Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 144

variation at the onset of the phylotypic stage ( Bininda-Emonds et al., 2003 ). If this
turned out to be true, the phylotypic stage would be about a common Bauplan of
organisms of a phylum/subphylum, and it does not necessarily imply that the stage
will follow the same tempo across the taxa.
Recall that scientific knowledge is always relative rather than final, and in hind-
sight, it can be criticized as imperfect. Such a viewpoint, also emphasizes his sci-
entific contribution: “Haeckel sensed a commonality in early embryos. Although
that sense could have biased his depiction of embryos, Haeckel was more correct
than the data of his day allowed.” ( Elinson and Kezmoh, 2010 ). The commonality
was rediscovered only almost half a century after his death, when Friedrich Seidel
(1897-1992) demonstrated that the third law was not valid for the earlier stages of
development and that many organisms tend to converge to a morphologically simi-
lar stage of the basic body pattern ( Stadium der Körpergrundgestalt ), which is now
known as the phylotypic stage . The phylotypic stage is considered to be “the central
event in early embryonic development” ( Sauer et al., 1996 ). However, the fact that
vertebrate embryos starting from different forms of early development converge later
at a common morphology at the phylotypic stage ( O'Farrell et al., 2004 ) to develop
more divergent morphologies later is incontestable ( Figure 3.20 ).
In humans, the phylotypic stage is between 25 and 30 days after fertilization;
in zebrafish, it occurs at 24-48 h; and in mouse embryos, at the E (embryonic day)
8.0-8.5. In Xenopus , the phylotypic stage is known as the tailbud stage . It corre-
sponds to the Nieuwkoop and Faber stages 20-44, which is divided into the early
tailbud stage (20-29) and the late tailbud stage (29-44). The modern hypotheses
based on von Baer's and Haeckel's concepts on patterns of embryonic development,
are presented in Figure 3.21 .
In vertebrates, generally, the phylotypic stage coincides with the onset of neurula-
tion and ends with the formation of the full set of somites ( Galis and Metz, 2001 ).
At the phylotypic stage, different chordates are similar in size, and this is valid for
both a mouse and a whale, although the latter is a millionfold heavier ( Kirschner and
Gerhart, 2005 ).
The phylotypic stage as the most conserved developmental stage in vertebrates
and the hourglass model are also supported by gene expression data. At the gene
expression level, it coincides with the most conserved expression pattern of the Hox
and other developmental genes, with the onset of intricate networks of both global
and local inductive signals for organogenesis, which make most changes lethal,
making the phylotypic stage highly conserved ( Irie and Sehara-Fujisawa, 2007 ).
Formation of the Neural Crest
The NCC is a transient vertebrate structure that evolved around 450Mya.
Developmental and phylogenetic evidence suggests it evolved from the epidermal
nerve plexus of protochordates ( Gans and Northcutt, 1983 ; see Figure 3.22 ). It is
involved in the development of almost all vertebrate organs and structures. It may
also be the most important event related to the evolution of vertebrates.