Amnion in chick embryos is derived from:
Avian amnion forms protective fluid-filled sac from somatopleure. Extraembryonic epiblast-derived ectoderm plus underlying somatic lateral plate mesoderm originating from epiblast ingressing through posterior primitive streak elevates as amniotic folds that fuse above embryo dorsally. Resulting inner layer ectoderm facing amniotic fluid secreting amniotic fluid, outer somatic mesoderm contacts chorion. Splanchnic mesoderm plus endoderm forms splanchnopleure of yolk sac and allantois, not amnion. This dual somatic mesoderm-ectoderm composition distinguishes amnion from chorion which shares same origin but encloses outermost cavity, ensuring cushioning and preventing desiccation.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Extraembryonic membranes - amnion from somatic mesoderm and ectoderm.