In adults, RBCs are formed primarily in the

Haemopoiesis is the process by which the formed elements of blood develop. Before birth, haemopoiesis first occurs in the yolk sac of an embryo and later in the liver, spleen, thymus, and lymph nodes of a foetus. Red bone marrow becomes the primary site of haemopoiesis in the last 3 months before birth, and continues as the source of blood cells after birth and throughout life.