Abstract: Constituent layers are harvested from a centrifuged material mixture contained in a centrifuge tube assembly. The centrifuge tube assembly includes a transparent tube, one end of which is closed by an elastomeric cap. The tube contains one or more detectable constituent layer boundary markers which will settle gravimetrically in the material mixture when the latter is centrifuged. The markers will identify boundaries of the constituent layers which will gravimetrically separate from each other during the centrifugation step. After centrifugation, a cannula is inserted into the tube through the elastomeric cap, and a fluid material such as a liquid or a gas is injected into the tube through the cannula. The injected material displaces the centrifuged mixture column and the boundary markers toward the end of the tube opposite the cap, which opposite end is open so that the centrifuged material is expressed from the tube therethrough.