In routine microscopic examination of tissue from premature infants post mortem, great variation in the amount of extramedullary hematopoiesis is observed.In infants of a given birth weight the number of sites of extramedullary hematopoiesis and the amount in each organ varies greatly. An attempt has been made to evaluate some of the factors affecting extramedullary hematopoiesis.
Since the widest variations seemed to occur in infants with birth weights between 500 and 2000 gm., this group was selected for study. In order to minimize changes in hematopoiesis due to age or postnatal disease, only those premature infants who died within 72 hours of birth were included. Within these limits of birth weight and age at death, consecutive cases were studied. Over a period of three years and two months, from January, 1950, to March, 1954, postmortem examination was made of 327 premature infants. Of this number, 205 fulfilled