CHANGES IN THE BONE MARROW AND BLOOD CELLS OF DEVELOPING RABBITS.

1. The full number of erythroid cells in the blood stream of the rabbit is reached by the 3rd week of life. 2. During this period, there is a predominance of erythrogenesis in the bone marrow. 3. During the 2nd week of life the bone marrow is in a state of hyperplasia owing to the needs of the body for blood and the small space available for the marrow. 4. This hyperplasia is reduced as the growth of the bone permits the marrow to spread. The control of the growth of the bones has an important bearing on hematopoiesis. 5. During the first 3 weeks of life, the chemical factors for the multiplication of red cells as well as for the elaboration of hemoglobin become available. 6. The amount of hemoglobin does not increase as rapidly as the number of cells, so that the macrocytic anemia of the fetus becomes reduced. The proportion of hemoglobin per red cell characteristic of the adult rabbit is reached by the 3rd month. 7. Further evidence on the intravascular origin of red blood cells is given. 8. The development of all of the white blood cells, with the exception of the monocyte, goes on at a slower rate than that of the red cells. 9. The monocytes reach their full number in the blood stream in the 1st week of life; granulocytes and lymphocytes by the 5th and 6th months. 10. Each of the three strains of white cells has a different rate of development. 11. The question as to whether the stem cell or primitive cell is identical with the lymphocyte is discussed.