Transheterozygotes are female sterile, exhibit no cuticular, testis or premeiotic defects but do exhibit some elevation of mitotic index and a low frequency of abnormal mitosis in the brain. Transheterozygotes with mgr^DD1 and asp^DD1 are viable but female sterile. Embryos derived from heterozygous mothers fall into two classes; some show few signs of nuclear divisions and die early, the remainder develop defects in the nuclear divisions in the cortex of the embryo, many undergo cellularisation and undergo gastrulation movements, but none ever develop into larvae.