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before a sperm can enter the ovum, it must first penetrate the multiple layers of granulosa cells attached to the outside of the ovum (the corona radiata) and then bind to and penetrate the zona pellucida surrounding the ovum itself. The mechanisms used by the sperm for these purposes are presented in Chapter 80.

Once a sperm has entered the ovum (which is still in the secondary oocyte stage of development), the oocyte divides again to form the mature ovum plus a second polar body that is expelled. The mature ovum still carries in its nucleus (now called the female pronucleus) 23 chromosomes. One of these chromosomes is the female chromosome, known as the X chromosome.

In the meantime, the fertilizing sperm has also changed. On entering the ovum, its head swells to form a male pronucleus, shown in Figure 82-1.D. Later, the 23 unpaired chromosomes of the male pronucleus and the 23 unpaired chromosomes of the female pronu-cleus align themselves to re-form a complete complement of 46 chromosomes (23 pairs) in the fertilized ovum (see Figure 82-1E).

What Determines the Sex of the Fetus That Is Created? After formation of the mature sperm, half of these carry in their genome an X chromosome (the female chromosome) and half carry a Y chromosome (the male chromosome). Therefore, if an X chromosome from a sperm combines with an X chromosome from an ovum, giving an XX combination, a female child will be born, as explained in Chapter 80. But if a Y chromosome from a sperm is paired with an X chromosome from an ovum, giving an XY combination, a male child will be born.

Figure 82-2

A, Ovulation, fertilization of the ovum in the fallopian tube, and implantation of the blastocyst in the uterus. B, Action of tro-phoblast cells in implantation of the blastocyst in the uterine endometrium.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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