In the blastoderm [stage 5], the prospective mesoderm occupies a midventral stripe of about 18 cell diameters in width. This region invaginates during gastrulation [stages 6-7] and gives rise to the mesoderm (ms; Poulson 1950; see Chasan and Anderson; Costa et al.; Bate; all this volume).

In the stage 8 embryo, the mesoderm forms a dorsal-ventrally flattened tube that is connected to the ventral ectoderm via two rows of cells, called mesectoderm (mec; see Goodman and Doe, this volume).

At the transition to stage 9, mesoderm cells rearrange and form a monolayer. In the ventral midline, this monolayer still contacts the mesectoderm. Mesoderm cells remain in the monolayered configuration until late stage 10 when they split up into different organ primordia. During stages 8-11, the mesoderm undergoes three waves of mitoses (Hartenstein and Campos-Ortega 1985). Another mitosis is seen in most, if not all, mesoderm cells during stage 12 (see Bate, this volume). The anterior part of the mesoderm (head mesoderm, hms) forms two vertical plates flanking the anterior midgut rudiment and, from stage 10 onward, the stomodeum. These plates lose their contact to the mesoderm of the trunk during later stages and move into the anterior tip of the head where they give rise to the musculature of the head and macrophages.

From early stage 12 onward, the mesoderm splits up into separate cell masses that give rise to the somatic musculature, visceral musculature, dorsal vessel, and fat body. The development of these organs is described separately in the following sections (also see Bate, this volume).