Fig. 4. Cellular kinetics during convergence and extension. (A) Kinetics of notochord domain convergence follows exponential decay. The rate of narrowing is represented by a single exponential fit to the data. The rate constant k_C is -0.0064. (B,C) Velocity versus position is linear for both convergence (B; k_C=-0.0059) and extension (C) (k_E=0.0062 for the notochord, green circles; k_E=0.0013 for the somite domain, red circles). The individual points represent velocities and locations of the entire field of tracked cells from one time sample, at 9 hpf (midgastrula stage). At this time point, k_C, is slightly lower than the value determined from the decreasing width of the field in A (-0.0059 min^-1 versus -0.0064 min^-1). We observed that under the conditions of our recordings, k_C, measured as the slope of the gradient, is not stable time point to time point, rather the slope is `jittery'. An `average'/integrated estimate (see Materials and Methods) over the entire time period is -0.0073 min^-1, agreeing within 15% with the calculation from the notochord's change in width (A). The line of best fit was determined using linear regression. Gradients were assumed to fit when r² showed evidence for substantial explanation of variance and the F-statistic exceeded the P=0.05 significance threshold. The blue arrow in B indicates the position of the midline; here, the ML velocity is approximately zero.

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