Fig. 2

Transmission electron micrographs of flagellar segments from 10 species in cross-sectional view. a Diagram detailing key ultrastructures, including axoneme, ODFs, fibrous sheath, and mitochondria (red modes), in three major structural domains. The outer area is indicated as the area surrounded by the blue line; the ODF area is indicated as the area around by the purple line; the area of the axoneme is indicated as the area around by the yellow line; and the green mode indicates the FS area in the principal piece. The numbers show the numbers of odf fibers 1–9. b Human. c Rabbit. d Dog. e Pig. f Guinea pig. g Goat. h Bull. i Mouse. j Golden hamster. k Rat. The measured areas were labelled by coloured lines, as shown in panel J. The number of measured segments was more than 30 in each species. The scale bars are 400 nm