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* Oocytes are produced by the ovaries (oogenesis) and expelled from them during ovulation. The fimbriae of the uterine tube sweep the oocyte into the ampulla where it may be fertilized.
* Sperms are produced in the testes (spermatogenesis) and are stored in the epididymis. Ejaculation of semen during sexual intercourse results in the deposit of millions of sperms in the vagina. Several hundred sperms pass through the uterus and enter the uterine tubes.
* When an oocyte is contacted by a sperm, it completes the second meiotic division. As a result, a mature oocyte and a second polar body are formed. The nucleus of the mature oocyte constitutes the female pronucleus.
* After the sperm enters the oocyte, the head of the sperm separates from the tail and enlarges to become the male pronucleus. Fertilization is complete when the male and female pronuclei unite and the maternal and paternal chromosomes intermingle during metaphase of the first mitotic division of the zygote.
* As it passes along the uterine tube toward the uterus, the zygote undergoes cleavage (a series of mitotic cell divisions) into a number of smaller cells-blastomeres. Approximately 3 days after fertilization, a ball of 12 or more blastomeres-a morula-enters the uterus.
* A cavity forms in the morula, converting it into a blastocyst consisting of the embryoblast, a blastocystic cavity, and the trophoblast. The trophoblast encloses the embryoblast and blastocystic cavity and later forms extraembryonic structures and the embryonic part of the placenta.
* Four to 5 days after fertilization, the zona pellucida is shed and the trophoblast adjacent to the embryoblast attaches to the endometrial epithelium.
* The trophoblast at the embryonic pole differentiates into two layers, an outer syncytiotrophoblast and an inner cytotrophoblast. The syncytiotrophoblast invades the endometrial epithelium and underlying connective tissue. Concurrently, a cuboidal layer of hypoblast forms on the deep surface of the embryoblast. By the end of the first week, the blastocyst is superficially implanted in the endometrium.

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