A detailed comparison of the postimplantation development of normal and double-sized mouse embryos, produced by aggregating two 8-cell stage eggs, revealed that size regulation occurred in the double embryos between 5 days, 16 h post coitum (p.c.) and 6 days, 16 p.c. Size regulation occurred simultaneously in all tissues, suggesting that a single regulatory mechanism may control size in the early embryo. Size regulation appeared to be brought about by alteration in cell cycle length. There was no obvious increase in cell death in the double embryos nor an increase in the non-dividing cell population. However, colcemid treatment revealed a significant difference in mitotic index between double and control embryos over the period of size regulation. Control embryos showed a proliferative burst around 6 days, 8 p.c. which did not occur in the double embryos. It is not yet clear whether this control of proliferativeactivity in double embryos is exerted by the embryo itself or by the uterine environment.
Histological analysis also suggested that proamniotic cavity formation, which occurs before size regulation, was dependent on total cell number and not on the number of cell cycles undergone since fertilization. Proamniotic cavity formation was observed to occur at different times but at similar cell numbers in double, control and half embryos.