The histogenesis of the popliteal lymph node in the rat and the popliteal and inguinal lymph nodes in the rabbit was examined by light microscopy. Special emphasis has been laid on the initial lymphocyte population in the lymph node anlage. In the rat on the seventeenth day of gestation lymphoid cells populate a limited mesenchymal area along the vein wall. The next day the mes-enchyme shows a bulb-shaped outgrowth causing an indentation in the wall of a lymph vessel, running parallel to the vein and having a saccular widening at this place. The bulb-shaped lymphoid outgrowth fills up the widened lymph vessel; the subcapsular sinus originates from the remaining parts of the lymph vessel. At birth the lymph node can be divided into a primitive cortex consisting of an area with evenly scattered lymphocytes among the basic network of reticular cells and a medulla. About three days after birth an ovoid area containing a dense concentration of lymphocytes is observed in the inner cortex. In the next days it expands in both lateral and medullary direction but not into the outer cortex. Primary follicles appear in the outer cortex 18 days after birth.
The development of the inguinal and popliteal lymph nodes in the rabbit shows the same characteristics as the histogenesis of the popliteal lymph node in the rat.
The morphogenesis of the lymph node is summarized in a schematic diagram.