Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 143

serotonergic neurons, differentiated earlier at stage 25 in the embryonic rostral brain
stem ( van Mier et al., 1986 ).
The Phylotypic Stage
In 1828, Karl Ernst Ritter von Baer (1792-1876) published the first volume of the
influential On the Development of Animals: Observation and Reflection . In this work,
von Baer pointed out that at his time, the prevailing opinion of biologists was “that
the embryo of higher animals goes through adult forms of lower animals” ( von Baer,
1828 ). He observed that different animals display morphological similarities dur-
ing various periods of their development: “The further we go back in the development,
the more we find also in very different animals such similarities” ( von Baer, 1828,
p. 223 ). 1 According to him, all the typical vertebrate characters appear very early during
development, so that “[t]he vertebrate embryo from the beginning is a vertebrate and in
no time it matches with that of an invertebrate animal” ( von Baer, 1828, p. 220 ). 2
There, von Baer criticized the idea that higher animals go through adult forms
of lower animals, but this concept was revived and popularized by Ernst Haeckel
(1834-1919) in his book General Morphology (1866), an attempt to support the
Darwinian idea of evolution and of common ancestry Haeckel (1866) . His concept of
“ontogeny as a brief and rapid recapitulation of phylogeny” is flawed, and support-
ing drawings that he published to popularize the idea received deserved criticism.
That said, the theory had a positive influence on the study of evolution, in the devel-
opment of the concept of homology, and in the modern concept of the phylotypic
stage, a central concept of modern evo-devo. Haeckel doctored his drawings to sub-
stantiate his biogenetic law to give Darwinism supporting evidence from embryology
( Richardson and Keuck, 2002 ).
He was criticized during his lifetime for exaggerating the similarities between
embryonic forms of higher animals and those of the adult forms of lower ones. Now,
a century later, this criticism about “inaccurate and misleading” drawings resumed,
and these inaccurate drawings are used to deny the existence of a phylotypic stage in
the animal world. While admitting that “Haeckel recognized the evolutionary diver-
sity in early embryonic stages, in line with modern thinking,” critics have identified
some “potential sources for several of the drawings, and find some evidence of doc-
toring,” yet they fairly admit that “Haeckel's much-criticized embryo drawings are
important as phylogenetic hypotheses, teaching aids, and evidence for evolution.
While some criticisms of the drawings are legitimate, others are more tendentious”
( Richardson and Keuck, 2002 ).
Richardson's main argument against a conserved phylotypic stage is the variation
in timing; that is, that different species and other taxa show considerable temporal
1 Je weiter wir also in der Entwickelung zurückgehen, um desto mehr inden wir auch in sehr ver-
schiedenen Thieren eine Uebereinstimmung.
2 Der Embryo des Wirbelthiers ist schon anfangs ein Wirbelthier, und hat zu keiner Zeit Übereinstimmung
mit einem wirbellosen Thiere.