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| Modern Genetic Analysis Chapter 16. The Genetic Basis of Development Sex Determination in Mammals: Coordinated Control by the Endocrine System Figure 16-6. Development of the mammalian urogenital system. (a) The embryonic genital ridge consists of a medulla surrounded by a cortex. Female germ cells migrate into the cortex and become organized into an ovary. Male germ cells migrate into the medulla and become organized into a testis. (b) In the initial urogenital organization at the indifferent gonad stage, precursors of both male (Wolffian) and female (Müllerian) ducts are present. If a testis is present, it secretes two hormones, testosterone and a polypeptide hormone called Müllerian-inhibiting substance (MIS, or anti-Müllerian hormone). This causes the Müllerian ducts to regress and the Wolffian ducts to develop into the male reproductive ducts. If an ovary is present, testosterone and MIS are absent and the opposite happens: the Wolffian ducts regress and the Müllerian ducts develop into the female reproductive ducts. (From U. Mittwoch, Genetics of Sex Differentiation. Copyright © 1973 by Academic Press.)

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