Practice question

Chorion is outermost extraembryonic membrane formed from somatic lateral plate mesoderm plus ectoderm, lining inner surface of shell membrane. While avascular alone, it fuses with highly vascular allantoic mesoderm creating chorioallantoic membrane where fetal capillaries appose shell pores. Oxygen diffuses inward and carbon dioxide outward through porous calcareous shell across this membrane driven by partial pressure gradients. Chorion also transports calcium from shell for bone ossification. Unlike yolk sac providing nutrition or allantois storing uric acid waste, chorion primary physiological role remains respiratory gas exchange throughout incubation.