The cellular basis of amphibian gastrulation.

Amphibian gastrulation is a complex integration of local cellular behavior to produce a supracellular system that, in turn, constrains and organizes the behavior of individual cells. Such behavior has fascinated and challenged embryologists for over a hundred years and has also perplexed some of them to the point of thinking it not reducible to part-processes. These thoughts were expressed by Walter Vogt (translated in Spemann, 1938), who did more than anyone to characterize the early morphogenesis of amphibians: It does not appear at all as if cells were walking in the sense, that single part movements were combining to form the movements of the masses; for even the most natural and plausible explanation by means of amoeboid moving of single cells fails utterly. We evidently have not the wandering of cells before us, but rather a passive obedience to a superior force.

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