Practice question

Allantois originates as extraembryonic diverticulum from posterior hindgut endoderm pushing into exocoelom lined by splanchnic mesoderm that becomes highly vascularized. It fuses with chorion forming chorioallantoic membrane apposed to eggshell allowing respiratory gas exchange via diffusion, and its lumen accumulates nitrogenous wastes as insoluble uric acid preventing toxicity. It also mobilizes calcium from shell for skeletal ossification. Amnion provides amniotic fluid, yolk sac nourishes, chorion alone insufficient. Thus allantois responsible for waste storage and gas exchange, multifunctional extraembryonic organ in cleidoic eggs. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.