Fig. 6. Models for the spatial relationship between myocytes and their precursors in the pool for two adjacent segments. The pool of precursors before the formation of the clonal boundary is shown above two consecutive myotomal segments. The colour gradient symbolises the regionalisation of the precursors. In the inverted model (A), cells in the extreme parts of the pool of precursors (closed circles) give rise to myocytes in the middle of the myotome, and cells in the middle part of the pool of precursors (open circles) to myocytes in the extreme parts of the myotome, resulting in an inversion of the gradient. Consequently, labelling of a precursor cell in the middle of the pool (open circle) before the establishment of the clonal separation will give rise to cells on both sides of the boundary, and will result in a clone contributing only to the two medial and lateral extremities of the myotome. In the direct model (B), precursors in the pool for two segments give rise to myocytes, keeping the orientation of the gradient. Labelling of a cell in the middle of the pool of precursors (open circle) will result in a clone that is contiguous across the boundary.