Contributions to the study of monocystid gregarines parasites ofOctolasium transpadanum. II. The fine organization ofApolocystissp. oocyst.

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Abstract
Contributions to the study of monocystid gregarines parasites of Octolasium transpadanum. II. The fine organization of Apolocystis sp. oocyst. The infrastructure is described of the developmental stages of Apolocystis sp. oocysts. The cyst, where all phases of sporogenesis took place, was spherical (Ø 45 μm); its wall of two thin dense layers (12.5 nm) was surrounded by flattened host cells. The early oocyst was roundish (Ø 5 μm), having a wall 170 nm thick made up of two layers: an outer like a thick “fuzzy coat”, and a thin inner one of dense material. A semilunar granular mass was situated between the plasmalemma and the wall. The single spherical nucleus was in meiotic prophase, the spindle being intranuclear. After meiosis the four-nucleated developing oocyst became elongated (8μm x 5 μm) and presented the granular semilunar mass at one of the poles; the wall and the cytoplasmic inclusions seemed the same as those found in the early oocyst. After postmeiotic mitosis the eight-nucleated oocyst assumed a typical boat shape (13 μm x 5 μm). The fully developed wall was about 350 nm thick and composed of three layers; the middle one might be made by the semilunar granular mass. Oocysts at this stage became impermeable. Nuclei were arranged around a large central vacuole and the individualization through endopolygeny of the sporozoites began. In mature oocysts containing eight sporozoites, the wall maintained the same appearance and structure as in the penultimate stage. Each sporozoite was invested in a three-membrane pellicle. We observed no specialized apical structures in sporozoites nor differentiated wall structures for oocyst dehiscence.