After ovulation most mammalian ova are surrounded first by follicle cells of the Culums oophorus so that the spermatozoon can only be recognized with difficulty while penetrating the fetal membrane. Only after the follicle cells have separated from the zona pellucida does the interior of the ovum become visible and spermatozoon can be observed in the pervitelline space. Under normal conditions only one spermatozoon reaches the ooplasma. Immediately after its entry the ovum shows clear activation phenomena. Especially noticeable are the shrinking of the ooplasma, the separating movement of the cumulus cells and the extrusion of the 2nd polar body as an external sign of the finished reduction division in the ovum. Then male and female pronuclei appear, merge with each other and become invisible. This introduces the prophase of the 1st mitotic division and the first segmentation division can be recognized approximately 24 hours after copulation. 4 hours after the first division the ovum reaches the 4-cell stage. After a further 6-12 hours the 8-16-cell stage develops.

fertilization / Mammalia