GERMFREE MICE granules with acid phosphatase activity, occasional fat droplets and autofluorescent lipofuscin and iron.(b) Immunologically competent cells (ICC) characterized by the presence of gamma globulin and sub-divided morphologically according to the criteria of Vazquez20; these exhibited basophilic cytoplasm with small, eccentric, cartwheel nuclei and perinuclear halos. In tallying ICC by conventional microscopy, only the Marschalko plasma cells were counted.(c) Lymphoid cells (lympho-cytes-small, medium, large), characterized by scanty cytoplasm con-taining none of the histochemical or immunologic features noted above. The shape, color and consistency of lymph nodes were similar in germfree and conventional mice, although in the former the nodes were slightly smaller. Microscopically all lymph nodes were similar in basic architecture and showed a division into cortex and medulla (Figs. i and 2). The submaxillary and mesenteric nodes exhibited an extensive medullary sinus system, but the axillary and popliteal did not (Figs. 3 and 4). With the exception of the popliteal nodes, in which the cortex was uniform, the lymph node cortex could be divided into a narrow peripheral and a wider intermediate zone. 21 The peripheral portion in nodes from both germfree and conventional animals contained nodules of densely packed small lymphocytes in which reaction centers and mitotic figures were seen. 21'22 The intermediate zone formed the major portion of the cortical tissue in all nodes but particularly in those of germfree mice; it was generally less cellular, and the small lymphocyte was the predominant cell type. In the mesenteric nodes of both germfree and conventional mice, the intermediate zone was frequently the site of con-spicuous leukocyte destruction, particularly of eosinophils and neutro-phils, with attendant macrophage activity. The submaxillary and axillary nodes showed only a few of such degenerating cells and rare macro-phages in the intermediate zone.

All nodes had perivascular medullary cords filled with cells which varied in type and number with the anatomic location of the node and the microbial status. The nodes in conventional animals showed larger and more cellularmedullary cords comprised of lymphocytes of varying sizes, eosinophils, macrophages and plasma cells. The medullary cords in germfree animals were thin and composed mainly of small and medium-sized lymphocytes (Fig. 5). Mitotic figures were most often seen in medullary cords of mesenteric nodes in conventional animals and were rare in thecomparable germfree nodes. Reaction centers, as defined by Ringertz and Adamson28 and by Con-24 way, were seen in the conventional state, most commonlyin mesenteric nodes (Fig. ro). They were rare in all germfree miceand virtually absent from axillary and popliteal nodes. The differences in the gross and microscopic characteristics of the