The formation of gametes (primary sex cells) is the basic process in embryology , it represents the first step in creation of a new human being and it is called gametogenesis . The gametes are produced from primordial germ cells , formed in the ectoderm during 2nd week  and move to the wall of the yolk sac up to the 3rd week  .These cells  are easily recognized because they contain large amounts of the enzyme alkaline phosphatase.

      During the 4th week , these cells migrate from the yolk sac to the genital ridge and arrive the developing gonads at the end of the 5th week.  The direction of growth varies after this , depending on the gender (sex) of the baby as follows:
In female , the primordial germ cells that enter the developing ovary to form the oogonia, the primary origin of ova and in male, the primordial germ cells divide to form the spermatogonia stem cells , the primary origin of sperms...

       Both the oogonia and spermatogonia contain 46 chromosomes    ( the diploid number) that is present in any human cell. The formation of mature gametes is called the gametogenesis which starts in the gonads at puberty, and during which  a complex series of changes occurs in the germ cells, including meiosis ; leading to reduction of the original number of chromosomes to a half ( the haploid number ) with recombination of the genetic material and cytodifferentiation to complete their maturation .

       Gametes are specialized cells and each cell contains a complete set of chromosomes that equals half the chromosome number which characterize the species , as the number of chromosomes varies in different species . The cells that contain half the chromosome number like gametes , is called haploid cells. The diploid cell , like zygote , is a cell having the two complete sets of chromosomes .

        In species that reproduction is sexual , there are two types of gametes according to their shapes : a large gamete produced by the female called the ovum , and a relatively smaller one produced by the male called the sperm.

       The process of gametogenesis in females occurs in the ovaries to produce ova , and it is termed oogenesis . On the other hand , gametogenesis in males occurs in the testes to produce sperms and it is termed spermatogenesis.