a preform of plastic material for manufacturing bottles through a blow - moulding process includes a substantially cylindrical central body having an open end and a closed end . in correspondence of the central area of the closed end there is provided a hollow appendix , projecting outwards of the preform , capable of internally housing the tip end of a stretching rod of the preform . both the hollow appendix and the remaining side portion of the closed end of the preform may be spherical , conical , elliptical or parabolic .