1. The thorax of Drosophila contains two main sets of muscles, the dorsal flight-muscles and a lateral set. Among the latter are the trochanteral depressors of the legs, of which those associated with the second legs are the largest. The development of all these muscles during the pupal period is described and an account is given of the effects of the contraction of the tendons of the longitudinal muscles in producing the curvature of the mesothorax and the formation of the post-notum.

  2. The dorsal flight-muscles develop from an interaction between (a) three pairs of larval muscles; (b) mesoderm cells derived from the mesothoracic imaginal buds; (c) cells which migrate from the developing trochanteral depressors of the second leg. It is the latter cells which are responsible for splitting the three larval muscles of each side into six. The fibrillar muscles of the lateral set have a similar origin.

  3. The trochanteral depressors are tubular, not fibrillar, in structure. They are formed from mesoderm cells of the leg imaginal buds.

  4. If the imaginal bud of a first or third leg is removed from the late larva, not only the leg but also the corresponding trochanteral depressor is missing from the adult. If a second leg bud is removed, the dorsal flight-muscles and the lateral fibrillar muscles on that side also fail to develop.

  5. In extreme bithorax phenotypes, in which the metathoracic bud has formed a structure externally resembling a mesothorax, this region does not contain a second set of flight-muscles, although there are sometimes small badly differentiated representations of them. The secondary scutellum is not provided with a post-notum.

  6. If the normal mesothoracic bud of one side is extirpated from a larva which is developing the bithorax phenotype, the transformed metathoracic bud may shift forward and fuse with the mesothoracic bud of the opposite side. In some such cases the metathoracic mesoderm, in association with cells from the second trochanteral depressor, has developed into flight-muscles.

  7. It is concluded that the migrating cells of the second trochanteral depressor provide a stimulus which is essential for the differentiation of the flight-muscles.

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