this invention relates to closing an open end of a pailette containing a biological element . the invention includes heating a dose of thermo - plastic material to a flowable condition and injecting the dose into the cold open end of the paillettes so that the material adheres to the inner wall of the paillette and solidifies to form a stable plug closing the end . the thermo - plastic material is preferably in the form of solid sticks . a valve in closing apparatus opens and closes communication with a reservoir containing the thermo - plastic material under pressure and defines a pre - determined volume for the dose . the invention is especially applicable to the production of paillettes for use in artificial insemination .