Tangential organization of olfactory, association, and commissural projections to olfactory cortex in a species of reptile (Trionyx spiniferus), bird (Aix sponsa), and mammal (Tupaia glis).

Small amounts of tritiated leucine were injected into the olfactory bulb or anterior olfactory cortex of softshell turtles, wood ducks, and tree shrews in order to compare quantitatively the laminar distribution of olfactory bulb, association, and commissural projections to olfactory cortex. In all three species, a similar colaminar distribution of olfactory and association projections was found: the olfactory projections are restricted to the superficial cortical layer Ia, while the association projections are distributed into the deeper cortical layers Ib, II, and III. Differences among these three species were found in the origin and distribution of commissural projections. Whereas in tree shrews these fibers originate from third-order neurons and project into the deeper layers of the contralateral cortex (with the homolateral olfactory bulb projections), in softshell turtles and wood ducks, they originate from second-order neurons and project into the superficial layer of the contralateral cortex (with the homolateral olfactory bulb projections). These results, in conjunction with those obtained previously in other species, indicate that the basic tangential organization of mammalian olfactory cortex is retained, albeit with some modification, from a remote, reptilian ancestor.