 Working in the US, Paul Weiss also elaborated field and gradient concepts to sketch a theory of development that anticipated key ideas of systems biology today (see the entry on  systems and synthetic biology).  His mature theory (“molecular ecology”) posited an array of molecular species in a cell, distributed, arranged, grouped in ways determined by their sterically-mediated interactions and physical environment (1968). Steric interactions are based on molecules’ shape and charge; biochemical features such as bonding and repulsion. Weiss theorized that these molecular interactions are organized by higher-level fields so as to produce patterns of cell differentiation, growth, and morphogenesis according to the principle of “gradual determination”. There is no pre-existing “seat” of embryonic organization in the egg or early embryo—instead, organization emerges in the process itself, through a physiological gradient. Weiss’s theory was based on extensive research on limb regeneration, neural development, grafting, and cell culture. A regenerated amphibian limb, for example, shows the correct orientation and organization of parts, even if derived from different cell types in another body part. How do the new parts acquire a “sense of direction” yielding correct morphological organization? Weiss’s answer is that some physiological gradient in the body, tells the cells “where they are and what to do”.