Starting from the correspondence held by Mumford and Muntañola, the article follows a critical trace developed by the same Muntañola with Saura Carulla, pointing out some key issues faced by Mumford in his vast work. Such issues allow to highlight, among the rest, the very contemporary, innovative and critical strength of the message left by the American Regionalist movement, centered on an integrated and complex relationship among man, nature and technology and a multidisciplinary vision considering planning as a ‘civic’ practice, emphasizing its ‘education’ and communication dimensions. Using the metaphor of “The Egg of the Snake” for the basic theoretical and practical errors of urban planning, the article highlights how Mumford had carefully analyzed such ‘eggs’ over time - a critique of contemporary urban planning in relation to ecological balances and wrong social practices, an analysis of the role of technology in relation to social welfare and so on - and for this had been charged, to his regret, of retrograde attitude. This critical reading of the typical approach to urban project, especially revealed in his last years, shows perhaps a state of anxiety that, in Mumford, was about to flow into a trans-disciplinary approach.