Fig. 9. Three models for the growth and splitting of laterals are shown (A-C). A lateral is shown before and after splitting near the distal tip. The expectation if subsequent growth is restricted to the distal tip, the proximal end or occurs throughout the length of the lateral, respectively. The shaded area represents material added by growth after splitting. Note that in A, the length of the arms distal to the split increases but there is no change in the distance from the branch-point to the proximal end. In B, the arms do not change length, but the distance from the branch-point to the proximal end increases. In C, growth takes place throughout the lateral. In this case we predict an increase in the length of the arms distal to the branch-point and an increase in the distance from the branch-point to the proximal end. Our observations on fry arista laterals are consistent with C.