2 STAGES IN THE DEVELOPMENT OF A MAMMALIAN RETINOCOLLICULAR PROJECTION

THE retinocollicular projection in the marsupial mammal the wallaby Ma cropus eugenii, has been investigated anatomically to determine the or der in the developing projection and electrophysiologically to determi ne the time of onset of synaptic transmission by recording evoked pote ntials in the colliculus in response to stimulation of the optic nerve . There are two clear stages: a protracted period when retinal axons g row into the colliculus in coarse retinotopic order with no recordable electrical activity followed by the formation of terminal zones in re tinotopically correct positions, the loss of more widely distributed a xons and the onset of evoked potentials. The two stages are not seen i n non-mammalian vertebrates where the projection is functional from th e beginning.