Fig. 3. Dual-colour in situ hybridisation for slow (dark blue) and fast (red) myosin heavy chain (MyHC). (A–E) Serial transverse sections of a stage 16 embryo (approximately 45 somites). (A) Posterior tail; fast and slow MyHC are still undetectable. (B) Middle tail; fast MyHC expression is evident in the medial domain of the somite. (C) Anterior tail; fast muscle cells now extend almost the full dorsoventral extent of the somite. (D) Posterior trunk (immediately anterior to the anal vent); the fast muscle cells fill the medial bulk of the somite. Slow muscle precursors are now evident and are mostly present in the medial domain of the somite (arrow). (E) Anterior trunk; the migrating slow muscle cells are now lateral to the differentiated fast muscle cells. (F) Transverse section through the middle trunk of an eyed-stage embryo (stage 20); the slow muscle cells have reached the outermost regions of the myotomes; other myogenic cells have differentiated into fast muscle. Scale bars, 45µm.