By bone-marrow tissue culture of hnman and guinea pigs, the author observed under a phase-contrast microcscope the morphological changes in megakaryoytes in the course of the culture and the mechanism of the platelet separation; and obtained the following results: 1. The progress of the degenerative changes in each megakaryocyte is not necessarily uniform even in the same medium conditions, and these megakaryocytes present varied features. 2. The duration in which megakaryocytes can carry on their functions in a simple culture medium is 90 hours at most in the case of human and up to 48 hours in the case of guinea pigs; and the peak of their activities is reached 18 to 30 hours after the culture in the case of human, and 15 to 18 hours in the case of guinea pigs. 3. It has been confirmed that tentacle-like processes indicative of platelet formation can be observed only when megakaryocytes are most actively moving. 4. The mottled appearance due to multiple small areas or so-called platelet separation of megakaryocytes as observable in the bone-marrow smear is due to the degenerative changes or artificial products of megakaryocytes and it is in no way associated with the mechanism of the platelet separtion. 5. Small tongue-like processes what appear to be platelets erupting from the margin of the megakaryocyts body are nothing but one of degeneration features of the megakaryocyte, and they are different from platelets. 6. As for the possiblity of nuclear substance of megakaryocytes moving into platelets, it is believed that at least from the morphological viewpoint the nuclear substance does not move into the platelet.