  In the linear theory, the galaxy angular momentum vectors which originate
from the initial tidal interactions with surrounding matter distribution
intrinsically develop perpendicular alignments with the directions of maximum
matter compression, regardless of galaxy mass. In simulations, however, the
galaxy spins exhibit parallel alignments in the mass-range lower than a certain
threshold, which depends on redshift, web type, and background cosmology. We
show that the observed three dimensional spins of the spiral galaxies located
on the void surfaces from the Sloan Digital Sky Survey indeed transit from the
perpendicular to the parallel alignments with the directions toward the nearest
void centers at the threshold zone, $9.51\le\log
[M_{th,\star}/(h^{-1}\,M_{\odot})]\le10.03$. This study presents a first direct
observational evidence for the occurrence of the mass-dependent spin transition
of the real galaxies with respect to the non-filamentary structures of the
cosmic web, opening a way to constrain the initial conditions of the early
universe by measuring the spin transition threshold.
