Figure 2.

Mouse blastocysts from time-lapse studies. Embryos in the top row (A–F) are zona-intact control embryos. Three groups of breaker cells are visible in the embryo in (A) at 1, 2 and 4 o'clock positions. The embryo in (B) is hatching, but a second group of breaker cells is seen at 11 o'clock. Three sites of hatching are seen in the embryo in (C). Typical hatching configurations are seen in (D) and (F). The middle row of pictures (G–L) shows embryos in which zonae were opened as illustrated in (G). Embryos (H–L) are hatching through the artificial hole without secondary groups of breaker cells. The zonae remain thick in all cases. The lower row shows embryos after zona thinning. Embryos (M–O) are hatching through non-thinned areas. Artificially thinned areas are indicated by arrows. The embryo in (P) is hatching from three areas and the cells at 2 o'clock are emerging through the thinned area. The embryo in (Q) (and the zona after it was isolated by manipulation) represents an anomaly seen after zona thinning. The embryo is trapped in the zona pellucida after hatching started in an area outside the thinned zone. The scale bar in (A) applies also to (B)–(Q). The scale bar in (R) applies only to (R).