During the primary six weeks of development, male and female embryos are indistinguishable.  In embryo form, the testes and ovum are made of the same tissue.  At the six week mark, the tissue begins to specialize to form a reproductive organ for a particular gender based upon the chromosomes given to the embryo during conception.  The woman will always provide an X chromosome; the man may supply an X or a Y chromosome.  If the embryo receives the genetic makeup of XX, a female will develop.  If an XY embryo is created, a male will develop.  During the sixth week, an XY composition will encourage testes to develop and descend; specifically at this moment when the testes descend the gender of the developing child can be distinguished.  If the reproductive tissues have not dropped, female ovaries will begin to develop around week ten. 

During the third month, the fetus possessing the XY chromosomes will start to have an increase in the hormone androgen.  This hormone promotes the growth and development of the Wolffian ducts, which in turn support the creation of the penis.  Meanwhile, the female fetus, XX, experiences a substantial rise in the hormone estrogen.  Estrogen encourages growth and development of external genitals.  

The internal female system, once developed, is known as the Mullerian ducts.  Mullerian ducts contain the ovaries, uterus, and vagina.  The internal male system or Woffian ducts include the prostate gland and the seminal vesicles for semen production.  

It is amazing how one tiny sex chromosome, X or Y, can have such a dramatic impact and influence upon every individual’s entire life structure!  Moreover, consider that the nutrition of the mother, the emotions of the developing fetus, and the future environment of the living child will greatly affect future gene expression.