<P> PGCs come from the epiblast and migrate subsequently into the mesoderm, the endoderm and the posterior of the yolk sac . Migration then takes place from the hindgut along the gut and across the dorsal mesentery to reach the gonads (4.5 weeks in human beings). Fibronectin maps here also a polarized network together with other molecules . The somatic cells on the path of germ cells provide them attractive, repulsive, and survival signals . But germ cells also send signals to each other . </P> <P> In reptiles and birds, germ cells use another path . PGCs come from the epiblast and move to the hypoblast to form the germinal crescent (anterior extraembryonic structure). The gonocytes then squeeze into blood vessels and use the circulatory system for transport . They squeeze out of the vessels when they are at height of the gonadal ridges . Cell adhesion on the endothelium of the blood vessels and molecules such as chemoattractants are probably involved in helping PGCs migrate . </P> <P> The SRY (Sex - determining Region of the Y chromosome) directs male development in mammals by inducing the somatic cells of the gonadal ridge to develop into a testis, rather than an ovary . Sry is expressed in a small group of somatic cells of the gonads and influences these cells to become Sertoli cells (supporting cells in testis). Sertoli cells are responsible for sexual development along a male pathway in many ways . One of these ways involves stimulation of the arriving primordial cells to differentiate into sperm . In the absence of the Sry gene, primordial germ cells differentiate into eggs . Removing genital ridges before they start to develop into testes or ovaries results in the development of a female, independent of the carried sex chromosome . </P> <P> Gametogenesis, the development of diploid germ cells into either haploid eggs or sperm (respectively oogenesis and spermatogenesis) is different for each species but the general stages are similar . Oogenesis and spermatogenesis have many features in common, they both involve: </P>

How many chromosomes does a germ cell have