The first appearance of muscle spindles in the iliofibularis muscle of the frog during metamorphosis

Abstract The iliofibularis muscle of the frog was studied histologically at different developmental stages during metamorphosis in order to determine when muscle spindles first appear. The earliest stage at which spindles were positively identified was stage XVIII, and they appear relatively complete as judged by their morphology. An analysis of the reports of variations in the numbers of neurons in frog spinal ganglia suggest that muscle spindles may in fact be induced as early as stage X.