The prenatal development of some of the visual pathways in the cat.

Lesions were made in the visual system in a series of cat fetuses of known gestational age, and fiber and terminal degeneration were stained by the Eager method. The times of development of the retinal projection, of the thalamcortical and corticothalamic projections of area 17 of the visual cortex, and of the intrinsic fibers in the visual cortex were examined. Enucleation of one eye resulted in degeneration being detected bilaterally in the lateral geniculate nuclei (LGN), superior colliculi (SC) and optic tracts. The optic nerves reached the optic chiasm by the thirtieth embryonic day (E30) and the optic tract connections with the LGN and SC were made by E37. The projection always appeared stronger in the contralateral LGN and SC, and the amount of degeneration increased in both sides with increasing age. A parasagittal knife cut was made in the dorsomedial crest of the visual cortex. Where the lesion passed through the cellular layers of the cortex, intrinsic fibers were cut when these were present. The deeper part of the incision through the white matter undercut the medial wall of the visual cortex, interrupting thalamocortical and corticothalamic fibers when these were present. The longer horizontal fibers that were intrinsic to the visual cortex began to develop during the last two weeks of gestation but were not fully developed at birth. In the undercut visual cortex distant from the place of entry of the lesion, and before the intrinsic fibers of the cortex had developed, degeneration was found in layers 1 and 4, demonstrating the presence of a thalamocortical pathway. The youngest fetus to show this degeneration was operated at E48. This degeneration was not present three days earlier at E45. Fiber plexuses that have been described earlier in development (Marin-Padilla, '71; Cragg, '75) do not appear to degenerate after undercutting the cortex. The corticothalamic pathway to the lateral posterior nucleus medial to the LGN was developed at E45. The descending pathways to the ipsilateral LGN and SC were developed by E48, but it is not known whether they are present before this. Thus degeneration has been used to detect the development of axonal pathways in the fetus for the first time; the major afferent and efferent pathways are developed at an earlier stage than has previously been described.