Final answer:
Twinning can occur as early as the two-cell stage or during the early blastocyst stage in zygote development, resulting in monozygotic twins with either separate or shared placentas, respectively. Additionally, twinning is conceptually mirrored in meiosis during prophase I when chromosomes exchange segments at the chiasmata.
Step-by-step explanation:
Twinning can start to occur at various points in early development. The first scenario of twinning occurs when a zygote divides into two separate offspring during the early development, particularly as early as the two-cell stage. In this case, the resulting twin embryos will have individual placentas. The second common point is during the early blastocyst stage, with approximately 70-100 cells present, where the twin embryos will usually share a placenta and a chorionic cavity. Finally, twinning can also be considered at a broader level in meiosis during prophase I, when the chromosomes pair up and form tetrads, with the exchange of chromosome segments occurring at the chiasmata.