Abstract
Unimplanted blastocysts decrease in viability in the absence of progesterone (Weitlauf & Greenwald, 1968). Progesterone may act by causing the uterine secretion of essential nutrients (Weitlauf, 1971). However, the nutritional condition of the blastocysts is influenced both by secretions from the uterine epithelium and by the relationship between the epithelium and blastocysts. The present paper reports transmission (TEM) and scanning electron microscopy (SEM) of uterine epithelium and blastocysts during delayed implantation, with and without administration of progesterone. The consequences of the findings for the nutrition of the blastocyst are evaluated. Virgin female albino NMRI mice (20–25 g body weight) were bilaterally ovariectomized under ether anaesthesia between 12.00 and 15.00 h on day 3 of pregnancy (day 1 = day on which a vaginal plug was found). They were then given either 1 mg progesterone (Leo, Helsingborg, Sweden) in 0·04 ml peanut oil, or the vehicle alone. On day 18 the uteri

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