Studies on the Mechanism of Meristic Segmentation: I. The Dimensions of Somites

The two major aspects of embryonic development are the appearance of differentiated tissues and of organized structures. Most recent investigations have been directed mainly towards the first of these two categories, and have attempted to elucidate the changes in the character of the living substance in terms such as the reactions between organizers and competent tissues, determination by means of gradients or ‘organ-forming substances’, the influence of genetic factors, and the like. Very much less effort has been devoted to the study of the changes of form undergone by the developing tissues. These geometrical changes must, of course, be brought about by the operation of physical forces, and embryonic development offers a whole range of biophysical problems about which our ignorance remains profound. This paper reports a study directed towards this general class of biophysical problem. In most embryonic changes of structure both the initial form and the later ones are geometrically complex and difficult to describe in reasonably precise terms.