Abstract
Presumptive red and white fibres originate from alternating multinucleated units throughout a single stack of fused myoblast plates in the myotome when muscular contractions first appear (1 1/2 days before hatching). At hatching the usual cytological differences between red and white fibres start to develop. The number of red and white fibres increases after hatching by splitting of preexisting fibres rather than by differentiation de novo from myoblast fusion. Intermediate fibres, on other hand, originate by multiplication, migration, and fusion of cells derived from the dermatome.
