The migration and fate of cranial neural fold cells of the chick embryo (White Leghorn strain) were followed radioautographically after replacing crestcontaining fold segments with comparable segments from donor embryos labeled with tritiated thymidine. The behavior of the neural crest cells was found to be remarkably similar to that of their amphibian counterparts. The movements of labeled crest cells into the upper facial region of the chick embryo, in conjunction with their proliferative activity, apparently gave rise to the embryonic facial (medial and lateral nasal, and maxillary) processes. As the cells moved into the visceral arches, they surrounded the core- like mescdermal muscle plates.

In the upper face and visceral arch regions, labeled crest cells participated in the formation of cartilage. Judging from the distribution of these cells in the remaining mesenchyme, it appears that the still undifferentiated skeletal and connective tisues of these regions are also of neural crest origin. Labeled crest cells were also found in the root ganglia of the VIIth and of the IXth and Xth nerves and the trigeminal and ciliary ganglia. While labeled neural crest contributed extensively to the formation of sheath cells, it made rather limited contributions to the meninges of the brain.