Spermiogenesis in Rajonchocotyle emarginata, fully formed zones of differentiation and median cytoplasmic process. A, longitudinal section of fully formed zone of differentiation showing long median cytoplasmic process at its distal extremity. B, C, side-by-side sections in zone of differentiation, with nucleus and/or mitochondrion, and median cytoplasmic process, with section of elongating mitochondrion; free flagella, with 9 + “1” trepaxonematan structure, are located along the median cytoplasmic process and parallel to it. D-G, sections of median cytoplasmic process with various numbers of peripheral microtubules. H, median cytoplasmic process with only one axoneme incorporated. I, transverse section of distal extremity of free flagella showing 9 + 0 structure. Scales in all figures: 500 nm.