Light micrograph of the embryo of a mouse at the eight-cell stage. The embryo in this phase is also known as the morula. It is completely surrounded by a thin, glycoprotein layer, the zona pellucida, which a spermatozoon penetrated in order to fertilise the egg. The embryo reaches this stage about three days after ovulation and at the same time it enters into the uterus. It becomes attached to the uterine epithelium 4.5 days after ovulation and the pregnancy lasts about 21 days. Magnification unknown.