ABSTRACT
I. Introduction................................................................................. 301 II. Examples of Nonhormonal Induction of Sexual
Differentiation ............................................................................. 302 A. Avoiding the Null Hypothesis ............................................. 303 B. A New Hypothesis ................................................................ 304
III. Evidence of Sex Chromosomal Effects on Brain and Behavior ............................................................................... 305
IV. Structure and Evolution of the Sex Chromosomes .................. 306 V. Possible Involvement of X-Y Genes in Sexual
Differentiation ............................................................................. 308 VI. Avian Sex Chromosomes ............................................................ 309
VII. Summary and Conclusion .......................................................... 310 VIII. Acknowledgments ...................................................................... 310 References ............................................................................................. 311
In the last decade, the question has arisen as to whether the classical model of vertebrate sexual differentiation of the brain is entirely correct. Since the work of Lillie
and Keller and Tandler,
sexual differentiation of somatic (nongonadal) tissues has been increasingly attributed to the action of gonadal steroid hormones. The basic concept is that the primary event in sex determination is
the differentiation of the primitive embryonic gonadal ridges into testes or ovaries, after which all subsequent sexual differentiation of other tissues is controlled by gonadal secretions. This idea followed from the studies of Jost,
who found that gonadectomy in rabbits during fetal life led to a largely feminine pattern of development of external genitalia and reproductive ducts (Wolffian and Müllerian ducts), and that testicular secretions of testosterone and Müllerian-Inhibiting Substance were responsible for inducing masculine patterns of development. The importance of gonadal secretions was expanded and applied to sexual differentiation of the brain, beginning with the work of Phoenix et al.
