1. Between the second larval and the first pupal stage,
Drosophila virilis of the vestigial genotype possess a substance, probably water-soluble, which functions directly in the production of the vestigial phenotypes, and which has been càlled the “vg-substance” by the author.
2. The concentration of one mashed third-stage larva of the normal vestigial type in 10cc of the ordinary food was arbitrarily designated as one unit.
3. The sensitive period for the vg-substance extends from the second larval to the first pupal stage.
4. Depending upon the units of vg-substance fed to the larvae of this period, the phenotypes of the vestigial alleles, namely, strap, antlered, vestigial, and no-wing, were induced in Wild flies in varying numbers. In like mánner, wings phenotypically simulating those of higher members of the series were called forth in flies of a lower grade.
5. An increase in temperature during the larval stage of vestigial flies lengthens the wing through all the intermediate stages up to the Wild-type.
6. The sensitive period for this temperature shock was found to coincide with that for the vg-substance, that is, between the second larval and the first pupal stage.
7. The relation between the vg-substance and heat is directly antagonistic. The higher the temperature, the greater is its destruction of the vg-substance until at 32°C. the destruction is generally almost complete.
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