  We consider the most general renormalizable chiral Yukawa model with
$SU(3)_{\rm color}$ replaced by $SU(N_c)$, $SU(2)_{\rm L}$ replaced by $SU(N_w
)$ and $U(1)_{Y}$ replaced by $U(1)^{N_w -1}$ in the limit $N_c
\rightarrow\infty$, $N_w \rightarrow\infty$ with the ratio
$\rho=\sqrt{{N_w}\over{N_c}} \ne 0,\infty$ held fixed. Since for $N_w \ge 3$
only one renormalizable Yukawa coupling per family exists and there is no
mixing between families the limit is appropriate for the description of the
effects of a heavy top quark when all the other fermions are taken to be
massless. A rough estimate of the triviality bound on the Yukawa coupling is
equivalent to $m_t \le 1~TeV$.
