Such bonds may be in the form of negotiable coupon bonds, payable to bearer, with the privilege to the holder of having them registered and upon such conditions in such manner as the trustees shall prescribe, and the principal thus made payable to the registered holder, unless the last registered transfer shall have been to bearer, or such bonds may be issued as fully registered bonds. If issued as fully registered bonds, it may be provided that they may thereafter be converted into negotiable coupon bonds of the tenor described above.