By using the method of quail-to-chick transplantation of neural crest in one series (VNG) and placodal ectoderm in a second series (VPG) we were able to determine the relative contribution of cranial neural crest and placodal ectoderm to the formation of the Glossopharyngeal-vagal complex. In chimeric embryos, quail cells originating from cranial neural crest grafts of postotic levels end up in the root ganglia, while quail cells originating from placodal ectoderm of postotic levels end up in the trunk ganglia. The results clearly indicate that the caudal levels of the medulla and rostral cervical segments represent the site, and the neural crest the source, for the neurons of the root ganglia. The neurons form a homogenous population of the small-cell type. This clearly rules out any contribution to the root ganglia from placodal ectoderm. On the basis of our experiments, it is also concluded the the neurons of the trunk ganglia are purely placodal in origin and are composed of a populatin of cells of the large-cell type. Our experiments also provide convincing evidence for a neural crest origin for Schwann cell and ganglionic Satellite cells.

Neurogenesis in the motor and mesencephalic nuclei of the trigeminal nerve was examined using autoradiographic techniques. Two groups of pregnant rats (control and experimental) were injected with two successive daily doses of 3H thymidine in an overlapping series starting from day nine of gestation in order to label in their progeny, the dividing precursor of neurons of the motor nucleus and mesencephalic nucleus of the trigeminal nerve. Control group of rats was raised on a standard diet, while the experimental group was made hypothyroid by propylthiouracil (PTU). At postnatal ages ranging between 20-30 days in the pups of both the control group and experimental group, the percentage of cells labelled and the proportion of cells added during each embryonic day were determined quantitatively throughout the rostro-caudal extent for both motor and mesencephalic nuclei. The neurons of the mesencephalic nucleus undergo their final cell divisions between gestational days 9 and 10 (E9 and E10). More than 80% of the population is generated by E10. The neurons of the motor nucleus undergo their final cell divisions between E9 and E11, and nearly 88% of the cells is generated by E11. In the thyroid deficient rats, in both nuclear centers, only 61% of the cells is generated by E12, and labelled cells are observed even as late as E18 and E19. In the hypothyroid state, there is a significant lengthening of the proliferative period. On the basis of absolute datings and duration of neuron production, it is postulated that in normal development, thyroid hormone determines the duration of the proliferative period, and push cells into the differentiative phase by taking them out of the proliferative phase.

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Degenerative changes in the entire third nerve nucleus were studied following selective removal of the ciliary ganglion, usually of the right side, in day-old chicks. The operated animals were allowed to survive three, six, nine, and twelve days after ciliary ganglionectomy. Retrogarde degenerative changes proceed rapidly between three and nine days post-operatively, and are strictly limited to the ipsilateral accessory oculomotor nucleus. The changes are at first observed in the medial division of the accessory oculomotor nucleus, followed by the lateral division of the accessory oculomotor nucleus. By nine days following the operation, both the divisions of the accessory oculomotor nucleus are virtually depleted of almost all neurons. The other components of the oculomotor complex were not affected at any stage by ciliary ganglionectomy. These observations provide conclusive evidence that the accessory oculomotor nucleus is indeed the avian homologue of the Edinger-Westphal nucleus.

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