The retrograde transport of horseradish peroxidase has been used to identify efferent cells in area 17 of the macaque. Cells projecting to the lateral geniculate nucleus are small to medium sized pyramidal neurons with somata in lamina 6 and the adjacent white matter. The projection to the parvocellular division arises preferentially from the upper half of lamina 6, while that to the magnocellular division arises preferentially from the lower part of the lamina. The projection to both superior colliculus and inferior pulvinar arises from all sizes of pyramidal neurons lying in lamina 58 (Lund and Boothe, '75); at least pyramidal neurons of lamina 5B send collateral axon branches to both destinations. Injections with extensive spread of horseradish peroxidase show that many cells of lamina 4B and the large pyramidal neurons of upper lamina 6 also project extrinsically but their terminal sites have not been identified. Other studies have indicated that cells of laminae 2 and 3 project to areas 18 and 19. Therefore every lamina of the visual cortex, with the exception of those receiving a direct thalamic input, contains cells projecting extrinsically. Further, each lamina projects to a different destination and from Golgi studies can be shown to contain cells with specific patterns of dendritic branching which relate to the distribution of thalamic afferents and to the patterns of intracortical connections. These findings emphasise the significance of the horizontal organisation of the cortex with relation to the flow of information through it and contrast with the current concept of columnar organisation shown in physiological studies.

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The development of the central optic projections in albino rat fetuses has been studied using light and electron microscopic degeneration techniques and the horseradish peroxidase method for demonstrating axonal projections of neurons. The first optic axons to reach the region of the optic chiasm arrive at day 15. By day 16, a substantial optic chiasm is seen and the optic tract can be traced into the epithalamus, having first passed through the ventral lateral geniculate nucleus and a thin lamina of cells which is thought to correspond to part of the future dorsal geniculate nucleus. A growth rate of 80-100 mum per hour is estimated for the fastest growing axons. By day 16-1/3 the first axons have entered the anterior border of the superior colliculus and in the next day have grown across the entire rostrocaudal extent with the exception of the medial and lateral edges. The optic axons are recognized at day 17 as bundles lying just below the surface, but in older animals they come to lie deeper, as the whole layer of optic innervation broadens. The first synapses to be formed in the superior colliculus (some of them of optic origin) appear on day 17. Subsequently, there is a gradual increase in the number of contacts, the great majority being formed by optic axons. Compared with previous studies on Xenopus and chick, one of the most striking features of the development of the central visual connections in the rat is the relatively long time before the first optic axons reach the brain and the speed with which they innervate the central structures once they have arrived.

In the primate, ganglion cells of the temporal retina project ipsilaterally and those of the nasal retina, contralaterally into the optic tract. The vertical meridian passing through the fovea defines the border between these two populations of ganglion cells and has been demonstrated in four Macaque monkeys after unilateral injection of horseradish peroxidase into the dorsal lateral geniculate nucleus and examination of the pattern of retrograde labeling of those ganglion cells projecting to the injected side. A median 1 degree vertical strip in which ipsi- and contralaterally projecting ganglion cells intermingle was found, confirming the report by Stone et al. ('73). In addition, occasional extrafoveal labeled ganglion cells were found as far as 2 degrees from the vertical midline in the otherwise unlabeled hemiretinae. These ganglion cells were not numerous and had somata of all sizes, suggesting that they do not constitute a separate class of ganglion cells as found in the temporal retina of the cat. In contrast to the description by Stone et al. ('73), the strip of vertical overlap did not show a constant width through the fovea, since mixing of labeled and unlabeled ganglion cells was found in a band approximately 1/2 degree wide along both the nasal and temporal rims of the foveal pit which is 500 mum (2 degrees) in diameter. Beyond these 1/2 degree arcs, the appropriate hemiretina was either completely unlabeled, or contained virtually every ganglion cell labeled on the side projecting to the injected dorsal lateral geniculate nucleus. The scattered labeled ganglion cells rimming an otherwise unlabeled hemifovea represent a possible anatomical basis for the phenomenon of "macular" or "foveal sparing" in which unilateral damage to the occipital cortex produces homonymous hemianopsia with sparing of a small island of centralmost vision extending about 1 degree from the foveal center. From this study, it is not possible to define the receptive fields or specific photoreceptor connections of the ganglion cells labeled with horseradish peroxidase, so that at the present time quantitative correlations cannot be made between the numbers of ganglion cells remaining on the affected side of the fovea and the extent of preservation of visual function in the spared zone. The presence of labeled ganglion cells rimming the fovea in its entirety is compatible with the sequence of foveal development in late prenatal life. After lateral displacement both nasally and temporally of ganglion cells which initially lay in the median vertical overlap strip of 1 degree, in the adult retina a strip approximately 1/2 degree wide around the perimeter of the foveola should contain a mixture of ipsi- and contralaterally projecting ganglion cells. The total population of ganglion cells beyond this 1/2 degree band should be completely ipsi- or contralateral in their projection patterns, as is observed...