SYNOPSIS. Thin sections of Plasmodium, cathemerium obtained 3 to 10 days after ingestion of blood by mosquitoes were examined by electron microscopy. Oocysts are encased in a relatively thick structureless capsule which becomes progressively thinner with advancing age and completely disappears in places at the time of release of the sporozoites. The capsule blends with and in most cases cannot be distinguished from the “elastic” layer of the mosquito stomach. For this reason the capsule is tentatively regarded as a product of the host rather than of the parasite. Capsular material appears to be shed into the interior of the oocyst and may serve as nutrient material in the later stages of development. Oöcysts and sporozoites contain mitochondria, endoplasmic reticulum and nuclei with nucleoli at all stages examined. All limiting membranes within the oocyst are feebly developed and no cell boundaries are seen until the sporozoites begin to form. At this stage, the sporozoites acquire very prominent surface membranes which are retained by the mature forms. Sporozoites contain long rods and cylinders of homogeneous material of moderate density. Their functional significance is not understood.

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