An elongated, pillow shaped prosthetic spinal disc nucleus body for implantation into a human intervertebral spinal disc, made of a hydrogel core and a flexible constraining jacket surrounding the hydrogel material core that permits the hydrogel core to expand and contract. The hydrogel core has a length approximating the sagittal diameter of a nucleus of the human disc, a width less than the length, and a height less than the length or width. The hydrogel core will expand and contract in a desired fashion as it imbibes and expels fluids in response to various loads placed upon the spinal tract. The constraining jacket is porous to allow fluids to pass through to the hydrogel core, but prevents the hydrogel from escaping, thus fostering the natural physiology of the human intervertebral disc. By implanting two prosthetic spinal disc nucleus bodies side-by-side into a damaged disc of a human spine, both height and motion can be maintained.