By experimentally manipulating Drosophia DNA the localization signal for oskar can be replaced by that of bicoid . Transgenic embryos containing the modified DNA have oskar mRNA localized at both the anterior and posterior ends of the egg. Germ cell develop at both ends of the egg. The oskar gene product is sufficient to initiate specification of germ cells. During cleavage, the germ plasm migrates to the floor of the blastocoel.
Germ cells form amid the embryonic endoderm.
Primordial Germ Cells originate outside of the developing gonad. They migrate to the gonad where they continue their development. Diagram of general migration pathway of vertebrate germ cells. In contrast to nematodes, Drosophila and amphibians which have distinct germ plasm in the zygote, mammals lack any visible germ plasm. In addition, individual blastomeres at cleavage stages retain developmental totipotency.