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Conception is the process whereby one male gamete (sperm) fuses with one female gamete (ovum) resulting in a new cell called the zygote. Since each gamete contains 23 chromosomes (1n), the diploid number (2n) of 23 pairs is restored in the embryo. The sex of the new individual is determined at the moment of conception by the type of sex chromosome in the male gamete. The zygote contains genetic information from both parents.

Conception occurs in the infundibular region of the uterine tube. The ovum is surrounded by a thick layer of glycoprotein called the zona pellucida. Sperm become firmly attached to the zona pellucida. After one sperm enters the ovum, permeability of the zona pellucida changes preventing other sperm from entering.

One male and one female pronucleus is formed within the one-cell zygote. The second meiotic division of the ovum is completed resulting in the production of a second polar body. Duplication of the inherited information (DMA) that resides in each pronucleus produces the 4n amount in the cell.

The spindle of the first mitotic division forms. Each paternal and maternal chromosome splits longitudinally with each half moving in a random fashion to opposite poles of the cell.

The one-cell zygote separates into two cells (Fig. 1–2), each containing a nucleus with the identical amount (2n) of inherited information (DMA). Cleavage begins with the formation of the two-cell zygote (schizolig).

Source: Atlas of Human Embryos.