In the early stage 12 embryo, the mesoderm forms a loose, multilayered sheet of cells that are about to split into a number of different organ primordia (see Bate, this volume). The superficial layer (somatic mesoderm, sm) contains the myoblasts of the somatic musculature. In this layer, cellular aggregates start to appear that develop into the individual muscle fibers. Muscle fibers are syncytia formed by the fusion of a variable number of myoblasts. It has been shown that the pattern of muscle fibers is foreshadowed in the stage 12 embryo by a set of early differentiating myoblasts; throughout later stages [12-13], other myoblasts fuse to form the syncytial precursors of muscle fibers.

At the end of stage 13, fusion of myoblasts is complete. Muscle fibers form tightly packed cell masses. During later stages, the muscle fibers insert at the apodemes formed by the epidermis. As the segments stretch during dorsal closure and head involution, individual muscle fibers become distinguishable.
The stage 17 embryo schematically depicts the fully developed larval muscle pattern. Groups of muscle fibers are named according to Crossley (1978).