Basic sexual trends in the development of vertebrates

The chain of events occurring during sexual development involves successive steps: genetic sex, gonadal sex and body sex. The latter comprises the genital tract, secondary sex characters and neural structures mediating sexual interest and appetite. Body sex obeys a hormonal control. In the absence of any hormone it develops in conformity with the homozygous sex type--feminine in mammals, masculine in birds, newts and one lizard studied so far. Similar differences have been observed for sex behaviour in some mammals and birds. It has been suggested that the sex of the gonads is determined by the presence (or absence) of the histocompatibility antigen produced by the sex chromosome of the heterozygous sex (Y or W). However, in newt or Xenopus graft chimaeras as well as in bovine freemartins, testicular dominance over presumptive ovaries is obvious whatever the mode of chromosomal control of sex. A unifying concept for sex differentiation in all vertebrates, accounting for the long series of recognized data, is still difficult to delineate.