Later Pattern Formations Morphogenesis
The early phases of pattern formation, as described in the two previous chapters, set the stage for all that follows—for the whole grand sweep of morphogenesis by which the phenotype is created. Right from the oocyte stage there is an ordering of the embryo, both in the sense of spatial patterning and in the sense of setting in place the components of developmental processes. As pattern formation continues, particularly in “regulative” embryos, the ordering of the embryo becomes more and more specific. A point is reached that is quite impossible to define but nonetheless real, when the embryo is set up in such a way that all the components are in place. At least, everything has been specified. Such an embryo, for example the amphibian neurula, may look very little like the final phenotype, but from this point onward morphogenesis represents a working out of potentials that have already been established. All the basic morphogenetic information is in place. Morphogenesis is both simple and complex. It is simple because relatively few processes will be involved. It is complex because of the diversity of cell and tissue types that is involved, because of the subtlety of control of differentiation and even size and shape in organogenesis, and because of the complexity of both embryonic and subsequent adult function. Finally, it is complex because of the interactivity and “wholeness” of the developing embryo as well as the multiplicity of the parts making up that whole. It is with the simplicity of morphogenetic processes, the relatively small number of cell and tissue-level processes involved (processes that are common to all morphogenetic systems), that we will be concerned first. In order to understand both the basic rules of development and the way in which they relate to mechanisms for the introduction of evolutionarily significant phenotypic variation (Chapter 2), we must understand first these common processes of morphogenesis, and the way in which they operate under the rules and mechanisms of pattern control discussed in the previous chapters. Morphogenesis involves a relatively small number of phenomena characteristic of all cells and tissues, and their relation to features of the extracellular environment.