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L. G A L L I E N
nature of the cortical and medullary mediators that, expressing the genie
effects, control the direction of the differentiation of the gonads. It is
reasonable to consider that these mediators are linked to the biosynthesis
of the steroid hormones in the course of sexual differentiation. This conclusion results: (a) from the existence of a neutral sex type; (b) from effects
observed upon administration of steroid hormones of exogenous origin; (c)
from the demonstration of very early stages of steroid hormones; and
(d) from the detection of enzyme systems that play a role in the synthesis
of the steroids and that precede them in time of appearance.
IV. General Conclusions
Recent progress in the study of sexual organogenesis in metazoans has
demonstrated three major tendencies.
A. Role of Endocrine Factors
In hydras, crustaceans, insects, and vertebrates, it seems that sexual
differentiation of the gonads and the somatic sexual characteristics are
controlled by chemical mediators. This conclusion emphasizes the unity of
the processes participating in sexual organogenesis in metazoans.
The study of these mediators is most extensive in vertebrates. It reveals
the decisive role of the steroid hormones. The chemical nature of the
hormones playing a role in the sexual differentiation of the invertebrates
is not yet known. However, the existence in the male of an androgenic
gland and of apical androgenic tissue, in crustaceans and insects respectively, testifies to the presence of endocrine structures with activity controlling sexual differentiation in the male. The female represents the neutral
condition of sex, which differentiates in the absence of hormones.
In more primitive forms, such as hydras, the nature of the differentiation
of the gonocytes into gametes corresponds to a definite regional alignment
along the body column. Blastogenesis and gametogenesis are controlled
by environmental factors (temperature).
B. Relationship between Genetic Structures and Epigenetic Effects
An important method of study, followed primarily in amphibians and
mammals, tends to clarify the relationship between the effects of genetic
structures and of epigenetic factors of a hormonal nature which control
sexual organogenesis.
The role of the Y chromosome as a carrier of genes for maleness has been
established in mammals.
The dysgenic effects of heteroploidy on sexual organogenesis have been
analyzed in amphibians and mammals.
The homo- and heterogametic nature of sex in amphibians and in the
L. G A L L I E N
nature of the cortical and medullary mediators that, expressing the genie
effects, control the direction of the differentiation of the gonads. It is
reasonable to consider that these mediators are linked to the biosynthesis
of the steroid hormones in the course of sexual differentiation. This conclusion results: (a) from the existence of a neutral sex type; (b) from effects
observed upon administration of steroid hormones of exogenous origin; (c)
from the demonstration of very early stages of steroid hormones; and
(d) from the detection of enzyme systems that play a role in the synthesis
of the steroids and that precede them in time of appearance.
IV. General Conclusions
Recent progress in the study of sexual organogenesis in metazoans has
demonstrated three major tendencies.
A. Role of Endocrine Factors
In hydras, crustaceans, insects, and vertebrates, it seems that sexual
differentiation of the gonads and the somatic sexual characteristics are
controlled by chemical mediators. This conclusion emphasizes the unity of
the processes participating in sexual organogenesis in metazoans.
The study of these mediators is most extensive in vertebrates. It reveals
the decisive role of the steroid hormones. The chemical nature of the
hormones playing a role in the sexual differentiation of the invertebrates
is not yet known. However, the existence in the male of an androgenic
gland and of apical androgenic tissue, in crustaceans and insects respectively, testifies to the presence of endocrine structures with activity controlling sexual differentiation in the male. The female represents the neutral
condition of sex, which differentiates in the absence of hormones.
In more primitive forms, such as hydras, the nature of the differentiation
of the gonocytes into gametes corresponds to a definite regional alignment
along the body column. Blastogenesis and gametogenesis are controlled
by environmental factors (temperature).
B. Relationship between Genetic Structures and Epigenetic Effects
An important method of study, followed primarily in amphibians and
mammals, tends to clarify the relationship between the effects of genetic
structures and of epigenetic factors of a hormonal nature which control
sexual organogenesis.
The role of the Y chromosome as a carrier of genes for maleness has been
established in mammals.
The dysgenic effects of heteroploidy on sexual organogenesis have been
analyzed in amphibians and mammals.
The homo- and heterogametic nature of sex in amphibians and in the