Xenopus embryos pass through five behavioural stages between the end of neurulation (stage 20) and the accomplishment of free swimming (stage 33). These are (I) non-motile (stage 20-22) when the myotome muscle begins to differentiate; (II) pre-motile (stage 22-24) when the first striated fibrils are visible and contractions are possible; (III) early flexure (stage 24-27) when reflex responses are given and peripheral nerves are present; (IV) early swimming (stage 28-33); and (V) free swimming

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... tage 32-46) when co-ordinated swimming is possible but the myotome muscles are still uninucleate. At the onset of metamorphosis (stage 48-50) the myotome muscle becomes multinucleate, possibly by fusion with satellite cells at the ends of the fibres, and has the appearance of adult skeletal muscle. The hind limb of Xenopus passes through similar behavioural stages but at a later stage in development: (i) non-motile (stage 48-52) when little differentiation of the limb-bud has occurred but nerves are present; (ii) pre-motile (stage 53-54) when the limb trembles and muscles are just beginning to acquire striated fibrils; (iii) motile (stage 55-58) when the limb can make stepping movements and the muscles are striated and multinucleate; and (iv) fully functional (stage 60-63) when the limbs are fully differentiated. Unlike the myotome muscle the limb muscle becomes multinucleate before striated myofibrils are assembled. By stage 60 myotome and limb muscle are similar in appearance except that the myotome muscle has larger fibres with fewer nuclei than the limb muscle. In Xenopus, myotome and limb muscle become multinucleate at more or less the same time in the development of the tadpole. In the myotome this is long after contractility and nervous control have appeared, in the limb it precedes the formation of striated fibrils and the ability to contract. It is suggested that the difference in development of the myotome and limb muscles with respect to the stage at which they become multinucleate may be due to some substance produced just before or during metamorphosis.