Fig. 6. The development of the Xenopus retinotectal map according to the retinal induction model. Twenty cells are added to both retina and tectum at timesteps 0, 20000, 35000, 55000 and 80000. Each newly arriving axon makes its initial contacts at random within a 90° arc, centred at the retinotopically appropriate tectal position available to it. Top row. To the left are the colour-coded distributions of EphA and EphB over the retina. Different colour scales are used for EphA and EphB. To the right are shown the location of individual cells within the retina (circle) and within the tectum (square). The shading indicates the growth of the retina, from centre to periphery, and of the tectum, from rostrolateral to caudomedial. The required orientation of the map is that temporal (T), nasal (N), dorsal (D) and ventral (V) retina projects to rostral (R), caudal (C), lateral (L) and medial (M) tectum, respectively. Second row. The initial mapping. The two plots to the left show the distribution of the reciprocal of ephrinA density and the actual ephrinB density over the population of tectal cells existing at that time. Plotting the reciprocal of the ephrinA density eases the comparison between the distribution of EphA over the retina and that of ephrinA over the tectum. The two plots to the right show the initial map of tectum projected onto the retina through the connections made and the same information shown as a map of retina onto tectum. Each node of the lattice comprising the map of tectum onto retina marks the retinal location eliciting maximal response from a different tectal cell; each link connects the retinal positions associated with neighbouring tectal cells; similarly for the map of retina onto tectum. Third row. Similar plots at an intermediate stage in development. Bottom row. The final mapping. In the retinotectal maps, the projection from the oldest retinal cell, from central retina, is marked by a red disc and that of the most dorsotemporal retinal cell (which should project rostrolaterally) by a green disc. The distribution of EphA density over the nasotemporal retina is as given for wild-type retina in the legend to Fig. 2; EphB density along the dorsoventral axis is assumed to vary linearly.