You perhaps regard the external genitals as the most definitive characteristics of a male or female, yet there is more similarity between the sexes than most people realize. In the fetus, the genitals begin developing from identical structures in both sexes. By 8 weeks, the fetus has the following (fig. 27.4):
• a phallus,8 a small shaft of tissue with a swollen glans (head);
• urogenital folds, a pair of medial tissue folds slightly posterior to the phallus; and
• labioscrotal folds, a larger pair of tissue folds lateral to the urogenital folds.
By the end of week 9, the fetus begins to show sexual differentiation, and either male or female genitalia are distinctly formed by the end of week 12. In the female, the three structures just listed become the clitoris, labia minora, and labia majora, respectively; all of these are more fully described in chapter 28. In the male, the phallus elongates to form the penis, the urogenital folds fuse to enclose the urethra within the penis, and the labioscrotal folds fuse to form the scrotum, a sac that will later contain the testes.
Male and female organs that develop from the same embryonic structure are said to be homologous. Thus the penis is homologous to the clitoris and the scrotum is homologous to the labia majora. This becomes strikingly evident in some abnormalities of sexual development. In the presence of excess androgen, the clitoris may become greatly enlarged and resemble a small penis. In other cases, the ovaries descend into the labia majora as if they were testes descending into a scrotum. Such abnormalities sometimes result in mistaken identification of the sex of an infant at birth.
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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.