Supporting Information

Files in this Data Supplement:

Movie 1. This time-lapse movie depicts the behavior of interneuromast cells in the developing posterior lateral-line organ of a homozygous colourless (cls) mutant zebrafish larva. The interneuromast cells can be observed to divide mitotically, coalesce, and eventually form two distinct pro-neuromasts. The principal horizontal structure, which is most prominent one-third of the field from the bottom at the outset of the sequence, is the horizontal myoseptum; above it lies the posterior lateral-line nerve. The two oblique features at the bottom of the field, which are most evident at the movie’s conclusion, represent vertical myosepta. The larva was labeled with fluorescein-coupled ceramide for 12 h, mounted in 1% low-melting-point agarose, and imaged under a ´ 20 objective with a BioRad-1024ES confocal system. Beginning 58 h after fertilization, 360 images of a single focal plane were acquired at 2-min intervals. The animal’s head lies to the left and its dorsal surface to the top of the field. Although raised at 28.5°C, the animal was maintained during imaging at 26°C.