Sperm and egg nuclei fuse during fertilization producing a zygote. The embryo becomes the fetus from week 8 post-fertilization until birth. Internal genitalia is used to transport gametes from the site of synthesis to the outside of body. Diploid chromosome number in humans is 46 (2n=46) 23 chromosomes form the mother, 23 chromosomes from the father. Encode things like hair color and eye color. The 23rd chromosomal pair are the sex chromosomes. Xx in females and xy in males. Encode genes that direct the development of internal and external sex organs. The x chromosome is larger than the y chromosome. Also contains more genes than the y chromosome. The y chromosome is an essential requirement for the development of male reproductive organs. In female cells, once the ovaries have developed one of the x chromosomes in each cell is inactivated. The inactive x chromosome is called a barr body.