External Genitalia of a 12 Week Old Male Fetus
Photograph of external genitalia of a 12 week old male fetus. The age is calculated from the day of fertilization. Up until 5 weeks after fertilization, the embryonic sex organs in males and females are essentially identical. Genetically, sex has been assigned but the external structures remain indifferent. During the third month, the external genitalia starts to differentiate. The formation of external genitalia that distinguishes between the male and the female sexes begins at the end of the ninth week of fetal development. The reddish indentation shown above the genitalia is the anus, the opening for solid waste. The external genitalia, seen from the backside, is comprised of the penis, scrotum, and urethra masculina.