Practice question

Imaginal discs are monolayer epithelial sacs invaginated during embryogenesis, proliferating throughout larval instars as diploid precursors held undifferentiated by ecdysone and juvenile hormone balance. At pupariation, ecdysone pulse triggers disc eversion, elongation, and differentiation into adult external structures: wings, legs, eyes, antennae, halters, genitalia. They do not contribute to embryonic segmentation, larval cuticle, or germline, which arises from pole cells. Disc patterning uses conserved morphogens Wingless, Decapentaplegic, and Hedgehog establishing anteroposterior and dorsoventral axes within each disc, analogous to vertebrate limb bud patterning, forming adult body after histolysis of larval tissues during metamorphosis.