How do males become male and females become female? And what are the consequences if the decision is not incisive? Drawing upon interests in animal genetics and molecular biology, the author endeavours to answer these difficult yet fascinating questions. Originally published in 1995, this book describes the genetic determination of sex and examines how sexual organs are differentiated. Using examples of intersexuality, chimaeras and asymmetries, the book describes the underlying molecular basis of sex determination and sexual differentiation, and focuses on the critical role of the rate of embryonic development in these vital processes. Male precocity is a recurrent theme, as is the involvement of Sertoli cells and their secretion of anti-Müllerian hormone. An invaluable book for reproductive physiologists, geneticists and developmental biologists whose interests may extend from animal science through veterinary medicine to human clinical medicine.