The primordial germ
cells (PGC) already appear at the time of gastrulation 6 in the epiblast and
complete an emigration out of the embryo into the wall of the yolk sac
(umbilical vesicle). Thanks to the collaboration of three factors - the folding
of the embryo, chemotactic factors and ameboid movements - they get from here
into the wall of the intestinal tract within the embryo again and, via the dorsal
mesentery, into the gonadal ridge. During their migration, which takes place
between the 4th and 6th weeks, they multiply through mitosis.