1. A method of producing human neural progenitor cells from human embryonic stem (hES) cells in vitro, said method comprising: (a) culturing undifferentiated pluripotent hES cells for 2-3 weeks under adherent conditions so as to generate differentiating cells, wherein said culturing does not induce extra embryonic differentiation or cell death; and (b) culturing said differentiating cells in the presence of serum free medium supplemented with growth factors which include epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF), thereby obtaining neural progenitor cells, wherein said neural progenitor cells are capable of further differentiation into neurons, into oligodendrocytes, and into astrocytes, and wherein said neural progenitor cells express polysialated N-CAM, nestin, vimentin and the transcription factor Pax-6.