AbstractWe previously showed that digit formation in mouse embryos from early morning mating seemed to progress faster than those from overnight mating. In this study, to confirm this phenomenon, we examine whether the embryos from normal (0 hr from ovulation to fertilization) and delayed matings (3, 6, or 9 hr from ovulation to fertilization) respond differently to some acute teratogens when they are treated at the same time point from mating.Five mg/kg of cytosine arabinoside (Ara‐C) was given to pregnant mice intraperitoneally at 246, 249, or 252 hr (day of gestation (dg) 10) after mating. The patterns of Ara‐C induced digit malformations in embryos from the delayed mating groups were those of more advanced stages, when compared with normal mating groups with the same time intervals from mating to Ara‐C treatment. In other words, oocytes fertilized up to 9 hr after the presumed time of ovulation could grow similarly to those of normally fertilized oocytes. Thiscatch‐up phenomenonsuggests that theovulation clockshould be used for the startpoint of the time scale of the growth and differentiation of embryos rather thanfertilization clock. © 1992 Wiley