The interaction between the trophoblast and the maternal epithelium at early implantation was studied by means of light and electron microscopy. The uterine horns were fixedin situand a double-embedding method was used to locate implantation sites. Observations were made on mice killed at 2 hour intervals 90–116 h.post coitumwhich covered the following stages: pre-attachment (i) with zona pellucida intact (ii) with zona pellucida in dissolution (iii) after loss of the zona; attachment; adherence; and invasio
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