The early stages in achieving precise retinotopy in the midbrain projection were studied by Marotte using the same combined laser lesion and HRP technique as described above. Even though a lesion had been made in the retina, a filing defect in the terminal label in the colliculus was not visible in animals younger than 40 days and it was incomplete for some time thereafter. This indicated that, unlike the adult, ganglion cells outside the region of the lesioned retina were also projecting their axon into the region occupied by axon of cells in the lesioned area. That is, the initial ingrowth of optic axon to the colliculus had not let them to precisely the appropriate spot. Was this because the collicular location of axon in the early stages was rather more loosely organized with respect to their neighbours from similar retinal locations than was the case later on? Or was it because some axon made gross errors? This was answered by Marotte by back-labelling ganglion cells in the retina with localised injection into their terminal zone in the caudal colliculus at progressively earlier ages, with a precision that would be difficult to approach in an intrauterine fetus. The answer was unequivocal. At no stages did ganglion cell axons make gross errors of growth. What might be called inappropriately projecting ganglion cells were always in a halo, diminishing in size with age, around those cells with axons ending in what would become the appropriate site of terminal in the adult projection. Those more widespread terminal branches may be removed by axon pruning, cell death, or both. It seemed that, even in the very earliest stages, axon of ganglion cells are distributed in the colliculus and perhaps even grow selectively along pathways, in a manner specified by the retinal location of the cells bodies, albeit less precisely than in the adult.