Lateral‐line neuromast development in Ambystoma mexicanum and a comparison with Rana pipiens

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Abstract
We have examined the embryonic development of the major neuromast lines of the lateral‐line system in the urodele Ambystoma mexicanum both in vivo (using microsurgical techniques to transplant placodes) and in preserved embryos using scanning electron microscopy (SEM). We have compared this to SEM observations of embryos of the anuran Rana pipiens. We have determined the approximate locations of the lateral‐line placodes in A. mexicanum and the approximate timing of both the migration of the lateral line primordia and the neuromast eruption in both species. We find that, at hatching, all primary neuromasts are present and fully formed in Ambystoma, while migration of the primordia is just beginning in Rana. The neuromast systems in both species are fully formed by the time feeding begins. If neuromast eruption is considered in relation to developmental events other than hatching, fewer differences are found between species, suggesting that hatching is precocious in Rana. We find no evidence of heterochrony to account for the morphological differences observed in these lateral‐line systems. Orthogonal neuromasts on the head, a derived feature of urodeles, appears to be the result of lateral neuromast movement subsequent to the rostral migration of the primordia. This process was not observed in the anuran. In addition, we observe that ciliated epidermal cells disappear from the area immediately around each of the lines and suggest that neuromasts cause the regression of cilia in their immediate vicinity.