Chemotactic models posit the existence of axon guidance molecules that direct the initial formation of the ocular dominance columns. These molecules would guide the axons as they develop based on markers specific to the axons from each eye. All chemotactic models must take into account the activity dependent effects demonstrated in later development, but they have been called for because several pieces of evidence make entirely activity dependent formation unlikely. First, it has been shown that the ocular dominance columns in squirrel monkeys have mirror symmetry across the cortex. This is very unlikely to occur by activity dependent means because it implies a correlation between the nasal retina of one eye and the temporal retina of the other, which has not been observed. Furthermore, work in achiasmatic Belgian sheepdogs has shown that columns can form between the projections from the temporal and nasal retina of the same eye, clearly suggesting a nasal-temporal labeling, rather than contralateral vs. ipsilateral, which would be much easier to explain with activity dependent mechanisms. Despite this, a molecular label that directs the formation of the ocular dominance columns has never been found.