Differentiation capacities of the prospective tail somite region of the neural plate in the embryos of Ambystoma mexicanum Available to Purchase

The hind part of the neural plate in amphibian embryos has a mesodermal significance although it occupies an ectodermal position till late neurula stage. In anurans it gives rise to the posterior tail somites (Smithberg, 1954) and in urodeles the somites of the posterior trunk region and of the entire tail together with several other mesodermal structures (Bijtel, 1931; Nakamura, 1938; Aufsess, 1941; Spofford, 1945; Chuang, 1947; Ford, 1949). Presence of mesoderm in the neural plate is an interesting developmental problem. During normal gastrulation this region is at first underlaid by the future anterior part of the archenteric roof which exerts a neuralizing inductive influence. It is only later that the posterior part of the notochord with its mesodermalizing influence comes to lie under it. According to Eyal-Giladi (1954), who worked on gastrula stages of the Axolotl, even a short and transient contact of the invaginating archenteric roof with the overlying ectoderm produces archencephalic induction in the latter. It is therefore relevant to ask whether the posterior part of the neural plate does or does not pass through a phase of neuralization before assuming mesodermal properties. This specific question has not been investigated although a number of workers have studied the determination, differentition capacities or inductive properties of the caudal portion of the neural plate in amphibians (Bytinski-Salz, 1931; Bijtel, 1936, 1958; Chuang, 1947; Nakamura, 1947; Spofford, 1948; Smithberg, 1954). Spofford (1948) demonstrated that in Ambystoma mexicanum neurulae the tail notochord induces the overlying ectoderm to acquire mesodermal properties. He replaced the posterior part of the neural plate by a piece of competent ectoderm which itself had never been subjected to the influence of a neural inductor. This technique could demonstrate that, under experimental conditions, the so-called transformation or mesodermalization need not be preceded by activation or neuralization. These results, however, do not show whether this is also true during normal embryogenesis.