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-µ long and 1- to 2-µ wide, is ensheathed in a complex double-layered 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 mµ 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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