Electron microscope studies of the erythrocytic forms, including gametocytcs and asexual schizonts, of the protozoa Plasmodium fallax, P. lophurae, and P. cathemerium, have revealed a "cytostome," a specialized organclle of thc pcllicular membrane which is activc in the ingestion of host cell cytoplasm. In matcrial fixed in glutaraldehyde and postfixed in OsO~, thc cytostomc appears in face view as a pore limited by two dense circular membrancs and having an inside diameter of approximately 190 m/~. In cross-section, the cytostome is a cavity boundcd on each side by two dcnse scgments corresponding to the two dcnsc circles observed in facc view; its basc consists of a single unit mcmbranc. In the process of feeding, the cytostome cavity cnlarges by expansion of its membranc, permitting a large quantity of red ccll cytoplasm to come into contact with the cytostome wall. Subsequcnt digcstion of erythrocyte cytoplasm occurs exclusively in food vacuoles which emanatc from the cytostomc invagination. As digcstion progrcsscs, the food vacuoles initially stain more densely and thcrc is a marked build-up of hcmozoin granules. In the final stagc of digcstion, a single membrane surrounds a cluster of residual pigment particles and very little of the original host cell cytoplasm remains. The cytostome in exoerythrocytic stages of P. fallax has been observed only in merozoites and does not seem to play the same role in the feeding mechanism.

The fine structure of the exoerythrocytic cycle of an avian malarial parasite, Plasmodium fallax, has been analyzed using preparations grown in a tissue culture system derived from embryonic turkey brain cells which were fixed in glutaraldehyde-OsO4. The mature merozoite, an elongated cell 3-to 4-/~ long and l-to 2-# wide, is ensheathed in a complex doublelayered pellicle. The anterior end consists of a conoid, from which emanate two lobed paired organelles and several closely associated dense bodies. A nucleus is situated in the mid portion of the cell, while a single mitochondrion wrapped around a spherical body is found in the posterior end. On the pellicle of the merozoite near the nucleus a cytostomal cavity, 80 to 100 m/~ in diameter, is located. Based on changes in fine structure, the subsequent sequence of development is divided into three phases: first, the dedifferentiation phase, in which the merozoite loses many complex structures, i.e. the conoid, paired organelles, dense bodies, spherical body, and the thick inner layers of the pellicle, and transforms into a trophozoite; second, the growth phase, which consists of many nuclear divisions as well as parallel increases in mitochondria, endoplasmic reticulum, and ribosomes; and third, the redifferentiation and cytoplasmic schizogony phase, in which the specialized organelles reappear as the new merozoites bud off from the mother schizont.

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