  We report the results of our three-dimensional radiation hydrodynamics
simulation of collapsing unmagnetized molecular cloud cores. We investigate the
formation and evolution of the circumstellar disk and the clumps formed by disk
fragmentation. Our simulation shows that disk fragmentation occurs in the early
phase of circumstellar disk evolution and many clumps form. The clump can be
represented by a polytrope sphere of index $n \sim 3$ and $n \gtrsim 4$ at
central temperature $T_c \lesssim100$ K and $T_c \gtrsim 100$ K, respectively.
We demonstrate, numerically and theoretically, that the maximum mass of the
clump, beyond which it inevitably collapses, is $\sim 0.03 M_{\odot}$. The
entropy of the clump increases during its evolution, implying that evolution is
chiefly determined by mass accretion from the disk rather than by radiative
cooling. Although most of the clumps rapidly migrate inward and finally fall
onto the protostar, a few clumps remain in the disk. The central density and
temperature of the surviving clump rapidly increase and the clump undergoes a
second collapse within 1000 - 2000 years after its formation. In our
simulation, three second cores of masses $0.2\msun$, $0.15\msun$, and
$0.06\msun$ formed. These are protostars or brown dwarfs rather than
protoplanets. For the clumps to survive as planetary-mass objects, the rapid
mass accretion should be prevented by some mechanisms.
