A modular nucleus pulposus prosthesis comprising at least two generally mirror image complimentary disc segments combined to form a generally discoid endoprosthetic disc and to associated tools and methods for replacing the intervertebral disc. The complimentary segments are substantially identical including outer circumferential walls roughly equal to a semi-circle aligned along concave-convex inner wall inner walls forming a common border and a generally symmetrical discoid congruent structure which is positioned within the annulus of a spinal disc section. Disc segments may include structures to support, position and secure the segments to one another intradiscally. Surgical tools include structures for inserting and aligning disc members together. Also disclosed is a system and method for replacing the nucleus pulposus using the surgical tools adapted for placement and alignment of the disc.