RYOICHI SATO and HITOSHI WATANABE : Pathways of oral infection in the four Microsporideae in the silkworm, Bombyx mori Pathways of oral infection with four Microsporideae in the 2nd instar larvae of Bombyx mori were reinvestigated. 1) Spores of N. bombycis extruded their polar filaments in every area of the mid-gut lumen. Sporoplasms and intermediate forms between sporoplasm and schizont of N. bombycis were found in the cytoplasm of the mid-gut epithelium and the circular muscle of the middle

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... rt of the mid-gut 5 and 24 hours after spore ingestion. Tow days later colonies of schizonts appeared in the cytoplasm of the cells of the mid-gut epithlium, circular muscles, longitudinal muscles, and adjacent tissues of the mid-gut.2) Sporoplasms of Nosema sp. M-11 were found in the cytoplasm of the mid-gut epithelium 6 hours after spore ingestion. Four days later, schizonts of M-11 proliferated in the mid-gut epithelium, muscle of the posterior area of the mid-gut, and their adjacent tissues. 3) Spores of Vairimorpha sp. M-12 germinated in every area of the mid-gut lumen and their sporoplasms were observed in the cytoplasm of the mid-gut epithelium and middle part of the mid-gut. Two days later, schizonts of M-12 were seen in the same tissues and sometimes in the adjacent tissues, 4) Spores of Pleistophora sp. extruded their polar filaments in every area of the mid-gut lumen. Based on these observations, the pathways of oral infection with these four Microsporideae were discussed.