During recent years Tiedemann and his co-workers have developed methods for extracting and purifying neural and mesoderm-inducing factors from homogenates of 9-day-old chick embryos (see Tiedemann, 1966). One type of extract, obtained initially by phenol treatment of material precipitated with ammonium sulphate from a pyrophosphate extract of chick embryos (Tiedemann, 1959) appeared to induce mesoderm in a high proportion of cases, when implanted into Triturus gastrulae. It has been purified by chromatography on CM-cellulose (Tiedemann, 1959; Tiedemann, Kesselring, Becker & Tiedemann, 1961) and by electrophoresis on Dextran gel (Kocher-Becker, Tiedemann & Tiedemann, 1965). Triturus larvae which have received implants of mesoderminducing factor at the gastrula stage show extra notochord, muscle and kidney within their ventrolateral mesoderm. From the illustrations in these authors’ papers, there can be no doubt at all as to the identity of these induced tissues. Since, however, they develop within areas that would normally have formed mesoderm of another kind, one may infer that in these cases there has been no transformation of cells other than mesoderm : rather, the type of mesoderm has been altered to somite myoblasts, chorda and kidney cells instead of lateral mesothelium. In tests by the ‘sandwich’ technique (Holtfreter, 1933) however, which were evidently also used but not reported in detail (Tiedemann, 1959) any mesoderm that formed would have to be by transformation of ectoderm cells. In earlier experiments on the effects of a crude protein extract from chick embryos on isolated amphibian ectoderm in tissue culture (Becker, Tiedemann & Tiedemann, 1959) transformations into mesenchyme, myoblasts and chorda cells were certainly shown to occur. It seems then that a ‘mesoderm inductor’ may have two kinds of effect: an enhancement of the growth of mesoderm, transforming it also into more dorsal, axial tissues; and a transformation of ectoderm into cells of all mesoderm types. Whether or not endoderm cells can be transformed by the inductor is not fully clear: Kocher-Becker et al. (1965) described an apparent migration of endoderm cells outwards over the surface of Triturus gastrulae which contained implants of highly purified mesoderm inductor, but they interpreted this as due to ectoderm transformation (with a resultant increase in affinity between ectoderm and endoderm) rather than suggesting that the endoderm itself had acquired any mesoderm-like properties.

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