  We present multi-epoch optical spectra of the $\gamma$-ray bright blazar
1156+295 (4C +29.45, Ton 599) obtained with the 4.3~m Lowell Discovery
Telescope. During a multi-wavelength outburst in late 2017, when the
$\gamma$-ray flux increased to $2.5\times 10^{-6} \; \rm phot\; cm^{-2}\;
s^{-1}$ and the quasar was first detected at energies $\geq100$ GeV, the flux
of the Mg II $\lambda 2798$ emission line changed, as did that of the Fe
emission complex at shorter wavelengths. These emission line fluxes increased
along with the highly polarized optical continuum flux, which is presumably
synchrotron radiation from the relativistic jet, with a relative time delay of
$\lesssim2$ weeks. This implies that the line-emitting clouds lie near the jet,
which points almost directly toward the line of sight. The emission-line
radiation from such clouds, which are located outside the canonical
accretion-disk related broad-line region, may be a primary source of seed
photons that are up-scattered to $\gamma$-ray energies by relativistic
electrons in the jet.
