  The process of gap formation by a growing planetary embryo embedded in a
planetesimal disk is considered. It is shown that there exists a single
parameter characterizing this process, which represents the competition between
the gravitational influence of the embryo and planetesimal-planetesimal
scattering. For realistic assumptions about the properties of the planetesimal
disk and the planetary embryo, a gap is opened long before the embryo can
accrete all the bodies within its region of influence. The implication of this
result is that the embryo stops growing and, thus, large bodies formed during
the coagulation stage should be less massive than is usually assumed. For
conditions expected at 1 AU in the solar protoplanetary disk, gap formation is
expected to occur around bodies of mass < 10^24 g. The effect of protoplanetary
radial migration is also discussed.
