Using the HRP retrograde transport technique in two different genera of owls (Speotyto and Tyto), we have studied the distribution of neurons projecting to the optic tectum and the visual thalamus. Small injections of HRP were made into these structures from the pial surface after they had been visualized directly by dissection of the overlying bone. In contrast to the findings in mammals, retinal ganglion cells were labeled only in the eye contralateral to the injection site, whether this was in the thalamus or tectum, and the labeled ganglion cells were found on both nasal and temporal sides of the vertical retinal meridian through the fovea. After thalamic injections, labeling was prominent in temporal retina representing the binocular field, temporal to the optic nerve head. Retinothalamic ganglion cells formed roughly concentric lines of isodensity centered on the fovea (Speotyto) or area centralis (Tyto); labeling from thalamic injections involved both large and medium-sized neurons, but did not involve the smallest nor a conspicuous class of very large neurons. Tectal injections led to prominent labeling along the horizontal streak region, with horizontally elongated isodensity contours in both Tyto and Speotyto; retinotectal ganglion cells were heterogeneous and included a group of very large neurons and anther group of small neurons, neither of which was labeled from the thalamus. In the visual Wulst, labeled neurons were confined to the supragranular layers after both tectal and thalamic injections. Corticotectal neurons were found in both ipsilateral and contralateral visual Wulst. They were characterized by large cell bodies and prominent dendrites. Corticotectal neurons were distributed throughout the mediolateral extent of the ipsilateral Wulst and therefore involved both the monocular and binocular representations of the visual field. Corticothalamic neurons, found only in the ipsilateral Wulst, were characterized by smaller cell bodies and fine dendrites. They were confined to the monocular crescent on the extreme medial edge of the World.

The topographic distribution of retinal ganglion cells and their cell body size have been studied in five Falconiform species, including predatory (chilean eagle Buteo fuscenses australis, and sparrow hawk Falco sparverius) and carrion-eating (chimango caracara Milvago chimango; condor Vultur gryphus, and black vulture Coragyps atratus) birds. All these species had a well defined nasal fovea and a horizontal streak. Instead of a temporal fovea as in eagles and hawks, an afoveate temporal area is present in chimango, condor, and vulture. The highest ganglion cell density was found in the nasal fovea of Falco and Buteo with 65,000 and 62,000 cells/mm2, respectively. A negative correlation between ganglion cell density and cell body size was found in all the species studied. The specializations of the temporal retina showed a rather homogenous population of medium sized neurons, while the nasal foveas showed a homogeneous population of smaller ganglion cells. Finally, the peripheral retina showed a heterogeneous population of large, medium, and small ganglion cells. Predatory behavior appears to be closely related to foveal specializations, and is best exemplified in the eagle and hawk and to a lesser extent in the chimango.

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Using a combination of anatomical and physiological techniques we have studied some of the neural connections subserving binocular vision in two species of artiodactyl ungulates (the sheep, Ovis sp., and the goat, Capra hircus). After monocular injections of tritiated proline, transsynaptic transport was observed bilaterally in layers 4 and 6 of visual cortical areas V1 and V2, but there were no sharply defined ocular dominance columns of the kind seen in cats and rhesus monkeys. In coronal sections there was a discontinuity in density of labelling between areas V1 and V2 corresponding to a point in the visuotopic map about azimuth –15° in the ipsilateral visual field. This discontinuity was most pronounced in the hemisphere ipsilateral to the injected eye. We conclude, therefore, that while the cortical representation of ipsilateral visual space can be explained by the retino-geniculo-cortical input pathway from the contralateral eye, the physiologically demonstrated cortical contribution to ipsilateral visual space from the ipsilateral eye cannot be explained in this way. This conclusion was reinforced by experiments using retrograde transport of horseradish peroxidase from the lateral geniculate nucleus (LGN) and medial interlaminar nucleus (MIN) to retinal ganglion cells in flattened whole mounts. These experiments revealed a sharp nasotemporal decussation in the ipsilateral retina, which could not thereby subserve any significant representation of the ipsilateral visual field. In contrast the contralateral nasotemporal decussation was smeared, with many labelled ganglion cells in the temporal retina which could subserve visual input from the ipsilateral hemifield. When we estimated the projection of the nasotemporal decussation line into visual space, we found that it was tilted from vertical by about 5°in each eye, in a similar way to that already reported in the cat. Neurophysiological recordings from binocular neurons in area V1 with different vertical eccentricities also showed that the vertical horopter (the midsagittal reference plane for binocular vision) would be tilted in life when the cyclotorsional position of the eyes was taken into account. Thus both anatomical and physiological methods concur in the prediction that ungulates have a tilted vertical horopter like that described for two other terrestrial species, the burrowing owl and the cat. Anatomical experiments reveal other similarities between the organisation of the ungulate''s visual pathways and that of the cat. For example, after tritiated proline injection in V1, we found visuotopic labelling in the claustrum, dorsal LGN, cortical area V2, and the superior colliculus. Horseradish peroxidase injections in V2 also revealed a direct input to that area from the MIN and dorsal LGN and also from layers 2, 3 and 5 of the splenial area.

The histogenetic development of the archi-, paleo-and neocortex of the rabbit has been studied using autoradiographic techniques. The analysis is based upon a series of 54 embryos and newborn rabbits, whose mothers were injected on successive days of gestation with a single dose of tritiated thymidine. Neurons destined to form the cerebral cortex originate in the matrix layer surrounding the lateral ventricles. From the matrix, cells migrate to form the different cortical layers in an inside-out sequence. This sequence appears reversed in the fascia dentata. In the anterior region of the brain, a single developmental gradient of histogenesis of the cortex is observed. This gradient starts in the ventralmost areas of the cortex and then advances in a dorso-medial direction. In the posterior region of the brain, there are two gradients of cortical histogenesis. These gradients start at the lateral aspect of the cortex. One advances in a ventromedial direction and the other in a dorsomedial direction. The archi and neocortex of both anterior and posterior regions of the brain appear to develop synchronously. However, the paleocortex of the anterior regions of the brain (prepyriform cortex) leads in its development the paleocortex of the posterior regions of the brain (entorhinal cortex and perirhinal cortex).