Formation of germ layers and roles of the dorsal lip of the blastopore in normally developing embryos of the newt Cynops pyrrhogaster

Three-dimensional relationships between tissues during the formation of germ layers were studied in sections of normally developing embryos of the newt, Cynops pyrrhogaster. In gastrulae, the inner postinvolution layer was not in direct contact with the outer preinvolution layer as a result of the presence of an intervening layer of cells. Only after the formation of the yolk plug, a narrow strip of primitive notochord, which consisted of columnar cells, established a close contact with the central part of the overlaying presumptive neural plate. The primitive notochord was also linked to endoderm at its right and left margins, facing the archenteron. Mesodermal cells other than notochord cells were mesenchymal until the neurula stage, when primitive somites appeared on both sides of the notochord. From a comparison of the relative locations of tissues in embryos at different stages of development, it was shown that the notochord elongates by a remodeling of the mass of the primitive notochord, and that, as the anteriorly directed translocation of the neural area and the invagination of endoderm occur, these processes keep pace with the elongation of the notochord. These observations suggest organizing or guiding roles for the notochord in the formation of germ layers. A role for the dorsal lip of the blastopore as the organizer is discussed in relation to the origin of the notochord.