Muscle cells in a nerve trunk of a frog muscle.

Three muscle fibers were identified by electron microscopy within a nerve of a frog muscle. They resembled extrafusal muscle fibers but were located in an endoneurial rather than in an endomysial compartment. To call these endoneurial muscle fibers the obvious continuation of extrafusal fibers of a muscle spindle is certainly unwarranted; to label these fibers ectopic and to let the matter rest there is probably an understatement of sorts.