Fig. 5. Models for the relationship of the precursor cells before and after bilateralisation. The pool of precursors (the large circle) before bilateralisation is represented above the myotomal segments of the left and right sides. Green arrows represent the ML axis of the left and right myotomes (below the myotomal segments) and red arrows represent the prefiguration of this axis in the pool of precursor cells before bilateralisation (above or to the right of the pool). Gradient of colours represents the ML regionalisation of the myotome, in the myotomal segments and in the precursor pool. (A) ML axis of the myotome is prefigured in ML axis of the pool, with an at least partial separation of the left and right precursors (red double arrow). Medial cells (white circle) in the pool contribute medially to the myotome, and lateral cells (black circle) laterally. Lateral cells contribute to a single side of the embryo, resulting in the absence of bilateral lateral clones. (B) No representation of the ML axis of the myotome in the pool before bilateralisation. Therefore, there is no relationship between the position of the precursors within the pool and the ML position of their myotomal descendants. (C) The ML axis of the myotome is prefigured in the ML axis of the pool, without separation between left and right precursors. Cells to the left of the pool (white circle) contribute to the lateral part of the left myotome and to the medial part of the right myotome, and this is inverted for cells to the right of the pool (black circle), resulting in complete asymmetry of the bilateral clones. (D) Orthogonal representation of the ML axis of the myotome in the pool before bilateralisation. All cells contributing either medially (white circle) or laterally (black circle) to the myotome can do so bilaterally. m, medial; l, lateral.

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