  Life presents fascinating examples of self-organization and emergent
phenomena. In multi-cellular organisms, a multitude of cells interact to form
and maintain highly complex body plans of well-defined size. In this thesis, we
investigate theoretical feedback mechanisms for both self-organized body plan
patterning and size control.
  The thesis is inspired by the astonishing scaling and regeneration abilities
of flatworms. These worms can perfectly regrow their entire body plan even from
tiny amputation fragments like the tip of the tail. Moreover, they can grow and
actively de-grow by more than a factor of 40 in length depending on feeding
conditions. These capabilities prompt for remarkable physical mechanisms of
self-organized pattern formation and scaling.
  First, we explore the basic principles and challenges of pattern scaling in
mechanisms previously proposed to describe biological pattern formation. Next,
we present a novel class of patterning mechanisms yielding entirely
self-organized and self-scaling patterns. This framework captures essential
features of body plan regeneration and scaling in flatworms. Further, we
analyze shape and motility of flatworms. By applying principal component
analysis, we characterize shape dynamics during different motility modes and
also identify shape variations between different flatworm species. Finally, we
investigate the metabolic control of cell turnover and growth. We identify
three mechanisms of metabolic energy storage; theoretical descriptions thereof
can explain the measured organism growth by rules on the cellular scale.
  In a close collaboration with experimental biologists, we combine minimal
theoretical descriptions with state-of-the-art experiments and data analysis.
This allows us to identify generic principles of scalable body plan patterning
and growth control in flatworms.
