Tritiated leucine was unilaterally introduced into the vitreous chambers of metamorphosing bullfrog tadpoles in an attempt to determine whcther the optic fibers may be traced by radioautography. After injection, the animals were sacrificed at varying time intervals from 4v2 minutes to 32 days. Serial sections of the brain were subjected to radioautography. The optic fibers were thereby traced to their known terminations and to a small thalamic cell cluster to which we found no reference in the literature. Significant differential radioactivity was detectable in the contralateral optic lobe as early as six hours postinjection, suggesting a flow, whether intraor extraaxonal, of at least 10-22 mm/day.
A single postmetamorphic frog underwent excision of the optic chiasin with positioning of the stumps of the optic nerves against the lateral wall of the thalamus. Fifteen weeks were allowed for regeneration. When the same histologic and radioautographic technique as above was used (sacrificing at eight days postinjection), a grain path was found to extend from the optic nerve into the brain. This path divided into two distinct branches, one branch crossing over and one remaining on the same side. These results indicate the practicality of tracing nerve distribution within the CNS by radioautography.