MBE growth on high symmetry surfaces depends to some extent on the shape of the terraces which form the embryos of each new atomic layer. The conditions for formation of compact or fractal terraces are discussed in two limits: large terraces, to which a continuous description applies, and small terraces, which require that the discrete nature of matter be taken into account. In both cases it is found that terraces are fractal if and only if diffusion of adatoms along steps is much slower than on the flat surface at the same temperature. Precise conditions are given which should be accessible to experiment