1. Ovum and sperms: (In vitro) From this photograph, it should be clear that the heads of human sperm are less than 1/20 the diameter of human eggs.  Arrows point to sperm heads

2. Fertilization

3. Stage 1 of fertilization: The acrosome reaction Occurs when sperms come into contactيتصل مع with the corona radiata of the ovum البويضة Passage مرورof sperm throughخلال the corona radiata depends onيعتمد على enzyme action: hyaluronidase released from sperm acrosome Tubal mucosal enzymes Flagella action also aidsيساعد corona radiata penetrationاختراق

4. Stage 2 of fertilization: Penetration اختراقof the zona pellucida around the ovum: Acrosomal enzymes: esterases, acrosin, and neuraminidase cause lysis تحللof the zona pellucida Once بمجرد انsperm penetrates zona pellucida, the zona reaction occurs: This reaction makes the zona pellucida impermeableغير منفذة to other sperms. When more than one sperm manages to enter the ovum (dispermy = 2; triploidy = 3), the fetus nearly always aborts يجهض.

5. Stages 3 & 4 of fertilization: Head and tail of a sperm enter the cytoplasm of the ovum

6. Stage 5 of fertilization: Formation of male and female pronuclei: Chromosomal material of the sperm decondensates and enlarges Chromosomal material of the ovum decondensates following the completion of meiosis

7. The male and female pronuclei are indistinguishable from one another. The second polar body can be seen (arrow). The plasma membranes of the two pronuclei are dissolving and one diploid nucleus will remain. Fusion of the pronuclei: (in vitro)

8. Stage 6 of fertilization: Membranes of the pronuclei break down, chromosomes condense and arrange themselves for mitotic cell division (cleavage) التفلج The first cleavage follows shortly, leaving 2 cells, each with 46 chromosomes.

9. The results of fertilization: Restores the normal diploid number of chromosomes (46). Results in variation of human species as maternal and paternal chromosomes intermingle. Determines the sex of the embryo. The sex chromosome (Y or X) carried by the successful sperm determines embryonic sex.

10. Twins: still 1 sperm per egg Monozygotic (monoovular): A fertilized, single egg splits into two developing zygotes at a very early stage. Identical twins; same sex. Dizygotic (polyovular): Result from the fertilization by two sperm of two separate ova that have reached maturation at the same time. Not identical twins; can be different sexes Incidence increases with age of the mother

11. Triploidy (in vitro) There are 3 pronuclei within this one zygote. In the laboratory, such embryos are discarded. In vivo, such embryos almost always abort spontaneously.