The flipping of this switch is the key step. The testis-determining factor signals the embryonic gonads to form into testes, which then begin to produce male hormones, and the hormones take it from there. The major hormone produced by the testes is testosterone, which stimulates the Wolffian ducts to start developing into the male duct system. At the same time, some of the testosterone is converted by the body into a second male hormone, dihydrotestosterone, which prompts the genital ridge to begin forming into male genitals. A female fetus, on the other hand, doesn't have a Y chromosome and so the master switch is never turned on.
Nothing happens in the eighth week.
Instead the fetus continues to grow and develop, and in the thirteenth week the gonads start to transform into ovaries. With no testes to churn out large amounts of male hormones, the rest of the sexual system, internal and external, develops in a female direction. The genital ridge evolves into the clitoris and labia, and the Mullerian ducts mature into the uterus, fallopian tubes and upper one-third of the vagina. The Wolffian ducts shrivel up. So sex is much more a matter of hormones than of chromosomes.