Formation of monozygotic twins: when does it occur?

An individual embryo from day 15 of gestation in the rhesus monkey showed partial separation of the cranial end of the embryonic shield into two separate cell masses with separate amnions but with a common yolk sac. Although what the fate of this conceptus might have been is not known, it demonstrates that the physical features of implantation in this primate are permissive of relatively late division of the epiblast, which suggests that monozygotic twinning may occur as late as the embryonic shield stage just prior to primitive streak formation. Copyright 2002 Elsevier Science Ltd.