The primordium of the VIIIth cranial ganglion (otic vesicle) was extirpated on the right side of 428 embryos of Ambystoma maculatum at stage 27. Mauthner's cell (MC) develops in close association with the VIIIth nerve roots and to a lesser degree with VIIth lateral-line roots. The primary objective of the experiments was to record from a large series of animals the number of cases in which MC failed to differentiate in the absence of VIIIth root fibers. In addition, all embryos were separated (stages 3, 5, 9) into various groups and maintained continuously at different constant temperatures (5°, 8°, 10°, 14°, 18°C) until operation. The various temperature groups were then compared among themselves for any constant or significant differences with regard to the suppression of MC on the operated side. Animals were examined at stage 46. The VIIIth root fibers were completely absent in every case on the operated side. Although the VIIth lateral-line ganglia were present in all animals, root fibers failed to develop in 173 cases. This afforded an opportunity to study the influence of VIIth lateral-line roots upon the differentiation of MC, as well as the influence of VIIIth roots. Mauthner's cell was absent in 119 cases (27.8%). In 55 of these cases only VIIIth root fibers were absent; in 64 cases both VIIIth and VIIth roots were missing. The importance of VIIIth root fibers in influencing the differentiation of MC, therefore, is obvious since in 46% of those cases in which MC failed to develop absence of the VIIIth root was the only deviation from normal. The presence or absence of MC correlated with the presence or absence of VIIth lateral-line roots gives a relatively high Chi-square value of 11.461. The temperature series data are inconclusive in demonstrating a critical or optimal thermal effect upon the sequential steps affecting the differentiation of MC. If the time in early development when temperature differentials were applied is ignored, there are no consistent or significant differences among the temperature groups with regard to the suppression of MC in the absence of the VIIth-VIIIth root complex. On the other hand, if only those animals are considered that were separated at stage 9 (the majority of the animals), then there is a low degree of correlation (significance slightly above the 1% level) between greater suppression of MC in the 8° groups as opposed to lesser suppression in the 14° groups. These results on temperature influence are in need of further experimentation and analysis. © 1969.
