1. Under normal circumstances blood platelets are largely derived from the megakaryocyte of the blood-forming organs.

2. The transitional leucocyte, representing a persistent form of the embryonic premegakaryocyte, is a circulating homologue of the megakaryocyte and probably plays some part in normal platelet formation.

3. Under conditions of excessive demand for platelet production, there may be a greater or less reversion to an embryonic mode of platelet formation in which less highly specialized cells than the megakaryocyte participate in platelet production.

4. In addition to the megakaryocyte, the cells that have been observed to take part in platelet formation are hyperplastic endothelial cells in the marrow, and mononuclear and transitional cells (premegakaryocytes) in the marrow, spleen, and blood.

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