Anatomy Embryology a83c3959 First germ layer to be formed is: A Ectoderm B Endoderm C Mesoderm D All of them develop simultaneously High-Yield Explanation Ans. B EndodermRef: Netter's Atlas of Embryology, 2012 ed. pg. 47* Epiblast is the source of all three germ layers and endoderm is the first germ layer to be formed.