1. In flies homozygous for thick there is a partial failure of the contraction of the leg from the highly inflated condition characteristic of the beginning of the true prepupal period; the same is true, to a lesser degree, of the wings.
2. This is preceded, and probably caused by, an over-contraction of the longitudinal larval muscles at the time of puparium formation, which is such as to give rise to an increased fluid pressure inside the body of the animal.
3. In humpy flies there is an over-contraction of the epidermis of the wings and thorax, which is responsible for the shortening of the wings and the formation of grooves and “vortices” on the thorax.
4. At the time of puparium formation, the larval muscles of humpy contract irregularly, and a similar irregular muscular contraction plays a part in producing the deformations during the beginning of the true pupal period- The abnormal larval contractions sometimes cause modification of the normal internal anatomy, which in their turn result in irregular eversion of the legs, leading to abnormalities in their later contraction during the pupal period.
5. It is pointed out that such a mechanism reduces the difficulty of understanding the apparently dissimilar effects of the different dumpy allelomorphs, some of which produce the humpy type of wing while others produce the humpy type of thorax.
6. The relevance of the data for the general theory of development is briefly mentioned.