The sites at which a mouse embryo sheds the zona pellucida in preparation for its hatching were investigated in vitro. Of all cases examined, 24% shedding occurred at the mural site, 24% at the inner cell mass site, 17% at the equatorial site, and 35% at other sites. It thus appears reasonable to conclude that there is no particular site destined for shedding and the site at which it does take ... [Show full abstract] place in vitro is irrelevant to that at which the uterus later becomes attached. Expressing its frequency in terms of a unit of area indicated shedding to occur markedly at the axis penetrating the inner cell mass and mural trophectoderm. The ratio of hatching frequency along this axis to that at other areas was determined as 7:1.