AbstractMaleDrosophila melanogasterembryos inheriting deficiencies of theNotchseries on the X‐chromosome fail to hatch from the egg, and are characterized in part by almost complete lack of differentiation of the mesoderm. In order to determine the developmental capacities of these tissues, male embryos containing N8 and N264‐40 deficiencies were culturedin vivoin the hemocoeles of wild‐typeD. melanogasterlarvae and adults. After periods of five and 12 days, theNotchembryos were compared with similarly cultured wild‐type control embryos. The deficient embryos of both types revealed considerable growth. The mesodermal derivatives muscle, fat, and gonad sheath were well differentiated. In every case, embryos of the short deficiency, N264‐40, grew to a greater size than did those of the much longer deficiency, N8. They generally did not exceed the size attained by normal embryos cultured for the same length of time. The growth of transplants in larval hosts always exceeded that seen in adult hosts in the same period of time. Neither of the types of deficient embryos was able to molt in either type of host, although control embryos always molted in larval hosts. The results indicate that the mesodermal tissues ofNotchembryos are capable of further differentiation than they are able to exhibit in their own environment, and that the genetic materal within the extent of the N8 deficiency may be involved in their capacity to grow i