Shortly before his untimely passing, the computing pioneer Alan Turing published his most cited paper The Chemical Basis of Morphogenesis (1952).
The central question for Turing was: how does a spherically symmetric embryo develop into a non-spherically symmetric organism under the action of symmetry-preserving chemical diffusion of morphogens (as Turing calls them, an abstract term for arbitrary molecules relevant to development)? The insight that Turing made is that very small stochastic fluctuations in the chemical distribution can be amplified by diffusion to produce stable (i.e. not time varying except slow increases in intensity; although also potentially time-varying with 3 or more morphogens) patterns that break the spherical symmetry.
The theory is beautifully simple and abstract, and produces very important qualitative results (and also quantitative results through computer simulation, which unfortunately Turing did not get to fully explore). However, even in the definition Turing discusses some potential limitations such as ignoring mechanical factors, and the inability to explain preferences in handedness. The particular models he considers -- a cycle of discrete cells and a circular tissue -- do not seem particularly relevant. As far as I understand, the key feature is his observation of symmetry breaking through small stochastic noise and instability.
What was the most important contribution of Turing's paper to developmental biology? Is his approach still used, or has the field moved on to other models? If his approach is used, how was the handedness problem resolved?