Studies of the Peripheral Blood of the Port Jackson Shark (Heterodontus portusjacksoni) with Particular Reference to the Thrombocyte

S ummary . Morphological features of the peripheral blood of the ancestral elasmo‐branch Heterodontus portusjacksoni , a shark species, are described. The red cell is nucleated and contains many membrane‐lined vacuoles which are derived from mitochondria. The granulocytic series resembles that of the mammals in appearance and in some phagocytic properties. The thrombocytes possess a number of similarities to mammalian platelets, but exhibit many differences. They aggregate readily, but this is temperature reversible and is independent of thrombin and ADP. They show a remarkable affinity for latex particles. Unlike many other species, fibrin formation and thrombocyte aggregation are apparently independent phenomena occurring in parallel rather than in series. Tritiated thymidine studies and the observation of mitotic figures in the peripheral blood suggest that at least part of the red cell and thrombocyte proliferation occurs in the circulating blood.