Phylotypic stage

The link between the two discoveries is the fact that the phyla defined by Cuvier, on the basis of the adult body plans, exactly correspond to the phyla defined by von Baer on the basis of the embryos phylotypic stages, a convergence that turns out to have a... 

Figure 7.3 The phylotypic stage in chordates (faringula) and in arthropods...

Such a conclusion is directly suggested by the very characteristics of the explosion. All adaptive radiations that came after the Cambrian have never modified the body plans, while the Cambrian explosion was characterised precisely by modifications of those plans. And, in a similar way, no adaptive radiations have ever changed the phylotypic stage of developing embryos, while the Cambrian explosion did precisely that. 

The embryonic development of the very first animals was almost totally hard-wired, and this had two important implications. The first is that all embryonic stages were controlled by genes, including the phylotypic stage, and this means that the body plans were modifiable by genetic changes. This was the period in which old body plans could be transformed and new body plans could be invented. The second implication is that those animals were necessarily small and relatively simple, because there is a limit to the number of characters that can be directly controlled by genes. 

Von Baer discovered the phylotypic stage of vertebrates at a time when earlier developmental steps were still unknown, and concluded therefore that the very first period of development was devoted to building the body plan. From this he derived the idea that embryonic development proceeds from the general to the particular. First it is the characteristics of the phylum that appear, then come the features that specify the class, the order, the family, the genus and the species, and only at this point do the signs appear that distinguish an individual animal from the others. 

The pattern which is observed before the phylotypic stage simply cannot be explained by the same evolutionary mechanism that produce the opposite pattern of the other stages, and this is a problem which has never been given a satisfactory answer. Haeckel, for example,... 

The crucial point is the idea that a body plan is simultaneously a phenotypic structure and a deposit of information. If information could be transported without three-dimensional structures, there would be no need to conserve three-dimensional patterns, but the information of a body plan is precisely about spatial organisation, and cannot be preserved without the three-dimensional structures which define that organisation. Traditional theories, in conclusion, have regarded the body plan exclusively as a phenotypic structure, not as a deposit of information (a supracellular memory), and it is this which has prevented them from explaining the conservation of the phylotypic stage. 

Let us now summarise the main points (1) there is a phylotypic stage in the development of the body and a specietypic stage in the development of the mind (2) there is a body plan in the development of the body and a universal grammar in the development of the mind (3) there is incomplete information in the development of the body and incomplete stimuli in the development of the mind. If we put these conclusions together, we obtain a semantic model of mind development just as we did for embryonic development ... 

So how do code and memory relate to the two-phase development of metazoans The first and most obvious connection, that Barbieri himself made, is that the construction of the phylotypic stage in each embryology represents a memory , what he calls the body plan supracellular memory. By using a tiny bit of code to arrange that the ovary makes oocytes of particular architecture from-and-within the structure of the adult, the next generation are started on their complexity-increasing way. 

Hall, B.K. (1997). Phylotypic stage or phantom is there a highly conserved embryonic stage in vertebrates Trends Ecol Evol. V. 12. p>p. 461-463. 

Schmidt, K. Starck, J.M.. (2010). Developmental plasticity, modularity, and heterochrony during the phylotypic stage of the zebra fish, Danio rerio. J Exp Zool B Mol Dev Evol. V. 15 No 314(2), pp. 166-78. 

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