Strategy

It is important, at this stage, to look at some of the ways in which developmental mechanisms work. Otherwise, the phylogenetic comparisons of development-controlling genes, to be presented in Chapter 7, will have little meaning. Differences between the DNA sequences of homologous developmental genes in different lineages are of some interest in themselves; but they are of much more interest if we can connect them with differences in the processes over which the genes concerned have a controlling influence. Ultimately, we want to be able to understand the whole chain of events from altered DNA sequence to altered gene product to altered developmental mechanism to altered morphology.

I will now proceed as follows:

1. In this chapter, my emphasis will be on cellular processes; genes do feature at various points, but they are not the primary focus of attention. In Chapter 6, the complementary gene-centred approach is adopted. These should be seen as convergent routes to a common goal: understanding developmental mechanisms as fully as possible.

2. Throughout this chapter and the next, I will be very selective, using only a few examples to illustrate key points. This approach allows me to deal fairly intensively with each example, without my overall account of developmental mechanisms becoming excessively long. However, it carries a potential danger. Within any one category of mechanism (e.g. induction) there is some variability in the details from one example to another.