5 day old human embryo "hatching" out of its protective membrane

A human embryo at the blastocyst stage, about six days after fertilization, viewed under a light microscope. The embryo is in the process of 'hatching' out of the zona pellucida - the tough outer membrane - just before implanting in the wall of the uterus. The actual size of the embryo is about 0.3 mm.

Dr Kate Hardy, Institute of Obstetrics and Gynaecology, Royal Postgraduate Medical School, Hammersmith Hospital, London .