Fine structure of tortoise skeletal muscle.

1. The fine structure of tortoise skeletal muscle has been studied with the electron microscope.2. The packing of the A filaments (1.6 mu long) and the I filaments (2.35 mu long) within the myofibril is similar to that in frog twitch fibres.3. The position of the N line within the I band is found to vary with the sarcomere length.4. The T-system tubules in these muscles form a three-dimensional network across the fibre at the level of the I band. On either side of the Z line it forms triad relationships with the other network of tubules of the sarco-plasmic reticulum.