The work is a doctor's thesis, which probably accounts for the long and repeated discussions of theories regarding origin and relations of blood cells and for the extensive tabular matter. The book is devoted to the problem of extramedullary erythromyelopoiesis in the spleen, liver and thymus of normal human fetuses of the seventh, eighth and ninth months, the prematurely born, the newly born and the infant, child and adult, with the object of determining the normal limits of extramedullary blood formation in these age groups. The material was by far the most extensive and best controlled of any series used for study of this problem. Trauma in one form or another was the most frequent cause of death. Only cases in which the necropsy revealed no pathologic changes were included in the series regarded as normal. A few pathologic cases were studied for comparison with the corresponding normal age groups.