Illustration of human genital development and sex differentiation. In the fetus, both male (left) and female (right) external genitalia pass through an undifferentiated state (upper frames). A glans is present in both (at top); a urethral groove runs along the phallus (middle); raised margins of the groove are genital folds which extend in a ridge to the anus. At the eighth week of pregnancy sex changes begin. In the male (lower left) the genital fold seals to form a penis shaft; glans growth occurs; genital swellings form the testes. In the female (lower right), genital folds remain open from which vagina and urethra open; the glans forms the clitoris; genital swellings the labia.