Final answer:
True, the yolk sac is the initial site of hematopoiesis in early embryonic development, producing the first blood cells before the placenta and subsequently the bone marrow take over this function.
Step-by-step explanation:
The statement that the yolk sac is the initial site of hematopoiesis (formation of blood) is True. In the early stages of embryonic development, the yolk sac functions as a crucial source for the creation of the first blood cells. Blood cells are produced in the yolk sac during the second and third week of development, before the placenta becomes the main source of nourishment for the embryo at around week four. As the embryo develops further, the spleen, lymph nodes, liver, and eventually the bone marrow take on the role of hematopoiesis, with the red bone marrow becoming the primary site of hematopoiesis in the later stages of development and throughout adulthood.