A method for predicting the cranio-caudal position of secondary embryonic structures

The morphogenetic events that give rise to a specific body pattern have to date avoided extensive elucidation. Extant models of pattern formation have dealt almost exclusively with the “primary patterning” of structures in the embryo. These “universal” models fail to explain many morphological conditions, such as the simultaneous change in position of the limbs, celom, mesonephros, and umbilical artery relative to the somites as a result of a single mutation. In the present paper, we propose that the relation between two non-periodic waves may function to determine the position of “secondary structures”, such as the limbs, in the embryo, relative to primary structures such as the somites. We propose that if two morphogenetic events are initiated at different times from the same region of the embryo, and are progressing in a cranio-caudal sequence at different rates, then the location of a given structure along the body axis can be described as a function of the two events. Applications and predictions based on the proposal are presented. Evidence from observations of morphogenetic events in the chick embryo, which tend to support the model, are also presented.