Imaginal discs are hollow sacs of cells that make adult structures during metamorphosis. They are so named because “imago” is the old term for an adult insect, and their shape is discoid (i.e., flat and round like a deflated balloon). They arise as pockets in the embryonic ectoderm and grow inside the body cavity until the larva becomes a pupa, at which point they turn inside out (“evaginate”) to form the body wall and appendages. In a D. melanogaster larva there are 19 discs (Fig. 1.1). Nine pairs form the head and thorax, and a medial disc forms the genitalia. The abdominal epidermis comes from separate cell clusters called “histoblast nests”. Unlike discs, histoblast nests remain superficial during larval life and do not grow until the pupal stage.

Given the diversity of cell types in the adult skin (e.g., bristles, sensilla, photoreceptors) and the commonality of their descent from one progenitor (the fertilized egg), it is natural to ask how cells specialize to adopt divergent roles. In principle, cells can acquire instructions from ancestors or contemporaries. More specifically, a cell can inherit predispositions from its mother (“intrinsic” mode), take cues from neighbors (“extrinsic” mode), or both. The predispositions could be gene states, while the cues could be diffusible ligands.

To the extent that fates are assigned intrinsically, there should be a rigid correspondence between (1) parts of the anatomy and (2) branches of the lineage tree.