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The mature labial disc, when implanted into a larva of the same age, undergoes metamorphosis along with the host and produces one lateral half of the medi- and distiproboscis. On the basis of results obtained from transplanted disc halves (including the separate peripodial membrane) a tentative fate map of the labial disc was constructed, which shows most of the presumptive mediproboscis to be located in the dorsal, and most of the presumptive distiproboscis in the ventral part of the disc. The distal protion of the peripodial membrane also contains imaginal anlagen, viz. part of the mediproboscis, prementum, and labellar cap anlagen. The involvement of this part of the peripodial membrane was checked by a careful histological analysis of labial disc development during the first ten hours after prepupation. The results were compared with the situation described forCalliphora imaginal discs.In addition, a detailed morphological analysis was made of the proboscis of the homoeotic mutantproboscipedia (pb). At 27°C,pb changes the distiproboscis into a "telopodite" (leg segments distal to the coxa); the (unchanged) prementum may therefore correspond to the coxa. At 15° C, the tarsus of this homoeotic "telopodite" is replaced to a greater or lesser extent by an arista. The present analysis thus confirms (a) the fundamental morphological correspondence of the medi- and distiproboscis with the labium of other insects, and (b) the fundamental developmental correspondence of the labial, antennal, and leg discs.

The mature haltere disc, when implanted into a larva of the same age, undergoes metamorphosis along with the host, and produces a haltere, haltere sclerite, notum III, and pleural process. Implants ordinarily form an inverted vesicle, into which the haltere is frequently everted, usually only partly. Many "adventitious bristles" may be formed which are not foundin situ. It was found thatsc may reduce, andmwh may increase the number of sensilla.On the basis of results obtained from transplanted disc fragments an organ map of the haltere disc was constructed, which shows the presumptive haltere area to be located in the posterio-central part of the disc, while the anterio-medial part forms the thoracic structures dorsal to the haltere, and the posterio-lateral part the thoracic structures ventral to it. The anlage of the "adventitious bristles" is probably within the notum area. The haltere anlage proper is concentric; the prospective distal capitellum is located in the center, and is surrounded by the anlagen of the more proximal segments.The organ map of the haltere disc is compared with that of other discs. Organ maps seem to be basically concentric. There is a close homology between the maps of the wing and the haltere disc.

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1. The wild-type mature eye disc, when implanted into a larva of the same age, undergoes metamorphosis along with the host, and produces exactly one half of the head capsule outside the frontal suture. Implants ofopht eye discs form fewer facets, but many additional vibrissae, and produce homoeotic wing tissue (Fig. 1) in 20% of the cases. 2. On the basis of results with fragmented discs an organ map of the wild-type eye disc was constructed (Fig. 3), showing the presumptive eye area to be located in the flat part of the disc, surrounded by the primordia of the hypodermal head structures in the peripheral folds of the disc. 3. Probably the whole presumptive eye area inopht discs is capable of wing formation; the protuberances later giving rise to wing outgrowths are localized in this eye area. The presumptive wing area was more closely determined by separate transplantation of protuberances. The anlagen of the supernumerary vibrissae inopht discs are located in the anterior border of the disc (Fig. 4). 4. Very young head segments were cultured in adult, wild-type hosts for some weeks, and transplanted back into full-grown wild-type larvae. They terminated larval development in the adult hosts, and beside the derivatives of the eye disc produced additional wing tissue, that was shown to be homoeotic (Fig. 5). It was concluded that the homoeotic wing tissue is not induced in the disc by the mutant larval environment, but develops autonomously.