The megakaryocytes in 5 normal and 20 thrombocytopenic individuals were studied. The stages ofdevelopment undergone by the cells were delineated and a new classification, according to maturation and function, was made.

Using this classification, the megakaryocytic reactions in the 20 cases of thrombocytopenia were grouped into three distinct types. The first is characterized by a rapid multiplication of cells and a marked inhibition of platelet-formation without change in the process of maturation. The second is similar to the first with this exception that the total number of cells is not increased. The third reaction shows a normal or low total number of cells, with inhibited platelet-formation and accelerated maturation.

From the collected facts, the following conclusions are inferred: (1) The factors of maturation and of platelet-production affecting the megakaryocytes are different. (2) Certain diseases may disturb either of the two processes or both. (3) The number of platelets in the circulating blood is directly related to the percentage of megakaryocytic "C" cells in general. (4) Lesions affecting the spleen usually check the fragmentation of platelets from the megakaryocytic cytoplasm. (5) Infection and primary blood disorders may cause changes in the megakaryocytes but the explanation for this is still to be investigated.

ACKNOWLEDGMENTS The writer owes a debt of gratitude to the members of the Eye, Ear, Nose and Throat Department of the University of Santo Tomas and to the medical staff of the North General Hospital for the use of some of their patients in this investigation.