The formation of the mesoderm in urodelean amphibians : I. Induction by the endoderm

The blastula [stage 8⁺ to 8/9 (Harrison)] ofAmbystoma mexicanum was subdivided into four successive animal-vegetative zones and the relative amounts of cellular material present in the successive zones were determined. The developmental capacities of the isolates I, II, III, IV, and I.II and III.IV as well as of the various recombinates of three of the four and of all four zones were studied, and their quantitative composition at the end of the culture period was determined. To this end the embryos were allowed to develop for only 5 to 6 days, during which period the primary organization and initial differentiation was accomplished, but without the appearance of marked changes in the volumes of the different components, which would have occurred upon extensive decomposition of intracellular yolk and subsequent cytoplasmic growth during a longer period of development.Comparing the differentiation of the recombinates with that of the corresponding isolates - in particular the recombinate I.II.IV with the isolates I, II and IV - it was concluded that the mesoderm arises as a result of an interaction between the pigmented, ectodermal and the unpigmented, endodermal "halves" of the egg, which initially [before stage 7 (Harrison)] constitute the only two components of the egg. A comparison of the quantitative composition of the recombinates with that of the corresponding isolates yielded strong arguments in favour of the statement thatthe mesoderm develops exclusively from the ectodermal "half" of the egg under the influence of an inductive action from the part of the endodermal "half". This statement was further corroborated by arguments collected from the literature.Whereas neither the endoderm nor the ectoderm alone are initially able to differentiate beyond a certain point - so-called atypical ectodermal and endodermal differentiation respectively - their interaction product, the mesoderm, apparently contains the information needed for differentiation into the characteristic mesodermal structures. Influences emanating from the differentiating mesoderm then enable both the ectoderm and the endoderm to proceed further on their path of differentiation.The role of the blastocoelic cavity - a cavity with a negative morphogenetic function - in thespatial interaction between the two primary components of the egg was elucidated. In the light of the conclusions mentioned above the centrifugation experiments ofPASTEELS (1953, 1954) were reinterpreted, whileSCHULTZE'S "Umkehrexperiment" byPENNERS andSCHLEIP (1928),PENNERS (1929) andPASTEELS (1938, 1939) andCURTIS' cortical grafting experiments (1960, 1962) were briefly discussed. The hypothesis was then advanced that the inductive interactions taking place in the early embryo preferentially spread through the most superficial layer of the egg, where the cells are tightly connected with each other. Finally, thetemporal aspects of mesoderm induction were discussed in relation to observations collected from the literature.Some parallels were indicated between the morphogenetic events taking place in early amphibian development, and recent biochemical observations on RNA and protein synthesis before the onset of gastrulation.Finally a general picture was drawn of the development of the amphibian egg on the basis of the principle of a stepwise increase in multiplicity by means of inductive interactions.