I<br > i<br > :Human life begins when a sperm cell and an egg eel1 unite. The sperm<br > cell develops within the father， the egg cell within the mother. The<br > union ta~es place within the reproductive tract of the mother.after sex-<br > ，uaFintereoursd: Ms in essence is the story of human reproduction.<br > Sex， always related to reproduction， also exists apart from it. The<br > meaning of sex ranges from the bodily differences between male and<br > ~female to the complex drive responsible for life itself and for many of its<br > lJleasures and problems. We will begin our discussion of reproductiori and<br > sex with the bodily differences between male and female. Sex in this<br > sense is fixed at the instant the sperm and egg cells unite (conception).<br > BODILY DIFFERENCES BETWEEN<br > THE SEXES<br > In every sperm and egg cell there are t~~ microscopically<br > small bodies called chromosomqs~ which constitute the principal working<br > machinery of hered~f~， ~ng the determination of sex. All egg cells<br > have similar "sex" chromosomes (x). Half of the sperm cells have one<br > small "sex" chromosome (y) and the other half have an x similar to<br > that of the egg cell. If a ~ fferm cell with a y chromosome fertilizes an<br > egg cell， the individual is male (xy); otherwise it is female (xx). This<br > 1<br >