The external genitalia of the male is distinct from those of the female by the end of the ninth week. Prior to that, the genital tubercle in both sexes is a phallus. The urethral groove forms on the ventral surface of the phallus early in development during the differentiation of the external genitalia. This is caused by the androgens produced and secreted by the testes. Androgen induced development causes the elongation and differentiation of the phallus into a penis, a fusion of the urogenital folds surrounding the urethral groove along the ventral surface of the penis, and a midline closure of the labioscrotal folds. This closure forms the wall of the scrotum the external genitalia. The external genitalia are completely formed by the end of the 12th week. At birth, the development of the prepubertal male reproductive system is completed. During the second trimester of pregnancy, testosterone secretion in the male declines so that at birth the testes are inactive. Gonadotropin secretion is low until the beginning of puberty. Summary The genetic sex is determined by whether a Y bearing or next bearing sperm fertilizes the open; the presence or absence of a Y chromosome in turn determines whether the gonads of the embryo will be testes or ovaries; and the presence or absence of testes, finally, determines whether the sex accessory organs and external genitalia will be male or female. This sequence is understandable in light of the fact that both male and female embryos develop within the maternal environment - high in estrogen secreted by the mother's ovaries and the placenta. If estrogen determined the gender, all embryos would become feminized. Puberty During puberty, increased gonadotropin secretion stimulates a rise in sex steroids creation from the testes. The increased secretion of testosterone from the testes during puberty causes the male secondary sexual characteristics to be manifested. Male secondary sex characteristics include: Growth of body hair, including underarm, abdominal, chest hair and pubic hair. Growth of facial hair. Enlargement of larynx (Adam's apple) and deepening of voice. Increased stature; adult males are taller than adult females, on average. Heavier skull and bone structure. Increased muscle mass and strength. Broadening of shoulders and chest; shoulders wider than hips. Increased secretions of oil and sweat glands. Secondary development includes the increased activity of the eccrine sweat glands and sebaceous glands along with the darkening of the skin in the scrotal region.