A morphologist's perspective on terminal growth and segmentation

Summary When approaching the study of terminal growth and segmentation, comparative morphology provides an important guide to formulate questions. There are often problems in unambiguously identifying the axis along which we wish to study terminal (often, actually, subterminal) growth, especially when the trunk axis is posteriorly prolonged in an appendage (as with the tail of vertebrates), or when the polarity of the “external animal” is other than the polarity of the “internal animal,” as in polypoid bilaterians. We cannot ignore that the rear end of the main body axis is possibly defined very early in development in some groups, for example, arthropods, whereas in others, vertebrates, for example, it is defined much later. We cannot think of segmentation as always corresponding to the sequential posterior addition of new units, thus ignoring the widespread occurrence of double segmentation. A more subtle problem is represented by the overlapping of different processes, all of them contributing to elongating the body, such as segmentation, cell proliferation, cell rearrangement, and cell growth. Within a segmented trunk, cell proliferation and differentiation may go on in parallel from as many growth points as there are groups of regularly spaced cells. The main consequence, however, is not so much to expedite elongation as to reduce the disparity of metabolic conditions, gene expression patterns and “relative age” of different body districts, otherwise possibly troublesome within the limited space of the embryo.