As an extension of studies on the chemical composition of rabbit blastocysts (Lutwak-Mann, 1966, 1971), and blastocyst-uterine relationships (Lutwak-Mann, Boursnell & Bennett, 1960), we have measured the content of calcium (Ca) and the uptake of 45Ca in free-lying (5- and 6-day-old) and attached (7-, 8- and 9-day-old) blastocysts, and endometrium, entire uterine wall, early placental tissue, blood plasma, peritoneal fluid and uterine fluid. Oestrous uterine fluid (Lutwak-Mann, 1962) was used because rabbit progestational secretion was too scarce for analysis.
Calcium content was determined by atomic absorption spectrophotometry using lanthanum chloride to eliminate interference from phosphates. Samples (50 to 150 mg) were incinerated with fuming nitric acid and hydrogen peroxide; the mean standard deviation of duplicates was 5 %. Some of the preliminary values for blastocysts were obtained by the method
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