Practice question

Chorion develops from extraembryonic somatic mesoderm plus ectoderm surrounding embryo, eventually fusing with allantois vasculature to form chorioallantoic membrane lying immediately beneath eggshell membranes highly vascularized via allantoic arteries. Diffusion distance minimal allows passive exchange: oxygen from air spaces through shell pores into chorioallantoic capillaries, carbon dioxide outward. Also mediates calcium mobilization. It does not store nutrients yolk sac does, protection amnion, waste allantois lumen. Hence in chick chorion mainly facilitates gas exchange essential for aerobic metabolism of developing embryo inside calcified shell. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.