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Extirpations of the dorsal mesothoracic disc carried out by Pantelouris and Waddington in mature larvae ofDrosophila melanogaster resulted in the formation of mesonotal chaetae on the operated side in 4 out of 20 cases. This experiment was repeated on a large scale in both mature and 72-hour third instar larvae. Special attention was paid to recording the degree of disc removal in these operations. Among the 137 survivors of the extirpations, 126 (92%) developed no more than one wing and one-half of the mesonotum. The 11 cases in which mesonotal chaetae or wing structures developed on the operated side all (except an unclassified specimen) belonged to classes in which partial extirpation of the disc had been scored. The structures which differentiated on the extirpated side generally conformed to the specific presumptive parts within the disc anlage which remained inside the larval body in the various extirpation classes. Both observations indicate that the structures which developed on the operated side arose from remnants of the partially extirpated disc. In contrast to the suggestion of Pantelouris and Waddington, evidence has been presented which indicates that the dorsal mesothoracic discs do not exist as members of a bicentric regulative field. Instead, it is concluded that determination of the pair of wing discs as right and left entities is complete, not only in mature larvae, but also in larvae in the middle of the third instar. Similar conclusions are drawn with respect to the paired eye discs. Among specimens in which chaetae did not develop on the operated side, regulation limited to cell proliferation and epidermal spreading may have occurred in 2 individuals in the Pantelouris and Waddington experiment and in 9 cases in the present study. This growth might be interpreted as a homeostatic response of trichome anlagen to absence of cellular material from the operated disc at the time of normal fusion of mesothoracic disc products.

Distal regions or “marginal ridge” areas of the dorsal mesothoracic disc of mature Drosophila larvae, not previously included in fate maps, were marked by microcauterization in situ. Structures assumed to have primordia in the distal half of the disc (mesopleura, pteropleura, arch, ventral scutellum, ventral wing hinge and wing proper) are either missing, deficient, or damaged among adults surviving the operation. In contrast, neither the dorsal wing hinge nor mesonotum, which are known to have primordia in the proximal half of the disc, are similarly affected. The pattern of distribution of deformities in surviving adults indicates that (a) the anlage of the mesopleural sclerite is located in the distal edge portion or “marginal ridge” of the disc, (b) anlagen of pteropleura, arch and ventral scutellum may also be tentatively assigned to this location, (c) there are significantly high associations between deformities affecting the pteropleura and the ventral wing hinge, and (d) the anlage of the deep phragma of the postnotum may be more closely related to the first abdominal tergite than to the dorsal mesothoracic disc. The distal localization of pleural, arch, and ventral scutellar anlagen corresponds to the physical separation from mesonotal anlagen predicted from an earlier statistical analysis of genetic mosaics; i.e., these presumptive sclerites are separated from the mesonotal anlagen by the anlagen of the wing in the “pouch” area of the disc. The new information has been incorporated into a hypothetical fate map of the disc, which includes anlagen plans suggested by previous authors.

The ultrastructure of the developing pupal leg ofDrosophila melanogaster is investigated at various stages from white prepupa to pharate adult. All the different cell types found in the leg at each stage examined are traced developmentally. This includes general epithelium which secretes both a pupal and adult cuticle and specialized epithelial structures such as apodemes, bristles and other sense organs. Muscle, nerve and tracheal tissue develop within the leg cavity. Particular attention was paid to the development of muscle tissue which was shown to differentiate from the adepithelial cells of the larval disc. The development of theDrosophila leg is compared with that ofCalliphora (van Ruiten and Sprey, 1974) especially with respect to the different stages at which new structures appear in the two species.