[Differentiation of the sartorius muscle during frog ontogeny]

Stages of morphological differentiation of muscular fibres in the musculus sartorius were studied at light optic and electron microscopic levels on tadpoles and mature frogs (Rana ridibunda). Three stages were detected in the development of the muscle--stage of mature myotubes, stage of young and mature fibres. At the 29th stage of development, the sarcomere length is statistically greater, at subsequent stages it is smaller and does not practically differ from that of mature animals. The T-tubes thickness and the A-discs width in the tadpole muscular fibres at different stages of development and those of mature animals do not differ statistically. Muscular differentiation and acquirement of features peculiar for mature animals continue after metamorphosis is completed.