Practice question

Yolk sac formed by extraembryonic endoderm from hypoblast plus extraembryonic mesoderm derived from primitive streak spreading over yolk surface encloses yolk mass. Endodermal cells exhibit microvilli and produce lipases, proteases digesting yolk lipoproteins, vitelline vessels transport lipids, amino acids and iron via vitelline circulation to embryo. Until hatchling absorbs residual yolk via midgut, yolk sac remains sole nutrient source. Gas exchange via chorioallantois, waste via allantois, so primary function nutrient supply not gas exchange or protection. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.