The view of embryonic development presented in the preceding nine chapters is rather different from accounts to be found in other modern developmental biology textbooks. We have focused on the phenomena of transitions between cell types, changes in the shape of tissues, and the generation of new arrangements of cells and have approached them as problems in physics. In contrast, when these subjects are dealt with in most contemporary treatments of development it is primarily as problems in regulated differential gene expression. While we have by no means ignored the roles of gene products and gene regulatory systems in our account of development, and while the notion of the embryo as a physical system is not entirely absent from the more standard accounts, the different emphases of the two perspectives could not be clearer.
Developmental biology has advanced to its current high level of sophistication with little explicit analysis of the physical dimension of the questions it treats. This is changing, however. The DNA sequencing initiatives of the last decade of the twentieth century confirmed that genes number in the range 10,000–50,000 in all the species traditionally studied by developmental biologists. Analysis with microarrays (computer-interfaced devices for quantifying the abundance of mRNAs in tissue samples) has shown that many genes are simultaneously expressed during any significant developmental event (Bard, 1999; Montalta-He and Reichert, 2003).