Concluding Remark

We end where this traverse of the regulatory mechanisms underlying adult body part formation began, with a thought about its evolutionary meaning. It should be obvious now how great is the distance between what it takes to carry out direct cell type specification, as in the Type 1 embryonic processes considered in Chapter 3, and what it takes to build the pieces of an adult bilaterian body plan. This distance must be considered in terms of the required genomic regulatory information: the number of regulatory linkages, the network depth, the specificity of cz's-regulatory modules involved throughout, and the number of diverse differentiation gene batteries ultimately needed. In architectural terms comparing the regulatory networks underlying direct cell type specification with

Dorsal Factors

Ventral Factors

Cell Lineage

Nkx3.1 Six3 Propl

Brn4 Ish P-Frk

Nkx3.1 Six3 Propl

Brn4 Ish P-Frk

(MSH)

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