Middle germ layer of the embryo (between ectoderm and endoderm) following gastrulation (Campos-Ortega and Hartenstein, 1997). Shortly after its formation, its cells undergo mitosis and it loses its regular epithelial architecture (Campos-Ortega and Hartenstein, 1997). In stage 9 it reorganizes into a regular monolayer of cuboidal cells that lines the ectoderm (Campos-Ortega and Hartenstein, 1997). At the end of stage 10 further cell divisions disrupt this structure and its cells reform into two layers in stage 11 (Campos-Ortega and Hartenstein, 1997). It finally splits up into separate cell masses in late stage 11 (Campos-Ortega and Hartenstein, 1997). Its derivatives include the muscles, heart and fat body (Campos-Ortega and Hartenstein, 1997).