UNUSUAL PATTERN OF RETINOGENICULATE PROJECTIONS IN THE CONTROVERSIAL PRIMATE TARSIUS

Studies of simian and prosimian primates have demonstrated a remarkabl e interspecific constancy in the pattern of innervation of the magnoce llular and parvocellular layers of the dorsal lateral geniculate nucle us (dLGN) by retinal afferents. The characteristic organization of thi s nucleus in primates, as well as its apparent phylogenetic stability, have led to the proposal that the distinct laminar arrangement of the dLGN is one of the diagnostic characters that define the Order Primat es. Here, we describe the distribution of retinal afferents to the dLG N of Tarsius, the single contemporary member of an ancient lineage of primates. In this genus, the more superficial layer of the dLGN receiv es projections from the ipsilateral eye, a unique situation among the members of the Order Primates. This observation adds support to the pr oposal that Tarsius does not share a more recent common ancestry with simians as compared with lemuriform and lorisiform primates.