Maturing and mature exoerythrocytic schizonts of Plasmodium berghei yoelii in the livers of laboratory-bred mice have been studied by electron microscopy.

Merozoites are produced by budding from pseudocytomeres. At the tip of each bud, a thickened region of pellicle appears to give rise to the apical rings and conoid; below these structures the paired organelle is formed. As development proceeds, the nucleus enters the base of the bud. Alongside the nucleus is an aggregation of smooth membranes. As no typical mitochondrion was seen, we suggest that this membranous structure may be its analogue.

In the pear-shaped free merozoites, there is, in addition to the above structures, a cytostome (“micropyle”), probably non-functional at this stage. The cytoplasm contains large, closely packed ribosomes; in the nucleoplasm are both fine and coarse granular elements. Subpellicular microtubules were not seen.

The development and structure of the exoerythrocytic merozoite resemble those described by other writers for avian malaria parasites, apart from the apparent absence of mitochondria and subpellicular microtubules.

After penetrating erythrocytes, the parasites lose one of their membranes, the apical rings, conoid and paired organelle; phagotropy commences and smooth and granular endoplasmic reticulum becomes prominent. In many of these parasites the nucleus is lobulated as if dividing.

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