  We study the stellar mass Tully-Fisher relation (TFR, stellar mass versus
rotation velocity) for a morphologically blind selection of emission line
galaxies in the field at redshifts 0.1 $<$ z $<$ 0.375. Kinematics ($\sigma_g$,
V$_{rot}$) are measured from emission lines in Keck/DEIMOS spectra and
quantitative morphology is measured from V- and I-band Hubble images. We find a
transition stellar mass in the TFR, $\log$ M$_*$ = 9.5 M$_{\odot}$. Above this
mass, nearly all galaxies are rotation-dominated, on average more
morphologically disk-like according to quantitative morphology, and lie on a
relatively tight TFR. Below this mass, the TFR has significant scatter to low
rotation velocity and galaxies can either be rotation-dominated disks on the
TFR or asymmetric or compact galaxies which scatter off. We refer to this
transition mass as the "mass of disk formation", M$_{\mathrm{df}}$ because
above it all star-forming galaxies form disks (except for a small number of
major mergers and highly star-forming systems), whereas below it a galaxy may
or may not form a disk.
