Genetic and hormonal factors in sex differentiation of the brain.

The mode of chromosomal sex determination is opposite in mammals and in birds, the heterogametic sex being male in the former and female in the latter. This difference is parallel by a basic program of development of sex characters in the absence of gonads; this program is feminine in mammals and male in birds. 'Defeminization' is produced by the testes in male mammals, and 'demasculinization' is produced by the ovary in female birds. The same difference prevails in the permanent organizational effects of sex hormones on the neural structures mediating sex behavior, during a critical period. In mammals, especially in the rat, testosterone released by the testis controls the various sex characters. It can work as such, or after being converted locally into dihydrotestosterone or estradiol, in the cells of the end organ. Estradiol seems to be the actual intracellular 'defeminizing' agent in the central nervous system. In birds, especially in quail, accumulating data suggest that estradiol is the intracellular 'demasculinizing' agent.