Endocytosis is the process by which a cell ingests material (solute, paniculate, living cells) and encloses it in a membrane-limited vacuole, the endosome. Protozoa may ingest or they may become ingested. In the latter case they may serve as food for the ingesting cell or they may, as parasites, survive and multiply at the expense of the ingesting cell. The present symposium was concerned with two topics: (1) endocytosis by free living protozoa, and (2) endocytosis of parasitic protozoa. In both cases the initial result of endocytosis is the formation of an endosome, whereas the further fate of the ingested contents differs (Fig. 1); in the first instance the contents are digested by the action of hydrolytic (lysosomal) enzymes and in the second instance the ingested parasite avoids digestion and becomes adapted to parasitism.
