The nasotemporal division of retina in the Siamese cat.

By sectioning one optic tract in Siamese kittens and allowing sufficient time for the affected ganglion cells to degenerate and disappear, we have obtained maps of the distribution of ipsi- and contralaterally projecting ganglion cells in the retina of the Siamese cat. As previous work predicted, many ganglion cells in the Siamese cat retina project contralaterally which, in the normally pigmented cat, would project ipsilaterally. The transition from the pattern of projection typical of nasal retina (all contralateral) to that typical of temporal retina (most ipsilateral) is much more gradual in the Siamese cat than in the normally pigmented cat, and is centred 1.7-3 mm temporal to the area centralis, instead of at the area centralis. In the Siamese cat only a few ganglion cells at the area centralis project ipsilaterally, as against nearly 50% in the normally pigmented cat. The proportion of ipsi-projecting cells reaches the 50% level 1.7-3.0 mm into temporal retina, and overall the zone of transition may be several millimetres wide, as against about 0.2-0.5 mm in in the normall pigmented cat. Evidence is presented that the degree of the abnormality of nasotemporal division varies between individual Siamese cats and that the abnormality is more severe among large or "giant" cells than among the population as a whole.