At approximately day 1

Primordial germ cells are visible in the yolk sac.

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

Mesonephric (Wolffian) ducts develop.

Reference

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

At approximately day 1

A two to four cell layer milk streak (or mammary band) is identifiable as thickening lines in the endoderm, extending from the axila to the groin.

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

Vascularity increases in the area of the milk streak.

Duration: 1 week

Reference [PubMed Link]

Laurence DJ, Monaghan P, Gusterson, BA. 1991. The development of the normal human breast. Oxford Reviews of Reproductive Biology 13:149-174.