In Kyoto 1924 Aschoff has suggested, that future researchs would prove, whether it has been justified to unite different but nevertheless cooperating cells to a special system. The conclusions of physiological and pathological examinations just as microscopical and submicroscopical investigations have substantiated, that this retothelial system, at first only functionally established, is characterized by a common morphological substratum. This cell system consists, in a closer sense, of the provinces of the reticular cell tissue which forms with its net of argyrophil fibres the stroma of lymphoreticular 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 basal border by a collagen basement membrane. The extended portion of the system is formed by mobile histiocytes in the undifferentiated circumvasal and subendothelial zones, where they can also be formed. Submicroscopically the reticular cell tissue, including the lining cells, show cell borders and continuous intercellular clefts which, owing to the spatial connection of the cells, communicats with the sinusiods. 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 extracellularly but sometimes are connected with the ectoplasm of the reticular cells, which has to be considered for the mode and place of fibrillogenesis. This system, which was first conceived from the agreement in functions of its elements, is seen to have an adequate morphological structure, which is maintained in both reactive and inflammatory manifestations of the system and is formed in the various neoplasias. These findings, when compared with the fine structure of fetal connective tissue, are an indication of the apparently slight differentiation of this cell system. This remainder of the embryonal multipotent mesoderm can be termed the retothelial system, since the ordinary endothelium of the blood-and lymph-parts can be excluded for its form and functions. On the examples of granulomatoses and neoplasias it is discussed, that the uniform texture of the reticular histiocytic tissue formations is the essential prerequisite for explaining the pathogenesis and prospective importance also of such retothelial determined diseases, whose origin is not yet known and whose nosological classification is still disputed. Demonstration of the retothelial substrate, including the cells in this stroma, and observation of developmental forms by cytological and histological examination also allowed to determine connections between the origin of regular and morbid hemopoiesis and the retothelial system. A synoptic evaluation of the findings presented confirms the anatomical integrity of the R.S, and its multifarious importance in health and disease.