The monograph edited by Drs. Wunder and Henon on "Peripheral Blood Stem Cell Autogtafts" is extremely useful as well as timely. It covers the "state of the arts" with respect to the use of hemopoietic stem cells collected from the peripheral blood for the reconstitution of hematopoiesis after myeloablative therapy. If is is accepted that hematopoietic function in the mammalian organism is the result of stem cell seeding of an appropriate stromal matrix, then the use of blood derived stem cells for hematopoietic reconstitution represents the "physiological form" of the (re)- establishment of a hematopoietic bone marrow. All observations to date are compatible with the assumption that stem cells migrate via the blood stream from extraembryonic hematopoietic tissue to the fetal liver to establish there a first intraembryonic site of blood cell formation and especially of stem cell replication and proliferation. This fetal liver tissue appears then to be the major source for the seeding offetal bone marrow stroma as it develops sequentially in all the bones of the skeleton - in other words during most of the entire embryonic development. There is a very high concentration of stem cells in the blood of the embryo (more than 20000 CFU-GM per ml in the 22nd week) and the stem cells in cord blood seem to be the "tail end" of a dramatic "stem cell traffic" in the embryo to establish the hemopoietic as well as lymphopoietic tissue.