Among the mammalia, the carnivora (dogs and cats) was chosen to make studies mainly by our clinical tissue culture method on megakaryocytes in the spleen of the successive stage from newborn up to adult. Followings are the results. 1) In dogs and cats, megakaryocytes are found in the spleen from newborn up to adult. During newborn, dogs have more megakaryocytes in the spleen than cats, but the reverse comes to be true during the following stages. 2) In dogs and cats, no significant morphological differences are noted between megakaryocytes in the spleen and those in the bone marrow. 3) By splenic tissue culture method, megakaryocytes in the spleen are found to have motility as those in the bone marrow. Ability of platelet separation from megakaryocytes in the spleen are found during newborn and infancy (1 month of age) in dogs, and during newborn in cats. During the successive stages of growth, the functions of megakaryocytes are noted to be decreased. 4) A correlation is noted in dogs and cats between the functions of megakaryocytes and erythropoiesis in the spleen, a rise and fall of the one being accompanied by the other.