Development of the eye-antenna imaginal disc and morphogenesis of the adult head in Drosophila melanogaster.

We have studied the organization and development of the eye-antenna imaginal disc of Drosophila melanogaster. We examined the pattern of gynandromorph mosaicism and determined the "sturt distances" between 42 different structures of the head, antenna, and maxillary palpus. A morphogenetic map based on these sturt distances resembles more closely in size and shape that of a single thoracic segment than that of two or more adjacent segments, suggesting that the eye-antenna disc is derived from a single embryonic body segment. We examined the morphology of the eye-antenna discs in situ in late third-instar larvae in serial cross sections. The two discs are connected medially by a thin cellular membrane that probably serves to join the two discs during evagination and morphogenesis of the adult head. A fate map of the imaginal disc was established by cutting the mature disc into fragments and transplanting the fragments into host larvae for metamorphosis. The peripodial layer of the eye-antenna disc is thickened in several regions, and our data suggest that these thickened areas represent primordia of adult head structures. A comparison of the location of precursors in the imaginal disc with those of the differentiated structures of the adult head revealed the nature of the morphogenetic movements that must occur during evagination and differentiation.