A sequential study was made of non-delayed and of experimentally delayed rat blastocysts. Alterations in shape, axis lengths and area of non-delayed blastocysts first appeared on the afternoon of Day 5 of pregnancy. Similar changes were observed in delayed blastocysts beginning 12 hr after the simultaneous administration of oestrone and progesterone. In both cases implantation occurred within the subsequent 24 hr. In contrast, delayed blastocysts maintained only with progesterone were markedly

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... arger than normal but failed to demonstrate typical pre-implantation changes or to implant. Therefore it was concluded that changes necessary for and indicative of impending implantation are induced by the synergistic action of oestrone and progesterone on the blastocyst. Blastocysts from non-delayed and delayed, oestrone-treated animals were consistently free of their zonae pellucidae at least 18 hr prior to implantation\p=m-\6 hr after pre-implantation changes were initially noted. Progesterone, although it appeared to influence zonae-loss during the period of delayed nidation, was ineffective in accelerating and synchronizing the shedding process unless associated with oestrone.