Summary: The spore of from the tissues of has a wall consisting principally of electron-dense exospore and electron-lucent endospore. The polar filament, composed of two membranes around an amorphous or tubular core, described eleven coils in the posterior half of the spore and joined anteriorly with the umbrella-shaped polar sac. The polaroplast consisted of close-packed single membranes and sacs disposed around the anterior straight part of the filament. There was no posterior vacuole. There were two nuclei in a central area of cytoplasm containing abundant ribosomes and cysternae of endoplasmic reticulum. The cytoplasm beneath the plasmalemma and around the coils of the polar filament lacked ribosomes. Various hypotheses for filament extrusion are discussed in the light of the present observations. It is proposed that the positions of the outer membranes of the polar filament are inverted during extrusion and that the core becomes a coat to the filament. Some of the cytoplasm, together with the nuclei, passes through the filament and probably acquires a membrane from the sealed tip of the filament to become the sporoplasm. The polaroplast does not participate structurally in extension of the filament.


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