The differentiation of neural crest cells into visceral cartilages and odontoblasts in <i>Amblystoma</i> , and a re-examination of the germ-layer theory

Abstract

Abstract A critical study and demonstration of the distribution of yolk globules and of pigment granules in normal development of the axolotl shows that these cell inclusions can be regarded as infallible evidence of the origin of cells from endo-mesoderm or from ectoderm layers of the embryo respectively. It is demonstrated that ectodermal cells of the neural crest differentiate into the cartilages of the visceral arches, into odontoblasts, and it is more than probable that they differentiate into osteoblasts of dermal bones. It is further demonstrated that the enamel organs of the teeth can be formed from the ectodermal cells of the stomodaeal collar, from the endodermal cells of the gut wall, or from both. The germ-layer theory is examined as regards its theoretical implications in connexion with the homology of structures in the adult and the presumptive organ-forming regions of the early embryo. It is found that there is no invariable correlation between the germ layers and either the presumptive organ-forming regions or the formed structures. It follows that the germ layers are not determinants of differentiation in development, but embryonic structures which resemble one another closely in different forms although they may contain materials differing in origin and in fate. The germ -layer theory in its classical form must therefore be abandoned.