Followings are the results of studies by our clinical tissue culture method mainly on motility and ability of platelet formation of megakaryocytes in the spleen of human embryo and a patient with osteomyelofibrosis. 1) Megakaryocytes are always found in the spleen of human embryo, and no significant morphological differences are noted from those in the bone marrow. 2) Deformated movement, pseudopodial movement, and tentacle formation for the platelet formation are noted on megakaryocytes in the spleen of human embryos. As for functions of megakaryocytes, motility and ability of the platelet formation are significantly found at 4 month of age, but are remarkably decreased after 5 month of age. 3) A correlation is noted in human embryo between the functions of megakaryocytes and erythropoiesis in the spleen, a rise and fall of the one being accompanied by the other. 4) In a patient with osteomyelofibrosis, megakaryocytes found in the spleen show a remarkable decrease in the functions.