Mouse blastocysts were cultured for 24 h in the presence of various substances at concentrations as high as possible, consistent with blastocyst survival. Lysis of the zona pellucida was produced by trypsin and pronase, but not by hyaluronidase, collagenase, β-glucuroni-dase, ribonuclease, neuraminidase or oestrogen. This suggests that the zona lysin secreted by the mouse uterus at the time of implantation consists of a proteolytic enzyme.

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