Fleming’s rule applies to all conductors in a magnetic field including the plasma arms of galaxies.
Depending on the direction of the current flow, there is a resulting inward or outward force on the plasma circuits. It must be outward as shown, otherwise extinction, not growth, would result.
This suggests that growth proceeds by the formation of a series of loops, giving the flat mesh structure seen.
Loop luminosity is perhaps a measure of current strength and of the order of formation.
The movie (above) is not of a huge slow-moving galaxy, but of a smaller fractal relative, star V838 in Monoceros. The growth action is the same but far quicker.
Technical note: Galaxy model
The alternator consists of conducting loops slicing through a magnetic field. Its output is lowest, but not zero, when the magnet and rotor are accurately coplanar and concentric. Any deviation improves efficiency dramatically, so fractal galactic wobbles can be expected to produce results from the sedate to pyrotecnic, depending on size.
The Hall effect nicely explains the separation of positive and negative flows in galactic plasma conductors ie. in the spiral arms. Note also that these are quantised, not linear and significantly, close observation reveals fractal behaviour. See www.nature.com/nature/journal/v422/n6930/fig_tab/422391a_F1.html.