Fig. 4.

Photomicrographs of sections reacted for ATPase activity in male laryngeal muscle at three developmental stages. An example of a small dark muscle fiber (slow twitch) is indicated (arrow) at early developmental stages. A larger light muscle fiber (fast twitch) is also indicated (arrowhead) at each stage. At the end of metamorphosis (top, 9 mg larynx), male laryngeal muscle is composed of a heterogeneous population of fibers; 51% are slow twitch. During masculinization (middle, 102 mg larynx), male laryngeal muscle has 21% slow twitch fibers. In adulthood (bottom, 517 mg larynx), male laryngeal muscle is composed only of fast twitch fibers, no slow twitch fibers are present. Calibration bar: 10 μm.