Autoradiogram of a frontal section through the brain of Tarsius bancanus after anterograde transport of tritium from the eye.
A number of features rule out any possibility that Tarsius is an anthropoid. Instead, Tarsius must have branched away from the main primate line early in evolution, before the lorises, lemurs and anthropoids branched off. The reversal of the magnocellular layers is shared between Tarsius and the colugo, further strengthening the primate-colugo link while emphasising the basal position of Tarsius within primates.
The medial terminal nucleus (MTN... white arrows), of the accessory
optic tract, is large, unlike any other primate. The size, shape and arrangement
of the MTN is very like a cat. The MTN is also well-innervated from the
ipsilateral eye (right side of diagram), a rare arrangement in any mammal..
The LGN is primate-like, but note that the external magnocellular layers.....black
arrows....lying next to the labelled optic tract........have the reverse
arrangement compared with all other primates. The ipsilateral magno cellular
layer (on right side of the illustration) lies externally, next to the
optic tract. On the contralateral side, (left side of diagram) there is
a dark gap (black arrow) where
the contralateral magno is located in other primates.(click here for comparison
to LGN of other primates)