EMBRYONIC ORIGIN OF THE IMAGINAL DISKS OF THE HEAD OF DROSOPHILA-MELANOGASTER

The embryonic development of the primordia of the Drosophila head was studied by using an enhancer trap line expressed in these structures f rom embryonic stage 13 onward. Particular attention was given to the q uestion of how the adult head primordia relate to the larval head segm ents. The clypeo-labral bud in the stage 13 embryo is located at a lat eral position in the labrum adjacent to the labral sensory complex ('' epiphysis''). Both clypeo-labral bud and sensory complex are located a nterior to the engrailed-expression domain of the labrum. Throughout l ate embryogenesis and the larval period, the clypeo-labral bud forms i ntegral part of the epithelium lining the roof of the atrium. The labi al disc originates from the lateral labial segment adjacent to the lab ial sensory complex (''hypophysis''). It partially overlaps with the l abial en-domain. After head involution, the labial disc forms a small pocket in the ventro-lateral wall of the atrium. The eye-antenna disc develops from a relatively large territory occupying the dorso-posteri or part of the procephalic lobe, as well as parts of the dorsal gnatha l segments. Cells in this territory are greatly reduced in number by c ell death during stages 12-14. After head involution, the presumptive eye-antenna disc occupies a position in the lateral-posterior part of the dorsal pouch. Evagination of this tissue occurs during the first h ours after hatching. In the embryo, no en-expression is present in the presumptive eye-antenna disc. en-expression starts in three separate regions in the third instar larva.