Cellular migration and morphological complexity in the caecilian brain

The morphology of the tectum mesencephali and the medial pallium is studied in species representing the six families of caecilians (Amphibia: Gymnophiona) in order to determine whether differences in brain morphology are related to function, phylogenetic history, or life history strategies. In general, the caecilian tectum is characterized by simplification in having little to no lamination and few migrated cells. The degree of morphological complexity differs between species and between brain regions. Our data suggest that changes in brain morphology are due to a mosaic of different influences. We did not find a strict correlation between visual system reduction and tectal morphologies. However, phylogenetic effects exist. The greatest degree of morphological complexity is found in members of the Rhinatrematidae, a family that is considered basal to the lineage. Thus, simplification of brain morphology in caecilians must be considered a secondary or derived rather than a primitive feature. Direct development and miniaturization are correlated with the greatest simplification in the tectum mesencephali and medial pallium. There is a relationship between differences in brain morphology and heterochrony in caecilians, as in other amphibians. J. Morphol. 231:11-27, 1997. © 1997 Wiley-Liss, Inc.