Histological determination of cell numbers in the mouse embryo between 4½ and 7½ days post coitum show that growth during this period, in which gastrulation occurs, is not uniform. Prior to primitive streak formation mean cell generation time is about 9 h. Co-incidental with the appearance of the primitive streak the embryo enters a period of rapid growth, lasting about 24 h, during which the mean cell generation time must be about 5 h in order to account for the increase in cell numbers. A more detailed study, in which variations in mitotic activity in different regions of the embryo have been analysed, has identified a small region, the so-called ‘proliferative zone’, constituting about 10% of the whole epiblast, in which cell generation time may average as little as 2–3 h over a 24 h period. The cell generation time for other epiblast regions is estimated at about 6·5 h.

It is calculated that the proliferative zone, in the 24 h period commencing with primitive streak formation, could generate about half the cells in the 7½-day embryo. The topographical consequences of such a rapidly expanding region in the embryo are discussed in the light of other, circumstantial evidence, and it is postulated that the cells generated in the PZ may constitute the ectoderm of later stage embryos.

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