Lethal no-differentiation and the development of the imaginal discs during the larval stage inDrosophilia.

1. Lethal no-differentiation, symboll-nd, is characterised by the failure of some or all of the imaginal buds to continue differentiation during the larval third instar. This is due to abnormal proliferation of the imaginal disc mesoderm. 2. There is a well-defined sequence in the sensitivity of the different organs to the lethal effect, the legs being the most commonly affected, and the genital disc the least commonly. 3. The abnormal proliferation of the mesoderm is preceded by degenerative changes in the lymph glands. 4. The mechanism of differentiation of the imaginal buds is discussed and it is argued that the organisation of the imaginal organs is determined primarily by the mesoderm. 5. It is suggested that the proliferation of the imaginal bud mesoderm is normally inhibited by a "bud-differentiation hormone" secreted by the lymph glands. The requirements of the different imaginal discs from this postulated hormone were represented by a histogram, in fig. 14.