The formation and regulation of the larval lateral line melanophore band of Brachydanio rerio was investigated. It is comprised of about 30 melanophores linearly arranged along the horizontal skeletogenous septum. This stripe forms in two distinct stages, an initial migration and orientation of pigment cells which outlines the stripe, and a later round of melanophore differentiation which regulates and perfects the stripe. From the initial melanocyte population which appears between approximately 24 to 48 hours of development and spreads ventrally over the body, some (0–21) melanocytes migrate to and reorient into the horizontal skeletogenous septum. At day 3 this initial lateral line stripe varies in melanophore number between individuals and even between left and right sides of the same embryo. By eight days of age however the lateral line bands of different fish are more uniform in appearance due to the entry of a second wave of melanophores which appear between days 4 and 8.

Some observations suggested the operation of a cell territorial-regulating mechanism. Second round melanophores tend not to appear in somite areas already occupied by 3-day cells but invariably appear in unoccupied locations. We propose that some “exclusion principle” operates in this embryonic cell population, controlling the positions along the stripe where second wave melanophores can appear.