A number of years ago Prof. J. Peiffer (Tiibingen/FRG) asked whether cerebros- nal fluid cells originate in blood cells. With this question, the first seeds were sown for the research which resulted in this book. The original question was eventually ex­ panded to whether extravascular hematogenous cells are present in the nervous sys­ tem. In the course of my research this question became more and more concentrated on the demonstration of monocyte derivatives (i.e., mononuclear phagocytes) within the nervous system. While these questions are certainly not new, they regularly crop up when micro­ glia are to be classified, particularly progressive microglia. Three extensive surveys were published during the past few years, all of which arrived at different conclusions (Cammermeyer, 1970b; Vaughn and Skoff, 1972; Fujita and Kitamura, 1976). In ad­ dition to progressive microglia, other active phagocytes were observed which I refer to as "perivascular cells," "free subarachnoidal cells," and "epiplexus cells." These cells were seldom described in the literature, and then predominantly in another context. A pre­ sentation based on the hypothesis that the cell types of the central nervous system have po­ tentially identical kinetics and functional potency has not been published.