The ultrastructure of the merozoite of Eimeria tenella has been studied by means of electron microscopy. The first-generation merozoite is approximately 3·4 μm in length, and 1·2 μm in width, while the second-generation merozoite is approximately 10·5 μm in length and 1·5 μm in width.
The cell wall of the merozoite consists of a double membrane, but at the anterior extremity the existence of a fluted collar gives the appearance of two narrow double membranes separated by a zone of less electron-dense material. Twenty-four surface fibrils are distributed around the periphery; they extend along the entire length of the organism and lie beneath the double limiting membrane.
The anterior end of the merozoite is distinguished by a conoid apparatus which includes several components. A bulbiform outer annulus is invested by a fluted collar, and itself encloses an extrusible papilla. Two osmiophilic fibrils, the paired organelles, arise within the extrusible papilla and extend longitudinally into the cytoplasm. Twenty-four smaller fibrils, or toxonemes, also arise within the conoid and pass back into the main body of the organism.
The cytoplasm of the merozoite includes mitochondria, glycogen, dense elliptical granules and endoplasmic reticulum, together with a definite Golgi complex. A nucleus is located in the posterior third of the organism and is enclosed by a perforated double membrane. At the posterior extremity the double membrane which bounds the organism is broken by a pore 700 A in diameter.
Our sincere thanks are due to Mr P. Richmond for technical assistance, and we are grateful to the Central Veterinary Research Laboratory, Weybridge, for supplying the strain of Eimeria tenella.