Abstract: An extravertable-wall container is formed by extruding a parison into a blow-mold cavity having a pair of oppositely diverging halves which are joined at their widest portions, one half being slightly smaller than the other so that the frusto-conical wall of the container formed in this half can be pushed (compressed) into the other half to produce a stackable cup-shaped structure. When the container is to be filled, the inner wall portion is extraverted to form an extension of the container, e.g. the bottom. The container may have a frustoconical upwardly widening base which is geometrically smaller but inverted with respect to the larger frustoconical head. A neck or filling opening may be provided on this head.