1. Lymph nodes from all parts of the body have been studied in a series of 58 rabbits. All of these studies have been on normal animals or on animals in which there were no specific pathologic lesions of the lymph nodes.
2. The supravital method of studying living cells has been employed together with the method of fixed sections. A correlation of the findings with these two methods is made.
3. Lymph nodes from different parts of the body possess marked differences in their cytology.
4. Developing monocytes have been found as normal constituents of all the lymph nodes of the rabbit except the mesenteric mass lying in the mesentery at the lower border of the stomach. When present the monocytes occur in varying numbers being most abundant in the more superficial nodes, particularly in the popliteal group, where they may constitute one-third to one-half of the total cells present.
5. The position of the developing monocytes in the lymph node is believed to be chiefly in the perifollicular area without definite relationship to the lymph sinuses. In some instances they are also found in the follicle itself or scattered in the medullary area. More accurate knowledge on this point must await further study.
6. During the first hours of life the monocytes are present in the lymph nodes in the same relationship as in the adult animal.
7. The monocytes are not normally present in the efferent lymphatics draining the popliteal nodes and are not normally present in the thoracic duct. It is suggested that the monocytes being extravascular in origin, enter the blood stream by means of their own motility in much the same manner as the granular leucocytes gain admission to the circulating blood. Unequivocal proof of this point can be obtained only by further investigation.
8. Many of the lymphoid cells of the lymph nodes have shown a definite division of cytoplasm into ectoplasm and endoplasm. The mitochondria and neutral red bodies are sharply limited to the less basophilic endoplasm.
9. Eosinophils are found in great numbers in the mediastinal lymph nodes and are less numerous in the mesenteric and superficial nodes.
10. Clasmatocytes are found in great numbers in the mesenteric lymph nodes where they are concentrated particularly in the medullary areas. They are also present in the spleen. They are much less numerous in other lymphoid tissue.
11. Several cell types not previously described are found to be normal constituents of a large percentage of normal rabbit lymph nodes.
12. Primitive cells are found throughout the lymphoid tissue, but are in greater numbers in the area of the germ center of the follicle.
13. No evidence of erythropoiesis could be found in normal lymph nodes.
14. The spleen and bone marrow in this series of animals have not been normally concerned to any significant degree in the formation and development of monocytes.
15. Further experiments are necessary to demonstrate conclusively whether or not the lymph nodes, other than the mesenteric group, are normally the exclusive source of the monocytes of blood.