,,1m Kleinsten die wirkliche Wahrheit gibt graBen Gedanken erst Klarheit" KARL THOMAS Each year sees the publication of hundreds of reports of experimental work on the lymphatic tissue, yet morphological studies of the cells involved can be counted on the fingers of one hand. Furthermore, anyone who tries to identify these cells by morphological criteria is accused of sophistry and hair­ splitting, whereas it is accounted scientifically correct and unbiased to speak of "lymphoid cells", "blast cells" etc. Not so many years ago things were different: there were too many names and too many classifi­ cations and everyone backed his particular fancy. People thought of cells in terms of rigid classes, nothing then being known about the transformability of mononuclear blood cells. Today we must look for the middle way: cells should be named and defined according to morpho­ logical criteria but their transformation potential should be borne in mind. Once the cells are analysed and subdivided, it will be simple enough to set up proper classifications afterwards. This book arose out of the conviction that there should be more criteria and more information available on the morphology of human lymphoreticular cells; previously such information had been restricted by the difficulty of the special hematological and cytochemical staining methods.