  Observations of line emission from water masers near the center of the galaxy
NGC 4258 have recently provided compelling evidence for rotating disk of gas,
viewed nearly edge-on, surrounding a massive black hole (Miyoshi etal. 1995).
  We show that the disk is only marginally stable to radial perturbations - a
stability regime where weak non-axisymmetric disturbances tend to grow via the
swing amplification effect, forming a ragged, multi-armed spiral pattern
similar to that observed in Sc galaxies.
  This suggests a natural explanation for the apparent clustering of the
high-velocity emission sources into several distinct clumps, and for the
observed regularity in the distance intervals between them. The clumps of
sources appear at the intersections of the spiral arms and the radial line of
longest coherent gain path, and are thus spaced apart at the characteristic
crest-to-crest radial distance between the arms. This interpretation implies an
H_2 density and disk thickness of about 1.8x10^{10} (Q/1.6)^{-1} cm^{-3}, and
0.003(Q/1.6) pc at a radius of 0.2 pc, respectively, where the local value of
the stability parameter is 1.2 < Q < 2.
