contrast and without labeled tracers. The discoveries of these workers passed almost unnoticed for a long time. This sounds amazing, but the reason I think, is fairly simple: pinocytosis is difficult to study without techniques that have been developed in recent years.The first problem I propose to discuss is whether we are right in t.hinking of pinocytosis primarily as an ingestion of fluid. The very word pinocytosis, as coined by Lewis, is derived from TLVELV, which means to drink, and it certainly emphasizes this side of the concept. However, it is becoming more and more clear that the substances that are dissolved in the fluid are even more crucial for pinocytosis than the fluid itself. I n order to discuss this question properly,

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