Negative staining of whole mounts of platelets preserves several structures not previously described. External to the plasma membrane there is a coat which contains electron dense bodies of varying shape. External to the coat is a region which stains intensely with phosphotungstate. Platelet granules coexist with tubules in the granulomere as well as with a large number of other membraneous structures.

A speculative interpretation of the function served by these structures in platelet physiology is proposed. The PTA halo denotes a cloud of adsorbed plasma proteins, including fibrinogen maintained in the platelet environment by the coat. During viscous metamorphosis, coagulation is catalyzed by discharge of the contents of the granules and shedding of the coat.

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