When blastocysts lacking zonae pellucidae were transferred to the reproductive tract of female mice, several implantations were subsequently found outside the tract, in the peritoneal cavity. These were presumably derived from blastocysts which had escaped from the reproductive tract. Eggs with and without zonae pellucidae were then injected directly into the peritoneal cavity : the incidence of implantation was low, and did not lend any support to the hypothesis that absence of the zona

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... da might facilitate extra-uterine implantation. In all, eight intraperitoneal implantations were found. Two contained recognizable embryonic structures, of which one closely resembled a normal 8to 8\ m=1/ 2\day embryo.