Fig. 4. Distribution of the crossing index (C) along the mediolateral axis. (A) Definition of the crossing index, C. The large rectangle represents a segment with a hypothetical clone, and the filled circles represent the ß-gal⁺ cells of the clone. The mediolateral axis of the segment is divided into parts of equal length. For each clone, the probability of crossing the limit between two parts, C(i), is equal to 1 if the clone has ß-gal⁺ cells on both sides of the limit, and is equal to 0 if all the labelled cells are on the same side of the limit. C is the sum of C(i) for all the clones. (B,C) Probability of crossing for the unilateral monosegmented (B) and bisegmented (C) clones. The mediolateral axis is divided into 20 parts of equal length. m, medial; l, lateral.