DEVELOPMENTS IN SEXUAL ORGANOGENESIS
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lished their decisive role in the embryonic differentiation of somatic sexual
structures.
From these studies the following major conclusions may be derived.
(1) The embryonic gonad of vertebrates is the seat of endocrine activity
at a very early age. The steroid hormones of the embryonic or fetal gonad
control the direction of sexual differentiation of somatic sexual characteristics, most particularly the reproductive tract, from the initial stage
when the two types of areas and primordia are established (ambisexuality).
This aspect of sexual differentiation can be considered as already essentially
explained. On the whole, it is the gonad that is the endocrine link, directing
the sexual differentiation of somatic sexual characteristics at a very early
stage.
(2) The problem of the determination of embryonic sex differentiation
of the gonadal primordia is not as well understood. In each sex it can be
considered as established that the original structure of the gonadal primordium is ambisexual. In the tetrapods, two areas, the cortex and the
medulla, of gynogenic and androgenic potency, respectively, can be identified by their topography, morphology, and histogenetic properties. In
fishes, although such a distinction may be morphologically difficult to
establish, the fact remains that in many cases it is possible to recognize, in
the primordium of the somatic tissues of the genital fold, a regional arrangement corresponding to the different sexual properties.
(3) The direction in which cortical or medullary dominance develops
determines the sexual differentiation of the gonads as testes or ovaries;
these are the inductive areas that guide the evolution of the primordial
germ cells, whatever their genetic constitution may be.
(4) An analysis of the sex ratio of the offspring of individuals of a sexreversed phenotype makes it possible to verify these conclusions and to
establish the homo- and heterogametic nature of sex, especially in the
amphibians.
(5) It appears that the dominance of one of the areas over the other
normally reflects the effects of the genetic makeup of the embryo.
(6) The direction of the genetic force can be changed and, within limits,
reversed by epigamic effects, the best known of which are those produced
by the steroid hormones. The action of these factors can be decisive in cases
of reversal if one of the somatic inductors has been completely inhibited.
Then the reversal is permanent. In cases of intersexuality, even of a very
pronounced degree, the persistance in a vestigial state of one of the components of the corticomedullary pair results in a return towards the genetic
sex of the individual. This means that the genetic sexualizing effects can be
dominated for a time, but that their potential capacity is still present.
(7) A point that has been controversial for a long time concerns the
311
lished their decisive role in the embryonic differentiation of somatic sexual
structures.
From these studies the following major conclusions may be derived.
(1) The embryonic gonad of vertebrates is the seat of endocrine activity
at a very early age. The steroid hormones of the embryonic or fetal gonad
control the direction of sexual differentiation of somatic sexual characteristics, most particularly the reproductive tract, from the initial stage
when the two types of areas and primordia are established (ambisexuality).
This aspect of sexual differentiation can be considered as already essentially
explained. On the whole, it is the gonad that is the endocrine link, directing
the sexual differentiation of somatic sexual characteristics at a very early
stage.
(2) The problem of the determination of embryonic sex differentiation
of the gonadal primordia is not as well understood. In each sex it can be
considered as established that the original structure of the gonadal primordium is ambisexual. In the tetrapods, two areas, the cortex and the
medulla, of gynogenic and androgenic potency, respectively, can be identified by their topography, morphology, and histogenetic properties. In
fishes, although such a distinction may be morphologically difficult to
establish, the fact remains that in many cases it is possible to recognize, in
the primordium of the somatic tissues of the genital fold, a regional arrangement corresponding to the different sexual properties.
(3) The direction in which cortical or medullary dominance develops
determines the sexual differentiation of the gonads as testes or ovaries;
these are the inductive areas that guide the evolution of the primordial
germ cells, whatever their genetic constitution may be.
(4) An analysis of the sex ratio of the offspring of individuals of a sexreversed phenotype makes it possible to verify these conclusions and to
establish the homo- and heterogametic nature of sex, especially in the
amphibians.
(5) It appears that the dominance of one of the areas over the other
normally reflects the effects of the genetic makeup of the embryo.
(6) The direction of the genetic force can be changed and, within limits,
reversed by epigamic effects, the best known of which are those produced
by the steroid hormones. The action of these factors can be decisive in cases
of reversal if one of the somatic inductors has been completely inhibited.
Then the reversal is permanent. In cases of intersexuality, even of a very
pronounced degree, the persistance in a vestigial state of one of the components of the corticomedullary pair results in a return towards the genetic
sex of the individual. This means that the genetic sexualizing effects can be
dominated for a time, but that their potential capacity is still present.
(7) A point that has been controversial for a long time concerns the