Complex primordium that is formed during gastrulation (stage 6-7), when anlagen invaginate from the monolayer of the cellular blastoderm (Campos-Ortega and Hartenstein, 1997; Hartenstein, 2004). The invaginated anlagen become organized into the middle two layers of a three-layered tube structure (Campos-Ortega and Hartenstein, 1997). These three layers correspond to the three germ layers of the endoderm, mesoderm and ectoderm (from inner to outer, respectively), with each layer having distinct cell division patterns and cell fates (Campos-Ortega and Hartenstein, 1997).