Routes of yolk utilisation in the newly-hatched chick.

1. This study was conducted to study the routes by which yolk is utilised in the chick during the initial posthatch phase. 2. Transfer from yolk to blood was examined by injecting, in the form of labelled compounds, oleic acid, triolein, inulin and dextran into the yolk; movement from yolk to blood was observed up to 72 h posthatch. 3. Transport of these molecules from blood to yolk was also observed by injecting them into the circulation and determining label in yolk. The yolk sac membrane was permcable in both directions for all labelled materials tested. 4. In the newly-hatched chick, blue dextran injected into the yolk sac could be seen moving in pulses into the intestine at irregular intervals. Transport of labelled materials from the yolk sac into the intestine was observed up to 72 h after hatching, and marker was found in the proximal small intestine and gizzard. The yolk stalk provided a pathway for transport to the intestine until lymphoid cells began to accumulate, with passage becoming partially occluded at 72 h posthatch. 5. Yolk utilisation was more rapid in fed than in fasted birds suggesting that the transport of yolk through the intestine could be increased by the greater intestinal activity found in fed chicks.