The formation of the mesoderm in urodelean amphibians : IV. Qualitative evidence for the purely "ectodermal" origin of the entire mesoderm and of the pharyngeal endoderm.

Xenoplastic recombinations of animal and vegetative parts ofAmbystoma mexicanum and Triturus alpestris blastulae, and similar recombinations of parts of3H-thymidinelabelled and unlabelledAmbystoma mexicanum blastulae demonstrate convincingly that the vegetative part (zone IV, see Nieuwkoop, 1969a) of such a recombinate does not contribute to mesoderm formation, but exclusively forms endodermal derivatives. In contrast, the animal cap of the blastula (zones I.II)-which only gives rise to atypical ectoderm if isolated-not only furnishesall the ecto-neurodermal derivatives, butall the mesodermal structures of the developing recombinate as well, and finally to a varying extent forms additional endodermal structures in the recombinate.In the recombinates endodermization of the ectodermal cap occurred at the anterior end of the invaginated archenteron-corresponding to the presumptive pharyngeal endoderm -, and along the dorsal side of the endodermal tube, while an endoderm-like epithelium is formed at the boundary between the caudal endoderm and the ectoderm (proctodaeum formation). These results suggest that in normal development also endodermization occurs in the "ectodermal half" of the egg. This occurs particularly on the dorsal side, leading to the formation of the presumptive pharyngeal endoderm situated above the dorsal blastoporal groove.These experiments show that the vegetative "half" of the amphibian blastula is firmly determined as the future endoderm, whereas the animal "half" is still virtually undetermined and pluripotent.