<prompt> Primordial germ cells are seen in --- by 3rd week of development?Neural crestSplanchnic mesodermGenital ridgeYolk sac  <response>: Primordial germ cells originate in the epiblast, migrate through the primitive streak, and by the third week reside among endoderm cells in the wall of the yolk sac close to the allantois. Germ cells do not appear in the genital ridges until the sixth week of development Ref: Langmans embryology; Chapter 16; Pg no 243 