Using injections of horseradish peroxidase (HRP) and the osmium-tannic acid method, megakaryocytic cells in the livers of rat embryos at 12-16 days of gestation were examined for the purpose of classification of the stages of formation of the platelet demarcation membrane. Megakaryoblasts were classified into the following three types according to the formation patterns of the demarcation membrane. 1) The P-type megakaryoblasts showed plate-like membrane invaginations in large localized areas at early stages. The invaginating membrane developed toward the periphery of the nucleus. 2) The L-type megakaryoblasts showed localized labyrinthine membrane invaginations but no definite direction in its development. 3) The T-type megakaryoblasts had tubular invaginations at multiple sites on the plasma membrane. The P- and L-type cells were observed at 12 and 13 days of gestation. The T-type cells were found after the 14th day. In all the types of megakaryoblasts the membrane invagination occurred in the areas making contact with hepatocytes. It was agreed that the cells of the megakaryocytic series in which the demarcation membrane developed contrary to the basic pattern were ordinary promegakaryocytes. The megakaryocytes forming networks of the demarcation membrane dividing into platelet areas were small in cell size. Examination of the patterns of formation of the demarcation membrane proved useful for classifying the megakaryocytic series at each stage of maturation.