Notochord formation in the Mexican salamander (Ambystoma mexicanum) is different from notochord formation in Xenopus laevis

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Abstract
The cytoskeletons of cells from gastrula and neurula stages of the Mexican salamander (Ambystoma mexicanum) and the African clawed toad (Xenopus laevis) were analyzed. Bundles of intermediate filaments were found in the superficial layer of the epidermis in neurulae (but not in gastrulae) of both organisms. The bundles (tonofilaments) were observed in the forming notochord of the axolotl but not in the differentiating notochord in Xenopus. Serial semithin sections demonstrated that the notochord develops from invaginated deep cells in Xenopus, but from invaginated superficial layer in the axolotl. Scanning electron microscopy of neurula stages from the axolotl and from Xenopus laevis confirmed this difference in notochord formation. This dissimilarity in notochord formation seems to be a typical difference between anurans and urodeles as was clearly pointed out by Ruffini ('25). This has implications for the fate maps published by Vogt ('29). The considerable differences in early embryonic development suggest to us that the salamanders (urodeles) and frogs and toads (anurans) are not monophyletic in origin.