The cytoskeletons of cells from gastrula and neurula stages of the Mexican salamander (A mbystoma mexicanum) and the African clawed toad (Xenopus laeuis) 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 laeuis 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.

We analysed the neurulation movements in the Mexican salamander Ambystoma mexicanum. Embryos were exposed to colchicine or nocodazole prior to neural fold formation. Exposure to these drugs prevented the anterior neural folds from closing. Neurulation however proceeded normally in the posterior regions of the embryo. We were unable to find apically constricted cells in the neural plate of colchicine-blocked neurulae. Only rounded-up neural plate cells were present (semithin sections). This situation was typical in embryos exposed to colchicine prior to neural fold formation. Concentrations of colchicine up to 2·5 × 10−3 were not capable of blocking neurulation once the neural folds were formed. The wedge-shaped cells were present in similar numbers to those found in controls. We quantified the cell shape changes in the neural plate and in the epidermis in both controls and drug-arrested embryos. The comparison of these to classes of data shows that epidermal spreading is prevented by colchicine but only slightly affected by nocodazole. Embryos blocked in late neurulation by exposure to these drugs can resume neurulation following neural plate excision in nocodazole but not in colchicine. We conclude from this observation that the epidermis contributes to raising and closing of the neural folds. The presence of neural folds in absence of wedge-shaped cells in the neural plate is also taken as evidence that neurulation is not exclusively driven by forces generated in or acting on the neural plate. Our view on the concerted interplay of various embryonic components is illustrated in a summarizing diagram (Fig. 11).