At approximately day 1

Gonadal sex (testes) apparent.

Reference

O’Rahilly R, Muller F. 2001. Human embryology & teratology. Third edition. New York: Wiley-Liss.

At approximately day 1

Primordial germ cells are incorporated into the primary sex cords. The testis-determining factor (TDF, found on the Y chromosome) initiates testicular differentiation; Sertoli cells arise from the primary sex cords.

Duration: 1 week

Reference

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