At approximately day 1

Gonadal (genital) ridges develop from a thickening of the mesothelium on the medial side of the mesonephros.

Reference

Moore KL, Persaud TVN. 1998. The Developing Human: Clinically Oriented Embryology. 6th ed. Philadelphia: WB Saunders Co.

At approximately day 1

Primary sex cords grow into the underlying mesenchyme. The gonad now consists of an external cortex and internal medulla. The medulla eventually regresses as the ovary develops.

Duration: 1 week

Reference

Moore KL, Persaud TVN. 1998. The Developing Human: Clinically Oriented Embryology. 6th ed. Philadelphia: WB Saunders Co.

At approximately day 1

External genitalia differentiate to form clitoris and labia.

Reference

Larsen WJ. 1997. Human Embryology. New York: Churchill Livingstone Inc.

At approximately day 1

Mammary crest and nipple primordium apparent.

Duration: 1 week

Reference [PubMed Link]

Gusterson BA, Stein T. 2012. Human breast development. Seminars in Cell and Developmental Biology 23(5):567-573.

At approximately day 1

Primordial germ cells migrate from the yolk sac, through the gut endoderm, to the gonadal ridge.

Duration: 1 week

Reference [PubMed Link]

Rabinovici J, Jaffe RB. 1990. Development and regulation of growth and differentiated function in human and subhumand primate fetal gonads. Endocrine Reviews 11(4):532-557.