The Evolution of Grammar

A blood collection and separator assembly of the type suitable for centrifuging to separate the plasma/serum or light phase from the heavy or cellular phase of blood is disclosed. The assembly includes a collection container and a piston slidably disposed therein for sealing off the plasma or serum phase from the cellular or heavy phase of blood after centrifuging is terminated. The piston is formed having a specific gravity greater than the specific gravity of blood and the interface-seeking piston is positioned in the container and includes at least two spaced sealing rings which contact the inner surfaces of the container. The piston provides a passage for conducting separated serum/plasma from below the piston to above the piston. A filter means is positioned on the piston and is in fluid communication with the passage therewith so that the serum/plasma is filtered as it passes through the passage. The lower sealing ring is deformable and acts like a valve to open the passage when the piston is subjected to centrifugation. The piston moves downward in the container and automatically stops at the serum/plasma-cellular inter face, when the filter means becomes clogged by the heavy phase. When the piston descent is stopped the deformed sealing ring closes the passage thereby forming a barrier between the heavy and light phases of the blood.