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Before this union both the spermatozoon (sperm) and the ovum have migrated for considerable distances in order to achieve their union. A number of actively motile spermatozoa are deposited in the vagina, pass through the uterus, and invade the uterine (fallopian) tube, where they surround the ovum. The ovum has arrived there after extrusion from its follicle, or capsule, in the ovary.

After it enters the tube, the ovum loses its outer layer of cells as a result of action by substances in the spermatozoa and from the lining of the tubal wall. Loss of the outer layer of the ovum allows a number of spermatozoa to penetrate the egg's surface. Only one spermatozoon, however, normally becomes thefertilizingorganism. Once it has entered the substance of the ovum, the nuclear head of this spermatozoon separates from its tail.

The tail gradually disappears, but the head with its nucleus survives. As it travels toward the nucleus of the ovum (at this stage called the female pronucleus), the head enlarges and becomes the male pronucleus. The two pronuclei meet in the centre of the ovum, where their threadlike chromatin material organizes intochromosomes.

Originally the female nucleushas 44 autosomes(chromosomes other than sex chromosomes) and two (X, X) sex chromosomes. Before fertilization a type of cell division called areduction divisionbrings the number of chromosomes in the female pronucleus down to 23, including one X-chromosome.

Themale gamete, or sex cell, also has 44 autosomes and two (X, Y) sex chromosomes. As a result of a reducing division occurring before fertilization, it, too, has 23 chromosomes, including either an X or a Y sex chromosome at the time that it merges with the female pronucleus.

After the chromosomes merge and divide in a process termedmitosis, the fertilized ovum, orzygote, as it is now called, divides into two equal-sizeddaughter cells. The mitotic division gives each daughter cell 44 autosomes, half of which are of maternal and half of paternal origin. Each daughter cell also has either two X-chromosomes, making thenew individual a female, or an X- and a Y-chromosome, making it a male. The sex of the daughter cells is determined, therefore, by the sex chromosome from the male parent.

Fertilization occurs in the uterine tube. How long the zygote remains in the tube is unknown, but it probably reaches the uterine cavity about 72 hours after fertilization. It is nourished during its passage by the secretions from the mucous membrane lining the tube. By the time it reaches the uterus, it has become a mulberry-like solid mass called amorula.

A morula is composed of 60 or more cells. As the number of cells in a morula increases, the zygote formsa hollow bubblelike structure, the blastocyst. The blastocyst nurtured by the uterine secretions, floats free in the uterine cavity for a short time and then is implanted in the uterine lining. Normally, the implantation of the blastocyst occurs in the upper portion of the uterine lining.

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