  Using 3D radiative transfer calculations on the reionization of an
inhomogeneous universe, QSO absorption line systems are simulated and they are
compared with observations of Ly$\alpha$ continuum depression at high
redshifts. By this comparison, it is found that the metagalactic UV intensity
decreases rapidly with $z$ at $z>4$ as $I_{21}=0.5\exp [3(4-z)]$, and the
reionization must have taken place between $z=6$ and 10. Based on this
time-dependence of UV background intensity, we explore the collapse of
pregalactic clouds in the UV background, and find that the self-shielding is
prominent above a mass scale as $ M_{BIF}=3.0\times 10^{11} M_\odot
[(1+z_c)/5]^{-4.2}(I_{21}/0.5)^{0.6}. $ This mass scale predicts the
bifurcation of galactic morphology, and by confrontation with observations it
turns out that the bifurcation mass successfully discriminates between
elliptical and spiral galaxies.
