The exoerythrocytic merozoites of Plasmodium fallax grown in a tissue-culture system have been investigated by negative staining and thin-sectioning techniques, and the respective results have been compared. Negative staining provided additional information, corroborated findings obtained with thin sectioning, and contributed particularly to the study of the pellicular complex of the merozoites which has been demonstrated as being composed of three layers: a thin outer membrane, a thick interrupted inner membrane, and a partial layer of microtubules. Observations made of negatively stained parasites revealed that the thick, interrupted inner membrane in thin sections is actually a labyrinthine structure and covers the entire surface of the merozoite, except at the regions of the conoid and the cytostome. The microtubules which radiate from the conoid to the posterior end demonstrated a transverse periodicity and filamental subunits parallel to the axis of the microtubule. The detailed structure of the conoid and the cytostome is also described.

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