Close-up view of a stage 3 embryo in vitro showing trophoblast herniation that is the beginning of the hatching process. The trophoblast begins to herniate through a small opening in the zona pellucida. The blastocyst is fully expanded and surrounded by a thin zona pellucida.
From: Veeck and Zaninovic´, 2003. Reproduced with permission of the publisher, Francis and Taylor Group.
Source: The Virtual Human Embryo.