Final answer:
During the last month of fetal development, hemopoiesis mainly occurs in the red bone marrow. This transition from other organs to the marrow is crucial for the sustained production of erythrocytes necessary for oxygen transport upon birth.
Step-by-step explanation:
Hemopoiesis, or hematopoiesis, is the process by which new blood cells are produced in the body. During the last month of fetal development, hemopoiesis predominantly occurs in the red bone marrow. Prior to this stage, the yolk sac, liver, and spleen are involved in the production of blood cells. However, as the fetus matures, the bone marrow becomes the primary site of erythrocyte synthesis, a process essential for maintaining oxygen transport after birth.
In the final stages of fetal development, particularly during weeks 21-30, the bone marrow almost entirely takes over the task of erythrocyte production from the liver and spleen. This transition is critical for the fetus to ensure a stable supply of erythrocytes which are necessary for efficient oxygen delivery to tissues post-birth. Thus, the bone marrow is the vital site for fetal hemopoiesis in the last month before birth.