The formation of the mesoderm in urodelean amphibians VI. The self-organizing capacity of the induced meso-endoderm.

During the years 1973 and 1974, non-dorsal/ventral (non-d/v)-polarized blastulae were made of animal (upper) ectodermal caps and vegetal (lower) endodermal yolk masses by means of the destruction of the d/v polarity of the yolk mass endoderm by disaggregation, stirring and subsequent reaggregation. A simultaneous dis- and reaggregation of the upper ectodermal cap, as performed in the 1970 and 1971 experiments, led to the same results. The destruction of d/v polarity in the reconstituted blastulae, under maintenance of their animal-vegetal polarity, did not therefore prevent dorsal axis formation. The formation of single, double, triple or multiple dorsal axes in these reconstituted embryos must be based upon the self-organizing capacity of the induced meso-endoderm, since the alternative assumption of the re-establishment of mesoderm-inducing centre(s) in the reaggregated yolk mass endoderm seems very unlikely. The formation of single, double, triple or multiple axes must be the result of a competition between mesodermal aggregation centres arising in an initially evenly-induced zone of meso-endoderm. These initial centres which compete for like cells, attract each other, resulting, e.g. in their fusion into a single dorsal axis system, or in the survival of two, three or more centres in an initially balanced spatial configuration. Intensification of the meso-endoderm-inducing capacity, by using dorsal instead of entire yolk endoderm mass and by using two instead of one dorsal yolk mass, led to a more complete type of dorsal axis formation and to a higher number of axes formed (a higher percentage of triple and multiple axes than double and single ones) in one and the same reaggregate. The possible nature of the self-organizing capacity of the induced meso-endoderm has briefly been discussed, emphasizing the fundamental significance of the self-organizing capacity of embryonic anlagen in developmental processes.