Analysis of the physical forces that influence the shape of chick somites

With experiments and measurements we support the hypothesis that chick somites are shaped by packing forces produced by increases in somite volumes. Volume measurements show that somites expand considerably during the period from initial segmentation to acquisition of the mature rectangular shape (as viewed from above). Anterior somites begin as roundish objects. Somite expansion flattens the intersomitic faces. Posterior somites emerge from a compressed segmental plate with the interface between somite and segmental plate already planar. In all somites, subsequent increase in volume causes expansion dorsally, ventrally and laterally. The dorso‐ventral expansion accompanies the increase in height of the neural tube. When a single somite is excised, neighboring somites or segmental plate fill in the available space. The bulging of somites into vacated spaces suggests the presence of compression which could be the product of previous increase in somite volumes. The pliability of somites is demonstrated with experiments that bend the trunk of the embryo to the left and right reversibly changing the shapes of the somites. Carbon marks placed in pairs in somites and in adjacent neural tube stay together; this suggests that somites do not move relative to the neural tube in response to the compressive forces. Segmental plates isolated without neural tube or notochord form long oval somites. In order to be shaped properly, somites must be in intimate association with, and probably bound to, the neural tube and also most likely the notochord.