The Effect on Development of a Lethal Deficiency in Drosophila Melanogaster: With a Description of the Normal Embryo at the Time of Hatching.

ECAUSE, as a rule, lethal genotypes have theireffect during emB bryogeny, where the tempo of development and the magnitude of the structural changes are most marked, they are particularly useful in investigating the role of the genes in development. Furthermore, a study of early-acting lethals offers the possibility of increasing our knowledge of the normal embryology of forms about which little is known at present. Investigations of the nature of lethal disturbances have involved two questions: I) whether the effect can be primarily ascribed to a single structure or group of structures which, because of their pathological condition, prevent further development, or; 2) whether the effect appears to be of a general character, expressed as a disturbance of the dynamics of development which may lead to malformations. The study reported below deals with a deficiency of the X chromosome in Drosophila melanogaster which is lethal when hemizygous. This lethal is interesting because it terminates development a t the very end of the egg stage, thus affording an opportunity of studying organs and anlage which are already well differentiated. Since the normal animal at this stage has not been described in any detail in the literature, an account of the developmental phenomena, and of the anatomy of the larva just prior to hatching has been included for comparison with the lethal embryos.