Embryos of the temperature-sensitive mutant shibirets 1 were given short exposures to the restrictive temperature during the stage when neuroblasts segregate from the presumptive epidermis. The resulting lethal phenotype, expansion of the nervous system at the expense of the epidermis, is characteristic of a group of mutants called neurogenic mutants. Exposures as short as 20 min were sufficient to promote the neurogenic phenotype. Cell masses from heat-pulsed embryos could be cultured in vivo as tumorous masses which retained some characteristics of neural tissue. An examination of the neurogenic region from heat-pulsed embryos revealed numerous packets of extracellular vesicles and coated pits blocked in endocytosis.