Precedence Relationships

Besides the direct embryological evidence, precedence relationships (PRs) in embryo development can be inferred from the gene control mechanism uncovered by Evo Devo research. Namely, that protein coding genes are controlled via signalling proteins which can bind to such genes' signature loci. If we indicate with the notation G(p1, p2, p3) a specific modulation of gene G when controlled by the signalling proteins p1, p2, p3, then PRs will exist between the genes which code for, respectively, p1, p2 & p3 and the specific modulation of G(p1, p2, p3).

In fact, the genes coding for p1, p2, p3 must be expressed in cell types that precede the cell type expressing the modulation G(p1, p2, p3). This is the fundamental reason why PRs exist between cell types: signalling proteins are produced by cell types. They in turn, modulate the expression of genes they control in "new" cell types that arise from the expression of such controlled genes.

One can then see that the interaction between the GeNets encoded in the genome and the process of cell duplication can be characterized as waves of progressive firings across the complete network of PRs, the PERT network previously discussed. Each node in the network being a distinct cell type formed by one or more modulations Gi(p1, p2,...pn) of differentiating genes. The effect of this interaction is the progressive growth and differentiation of the embryo. The number of cells and cell types growing as times passes.

This process is a bootstrap process, in the sense that earlier steps make it possible to execute subsequent steps of increasing complexity both in terms of increasing variety of cell types and richness of tissues structural interactions (architecture).

The graphic representation (PERT) of the PRs prevailing in a specific embryo development will be very complex and not likely to be clearly representable as a simple planar (2 dimensional) graph without having a very large number of arc crossovers. Two techniques could be used to obtain "readable" graphs:

Use of more than two dimensions, three dimensional PERTS would be quite readable and avoid a great deal of the messiness of 2-D PERTs. One could also, in principle, use four or five dimensions, especially the latter case is known to avoid the creation of knots. Such higher dimensionality PERTs would present no particular problems to computer based systems. The computer, in turn, could readily create to 2-D or 3-D cross sections which people could easily read.
Resort to using sub-PERTs based on the anatomy of the specific species. It is very likely, in fact, that the development of body appendages, internal organs & sense organs, will prove out to be characterized by PR networks that can, to a good approximation, be considered fairly independent sub-PERTS. The model of the entire embryo development process would then consist of an abstract PERT linking the major components of the phenotype, together with a number of sub-PERTs one for each body part or organ.

An alternate form of the PERT would use the MCGs as its nodes. Such MCGs based PERT would clearly depict the control architecture of the embryogenesis process. The version based on cell types would more directly relate to the embryological evidence. The former would provide a clearer roadmap to underlying molecular genetics processes. Both versions of the PERT would be essentially invariant for members of a species as discussed previously.