The Primary Organizer dorsalizes the mesoderm, resulting in dorsal mesoderm (gives rise to somites) and intermediate mesoderm (gives rise to kindey and gonad). Intermediate mesoderm is located between the paraxial mesoderm and the lateral plate. It develops paired epithelial thickenings.

These paired epithelial thickenings are known as genital ridges, from which primordial gonads arise. During the indifferent stage, the genital ridge epithelium proliferates and extends into the loose mesenchymal tissue above it. The epithelium that proliferates into the mesenchyme forms the primitive sex cords.

The primary sex characteristic is the sex of the gonad: in males, it is the testes; in females, it is the ovary. Secondary characteristics are all other differences between males and females.

These are referred to as secondary, because the primary sexual characteristic sets in motion all of the events associated with the development of the secondary sex characteristics.

Secondary sex characteristics can be internal or external. Internal secondary sex characteristics include the accessory organs used to transport gametes to the site of fertilization. External secondary sex characteristics include differences in overall size, musculature and bone shape, mammary glands, hair-growth patterns, etc.

In many organisms, behavioral differences are also secondary sex characteristics. For example, in song birds, only the males sing, and this is due to the effects of testosterone on development of specific neurons in the brain. In humans, social factors cloud what behaviors are or are not secondary sexual characteristics.

1. the mesonephric (Wolffian) ducts regress
2. the Müllerian ducts differentiate into:
   • oviducts (Fallopian tubes)
   • uterus
   • upper vagina

1. the Mullerian ducts regress
2. the Wolffian ducts differentiate into:
   • epididymis
   • vas deferens
   • seminal vesicle

1. genital tubercle forms clitoris
2. genital fold forms labia minora
3. genital swelling forms labia majora

1. genital tubercle forms head of penis
2. genital fold forms shaft of penis
3. genital swelling forms scrotum

A series of experiments in fetal rabbits led to the conclusion that the sex of the gonad controlled the development of all other sexual characteristics. These experiments showed:
Removed	Overview
Testes	Removing testes from a male fetus will cause it to develop a female phenotype without gonads.
Ovaries	Removing ovaries from a female fetus will cause it to develop a female phenotype without gonads.
Conclusion: The ground state is female. Testis are required for male differentiation. Without testis then the phenotype is female.
These results led to the hypothesis that a substance produced by the male gonad causes male differentiation. We now know that the key products of the testes that allow male development of secondary sexual characteristics are hormones.

Injection	Overview
Testosterone	Testosterone injected into female fetuses will cause them to develop as males, except that the ovary does not differentiate into a testis and produce sperm, and the paramesonephric (Mullerian) ducts do not degenerate.
Estrogen	Estrogen injected into fetuses will not cause any change in the fetus’ development. Males still develop as males and females as females.
AMH	Regression of the Mullerian ducts in the male is under the control of anti-Mullerian duct hormone (AMH, a TGF-β) secreted by testis’ Sertoli cells.

Conclusion: The development of the male phenotype in mammals depends on the development of the testis, which then controls secondary sexual development by producing testosterone and AMH. As a side-note, testosterone is secreted by the Leydig cells of the testis and estrogen is produced by the ovary.

Amphibian primordial germ cells (PGCs) originate in the endoderm of the gut. Mammal and bird PGCs originate in the epiblast and then migrate to the yolk sac. Later, PGCs perform directed cell migration to the genital ridge.

Stromal-derived factor-1 (SDF-1) guides PGCs in many vertebrates. SDF-1 is expressed along the PGC pathway, and PGCs express the SDF-1 receptor. In SDF-1^-/- or the SDF-1-receptor^-/- mutants, germ cells do not reach the genital ridge.

The PGCs invade both the primitive sex cords (medulla) and cortex of indifferent gonad (at week 6 in humans). The different fates of the cortex, medulla and PGC’s in males and females is determined by the sex of the gonad.

The gene SRY has been cloned from the human Y chromosome and is required to confer maleness in mammals. This gene has a close homolog in mice, is expressed in gonad at the time of testis differentiation, is not expressed in ovaries, and is a DNA or RNA-binding protein. Transgenic SRY mice always had a male phenotype even when they had an XX genotype. However, male XX^SRY were incapable of spermatogenesis since sperm cell differentiation requires additional Y chromosome proteins. This shows that the presence of the SRY gene is sufficient to drive almost all aspects of male sexual differentiation.

Sox9 is an autosomal gene necessary for maleness in all vertebrates. Sox9^-/- causes a female phenotype regardless of genotype. The current hypothesis is that in mammals and marsupials, SRY controls the expression of Sox9. The role of Sox9 as a determinant of testes formation appears to be conserved among all vertebrates. This is true even in vertebrates other than mammals, which do not have the Sry gene. It is not known why expression of Sox9 came under the control of Sry in mammals. In vertebrates such as fish and reptiles, where sex is determined by hormones or the temperature at which the embryo develops, it is thought that Sox9 activity is influenced by sex hormones.

Researchers have not deciphered how sex is determined in animals that use a ZW system (including birds). This is largely due to the impossibility genetic screens or other genetic manipulations in birds. However, RNAi has revealed that the DNA-binding protein DMRT1 is sex-determining. DMRT1 is required for the gonad to differentiate as a testis in birds. Interestingly, DMRT1 is highly conserved. It is expressed in mammals and in Drosophila its homolog (Doublesex) determines sex in flies. As a side-note, monotreme mammals (egg layers such as platypus and echidna) use a ZW system, indicating that the XY system evolved rather recently.

The female phenotype is not simply a default state. The development of secondary sexual characteristics in females requires estrogen and other hormones. Also, the Wnt4 gene is required for the development of the female gonad as an ovary.

In female mice lacking Wnt4, the gonad develops male characteristics (Sertoli cells) and expresses genes involved in testosterone production. Wnt is a member of the Wnt family of genes related to the Drosophila wingless (wg) gene.