Practice question

Yolk sac in chick originates from splanchnopleure consisting of definitive endoderm derived from primitive streak ingression plus splanchnic lateral plate mesoderm spreading over massive yolk mass, developing extensive vitelline vasculature. Endodermal cells express cathepsins and lipases endocytosing lipoprotein yolk platelets, hydrolyzing triglycerides and vitellogenin, transferring amino acids, lipids, iron and vitamins into embryonic bloodstream. Although chorioallantois later handles gas exchange, yolk sac remains primary nutritional organ throughout incubation and early hematopoietic site via blood islands, analogous in nutritive function to mammalian placenta but utilizing stored yolk.