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Cell cultures of embryonic bovine liver, lamb kidney, whole ground-squirrel embryos and adult Madin-Darby bovine kidney were used to study the in vitro development of Eimeria larimerensis from the Uinta ground-squirrel, Spermophilus armatus. Leighton tubes were inoculated with sporozoites and the monolayers were examined with phase-contrast microscopy at various intervals; fixed and stained material also was studied. In all cell types, sporozoites penetrated and developed to trinucleate second-generation schizonts. The intracellular sporozoites increased in size and underwent nuclear divisions. The sporozoite-shaped schizonts had motility similar to that of the sporozoites. After the formation of 4–10 nuclei, transformation from the sporozoite-shaped schizont to a spheroidal schizont occurred by a lateral outpocketing in the anterior region or a progressive increase in width of the body. In 10 specimens, completion of the transformation process at 37 C required an average of 57 min. The greatest numbers of spheroidal schizonts were seen at 48 hr. Merozoites were apparently formed by a radial budding process, which required an average of 72 min at 37 C. The largest numbers of mature first-generation schizonts were seen at 60 hr after inoculation; these had 4–36 merozoites. Each merozoite had anterior and posterior refractile bodies; some also had small posterior refractile granules. The refractile bodies of the merozoite were derived from the posterior refractile body of the sporozoite. Second-generation trophozoites were seen at 60 hr, and second-generation bi- and trinucleate schizonts were seen at 60 and 72 hr after inoculation.

The fine structure of the macrogametes of coccidia was investigated, described and diagrammatically depicted. A comparative analysis was made of representative species belonging to various genera. The following species were investigated: Eucoccidium dinophili, Aggregata eberthi, Eimeria perforans, E. stiedae, E. falciformis, E. bovis, E. auburnensis, E. tenella, E. maxima, Toxoplasma gondii, and Klossia helicina. The macrogametes of most of these Eimeria species are relatively uniform in their fine structure, but E. falciformis shows some differences and the macrogametes of T. gondii are also different in some respects. In the macrogametes of E. dinophili, A. eberthi, and K. helicina are found a number of special structures not seen in the others.

The microgametogenesis of Eimeria auburnensis from experimentally infected calves killed 16 to 18 days after inoculation was investigated with the electron microscope. Early microgametocytes had nuclei with compact nucleoli and relatively few electron-dense masses. In this stage, deep invaginations from the surface evidently served for the intake of nutrients from the parasitophorous vacuole. In older stages, the nuclei had no compact nucleoli and had more elctrondense material. An intranuclear fiber apparatus was present in some nuclei, apparently in an early stage of division. Nuclear division appeared in electron micrographs to occur by a kind of fission, in which the intranuclear fiber apparatus may participate. Numerous fissures appeared in the interior of the microgametocyte and the nuclei were arranged in irregular rows in association with these. Centrioles, either single or double, were observed between the nuclei and the membrane lining the fissures. Such nuclei often had an intranuclear fiber apparatus; one of the osmiophilic poles of this apparatus protruded outward from the nucleus in the vicinity of the centriole. Directly over this pole, electron-dense material, probably representing the anlage of the perforatorium, occurred immediately beneath the surface membrane of the fissure. As many as 9 closely arranged micropores were observed in this membrane in some specimens. In the nearly mature microgamete, the basal bodies of the flagella lay at the anterior end; the mitochondrion, with numerous, regularly arranged tubules began slightly posterior to this. The strongly osmiophilic, condensed nucleus was a little farther posterior; this remained connected by a narrow stalk with the uncondensed portion of the nucleus in the residual cytoplasm until the microgametes were almost completely mature. Usually, 2 flagella were present. Rarely, 3 flagella were observed, and evidence of a rudimentary middle flagellum was found in a number of specimens.

Mature macrogametes of Eimeria bovis in the mucosa of the colon of calves killed 17 days after inoculation had a characteristic fine structure. Numerous osmiophilic wall-forming bodies of type I were located peripherally in the macrogamete, whereas the wall-forming bodies of type II were relatively large and located more centrally. Numerous polysaccharide bodies and lipid inclusions were also present. The parasitophorous vacuole, when present, was narrow and had electron-pale contents. The host cell cytoplasm was vacuolated; in some specimens the host cell appeared to have disintegrated. A microgamete was observed within a vacuole in the cytoplasm of a macrogamete from a calf killed 19 days after inoculation. This microgamete had two flagella and a homogeneous osmiophilic nucleus. The wall-forming bodies of type I and type II were altered in this macrogamete as compared with those of mature macrogametes, indicating that oocyst wall formation had begun. Many microgametes were observed in the cytoplasm of the host cell as well as in the parasitophorous vacuole of the macrogamete.

The fine structure of the microgamonts and microgametes of coccidia and related groups was investigated and is herein described and diagrammatically depicted. Particular attention is given to the developmental processes during the differentiation of the microgametes. The species included in the study are: Eimeria perforans, E. maxima, E. tenella, E. auburnensis, E. falciformis, and Toxoplasma gondii. A comparative analysis of the developmental processes as well as the fine structure of the microgametes of these species and those of other Sporozoa is presented.