The developing vertebrate gut tube forms a reproducible looped pattern as it grows into the body cavity. Here we use
developmental experiments to eliminate alternative models and show that gut looping morphogenesis is driven by the
homogeneous and isotropic forces that arise from the relative growth between the gut tube and the anchoring dorsal
mesenteric sheet, tissues that grow at different rates. A simple physical mimic, using a differentially strained composite
of a pliable rubber tube and a soft latex sheet is consistent with this mechanism and produces similar patterns. We devise
a mathematical theory and a computational model for the number, size and shape of intestinal loops based solely on the
measurable geometry, elasticity and relative growth of the tissues. The predictions of our theory are quantitatively
consistent with observations of intestinal loops at different stages of development in the chick embryo. Our model also
accounts for the qualitative and quantitative variation in the distinct gut looping patterns seen in a variety of species
including quail, finch and mouse, illuminating how the simple macroscopic mechanics of differential growth drives the
morphology of the developing gut.