AbstractEggs ofAmbystoma maculatumwere separated into three groups and maintained at 5°, 8° and 14°C, respectively, until stage 27. At this stage the otic vesicle was extirpated from the right side, thus eliminating the VIIIth cranial ganglion and root. The number of cases in which Mauthner's cell (MC) failed to differentiate in the absence of VIIIth roots was compared among the three temperature groups.In a total of 713 animals, MC was absent on the side of operation in 200 cases (28.05%). Comparison of the three temperature groups with respect to the absence of MC is as follows: 14°, 39 of 212 cases (18%); 8°, 83 of 266 cases (31%); 5°, 78 of 235 cases (33%). Statistical analysis of these data demonstrates a significant difference between either one of the colder groups and the 14° group with regard to suppression of MC. When the two colder groups are tallied as a single sample, the difference is even more significant. There is no significant difference between the 5° and 8° groups. These results indicate that the differentiation of MC is more dependent upon the ingrowth of VIIIth root fibers when morphological development has been delayed by prolonged hyp