In the coagulation cascade the clot is initiated by negatively charged paicles on the glass bead. It is in-vivo initiated by:

The coagulation cascade is series of amplifying enzymatic reactions resulting in deposition of an insoluble fibrin clot. Each reaction step involves: Enzyme (an activated coagulation factor) Substrate (an inactive pro-enzyme form of a coagulation factor) Co-factor (a reaction accelerator) These components are added on a -vely charged phospholipid surface, which is provided by activated platelets. In vivo, tissue factor is the major initiator of coagulation, which is stimulated by feedback loops involving thrombin. Clotting is initiated in the laboratory by adding phospholipids, calcium and either a -ve charged substance such as glass beads (intrinsic pathway) or a source of tissue factor (extrinsic pathway).