"Constraining" the probability toward a specified attractor: Comment on: Morphogenesis as Bayesian inference: A variational approach to pattern formation and control in complex biological systems

In an old paper, dating back from 1945, the German embryologist G. Fankhauser demonstrated how relevant could be top-down constraints in shaping organization structures at lower levels. In the particular case considered by this author, a polyploid triton – while having a doubled cell size with respect to the diploid – had exactly the same dimension of organs and ducts[1]. Simply the polyploid triton used half of the cells to build up its organs. This is crucial for life: the optimizationof the caliberof a biological structure (the duct) is finely tuned to fit with the flow of biological fluids (a top-down constraint) and cannot be established by its constituent cells or the genome neither. While this is an intuitive tenet (after all we do not decide our home size based on the size of the bricks!), this is a largely overlooked issue in Biology that, in the last fifty ears, concentrated almost exclusively on the investigation of bottom-up control. Indeed, this reductionist stance resulted inadequate – to say the least – in accounting for the emergence of complex patterns, in both morphology and molecular networks