The reticular cell tissue forms with its net of argyrophil fibres the stroma of lympho-reticular tissue and bone marrow and the structural element of the spleen. Reticular coating cells are the endotheloid elements lining the sinusoids of the bone marrow, spleen, liver and lymph nodes; the essential feature distinguishing them from the general vascular endothelium is the absence of a collagen basement membrane. Submicroscopically the reticular cell tissue, including the lining cells, show cell borders and continuous intercellular clefts which, owing to the spatial connections of the cells, communicate with the sinusoids. Cross striated fibrils are seen in the amorphous ground-substance of the widened clefts; as oriented bundles they correspond to the histological argyrophil reticular fibre. These fibrils occur not only extracellullarly but sometimes are connected with the ectoplasm of the reticular cells, which has to be considered for the mode and place of fibrillogenesis.