1. Trophoblast development in utero in the first half of gestation is accompanied by a rise in the ploidy levels in giant cell nuclei. Giant cells constitute up to one-third of the trophoblast population over this period.

2. The expression of polyploidy is independent of the maternal environment; the extent of polyploidy and the degree to which trophoblast develops morphologically under various conditions appear to be related.

3. The following events are not necessary for the initiation of polyploidization in trophoblast: (a) hatching from the zona pellucida, (b) oestrogen sensitization, (c) implantation, (d) postimplantation changes in topology.

4. That earlier developmental events such as the formation of the blastocoel cavity may in some way trigger polyploidization of trophoblast is not inconsistent with the studies described.

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