Fig. 3. Chick commissural growth cones decrease in complexity as they execute their final turn. (A) Schematic representation of the most predominant contralateral commissural pathway. The numbers indicate the positions along the pathway at which growth cone morphology was examined. In B, rows 1-5 show the morphology of growth cones at the corresponding regions of the contralateral pathway depicted in A. Commissural growth cones are large and highly elaborated with numerous filopodial extensions and well-spread lamellipodia within more proximal regions of the contralateral pathway (rows 1 and 2). By contrast, upon reaching the dorsal limit of their extension (row 3), axons terminate in growth cones that are simple and elongated. This simple morphology is maintained in more distal segments of the pathway (rows 4 and 5), as axons project rostrally in the longitudinal plane. Scale bar: 100 µm.