The embryonic organization of the imaginal wing disc ofDrosophila melanogaster.

1. Gynandromorphs were recovered fromIn(1) X ,w /y Hw f females. The distribution and frequency of the adult male and female areas indicate a very early loss of theIn(1) X ,w chromosome. An analysis of mosaicism of the dorsal thoracic structures was carried out in order to derive a morphogenetic map of the location of their presumptive cells. It was constructed using 12 landmarks in the notum, 16 in the wing surface, the humerus and the haltere. 2. By several methods, the minimal number of cells populating the dorsal mesothoracic anlage was estimated to be about 40. 3. A map of the distances in trichomes-adult cells-between landmarks in the notum and the wing is presented. A comparison of the morphogenetic map and the adult map shows regional differences in growth. Whereas in the notum growth is isodiametrical, in the wing growth is preferentially along the proximo-distal axis of the anlage. However, the calculated mean mitotic rate is the same in both wing and notum. 4. Mosaicism in the notum is undetermined with respect to the adult structures. In the wing a virtual line along vein IV separates two wing regions, anterior and posterior, with different morphogenetic characteristics. 5. Possible morphogenetic mechanisms controlling the mesothoracic disc development are discussed.