DAVID SPEARS LTD / SCIENCE PHOTO LIBRARY DAVID SPEARS LTD / SCIENCE PHOTO LIBRARY
Light micrograph of the embryo of a mouse seen at the blastocyst stage. It reaches this phase about three days after ovulation when the embryo enters the uterine cavity. It is completely surrounded by a thin, resistant glycoprotein layer, the zona pellucida, which a spermatozoon penetrated in order to fertilise the egg. The blastocyst is characterised by a cavity which is filled by fluid penetrating through the zona pellucida. The embryo attaches to the uterine epithelium 4.5 days after ovulation and the pregnancy lasts about 21 days. Magnification unknown.
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