Apparatus for forming hollow fibers and said fibers.

A nozzle assembly for forming hollow fibers, particularly a "Z-fiber" having longitudinally oriented trabeculae with the fiber wall, is disclosed. The nozzle assembly contains a replaceable nozzle cap, a replaceable seal-guide, and a retractable central bore used to supply core material for the hollow fibers. The Z-fibers are particularly useful in enabling cells, solutes, or the like, to enter the fiber wall and move longitudinally into the center of the fiber wall.