Energy requirements of avian embryos during incubation are fulfilled by lipids stored in the yolk. Approximately 50% of yolk material consists of lipid. Yolk contents of chicks and poults are utilized by two different concurrent processes. First, lipids are transferred from the yolk to blood following endocytosis by endodermal cells of the yolk membrane and packaging into lipoproteins for release into the bloodstream. This process begins during early embryonation, accelerates during the last week of incubation, and continues after hatching. Second, yolk material is secreted through the yolk stalk into the small intestine as irregular pulses during the first 72 hours after hatching in chicks and for 120 hours after hatching in turkey poults.
Peristalsis and antiperistalsis of the small intestine disseminate yolk materials throughout the small intestine and gizzard. Yolk lipids reaching the proximal small intestine are hydrolyzed and utilized, whereas hydrolysis and therefore utilization do not occur in the ileum and cecum. By 72 hours after hatching, lymphocytes accumulate in the subepithelial connective tissue of the yolk stalk. The lumen of the yolk stalk becomes partially occluded and passage of yolk into the intestinal lumen from the yolk sac ceases. The vitelline stalk of chicks is occluded by lymphocyte aggregations by 4 days after hatching. The yolk stalk converts to lymphopoietic tissue after 14 days and may become a site for extramedullary hematopoiesis. The embryonic remnant of the yolk stalk is often referred to as the Meckel’s diverticulum.