Multiple choice What part of the embryo is the origin of the primordial germ cells? Extraembryonic ectoderm Coelomic epithelium Extraembryonic mesoderm Gonadal ridge Endoderm of the yolk sac Reveal answer Fill a bubble to check yourself E Correct answer Explanation In gonadal development, undifferentiated primordial germ cells arise from the endoderm lining the yolk sac.