Cell Division in the Early Growth and Morphogenesis of the Chick Limb

Abstract

THE confusion concerning the relationship between growth and form in animal development is quite deep and widespread and possibly stems from the title of D'Arcy Thompson's great book¹, together with the botanists' emphasis on localized growth zones such as the apical meristem. Our attention was drawn to this during our studies on budding in hydra², where it had always been assumed that formation of a new hydra by budding was partly the result of localized growth at the base of the bud. From our experiments it now seems that bud elongation does not require cell growth and division but is a morphogenetic process involving cell movement and changes in cell contact. This led us to question whether cell growth and division ever determine in a significant manner the form, as distinct from size, of animal tissues and we have had difficulty in finding examples where this is obviously the case. Orientated cell divisions in early development contribute to the morphogenesis of the blastula³ and are almost certainly important for the determination of form in mosaic embryos. In these cases it is the plane of cell division and not growth that is important. The growth of certain insect structures involves orientated cell divisions and is one of the very few cases where growth seems to determine form⁴. Form seems to be determined by the spatial relationship between the cells which may be largely determined by cell movement and cell contact³.