During the past 20 years, celJ biology has made immense strides which have completely transformed the time-honored morphological hematology of yesterday. This progress is primarily due to the introduction of new techniques which allow functional rather than anatomic studies: labeling techniques have made possible the study of celJ kinetics from birth to death of a celJ: culture techniques (both in vivo and in vitro) have made it possible to establish the progeny of certain stern celJs, their growth poten tiaL and the mechanisms of their regulation. The results have been so impressive and have...