The relationship between spiral patterns in the inner wall of the micropyle and attaching and non-attaching filaments on the chorion was investigated by electron microscopy in eggs of the medaka, Oryzias latipes. There were both right- and left-handed spiral patterns of attaching filaments that arose from the chorion at the vegetal pole area. In the majority of eggs examined, however, the prominent folds in the inner wall of the micropylar vestibule descended toward the micropylar canal in a left-handed spiral. This spiral pattern in the micropyle may reflect surface indentations in the cytoplasmic extension of the micropylar cell during formation of the micropyle apparatus. The present observations suggest that the patterns of these spiral structures of the chorion are casused by the rotation of the oocyte due to the movement of follicular cells during oogenesis. © 1993 Wiley-Liss, Inc.