313of embryology, which Stephen J Gould paraphrased succinctly: “Development is
individualization; it proceeds from the general to the special; it is a true differen-
tiation of something unique from something common to all.” (p. 55) (Gould 1977 ).
In other words, embryos of the same phylum start out looking very similar to each
other, displaying the general characteristics shared by all members of that phylum,
and subsequently diversify as the more specific traits of each species develop (Baer
and Stieda 1828 ). von Baer built a classification system of developmental history
based on shared embryological characteristics. For example, animals containing an
embryonic notochord were grouped together, and this group further divided into
those with vertebrae and those without vertebrae. In his private notes, he expressed
this classification system as a “tree of developmental history,” which bears a strik-
ing resemblance to Darwin’s evolutionary tree published 31 years later (Brauckmann
2012 ; Abzhanov 2013 ). The Germ Layer Theory was highly controversial for most
of the nineteenth and early twentieth centuries, with much discussion focusing on
which organs and tissues were true homologues in different species (Braem 1895 ).
Modern methods of marking specific cell types have resolved this issue, and have
shown that germ layers in different species do, in fact, produce homologous organs.
von Baer left embryology in 1838 to study ecology, but his work inspired a new
generation of scientists to use comparative embryology to understand the evolution
of species (Brauckmann 2012 ).
7.2.2 The Developmental Hourglass
The stage in which all embryos of a phylum resemble each other was eventually
called the phylotypic stage (Fig. 7.3b). After the phylotypic stage, the embryo mor-
phologies increasingly diverge as the unique features of each species, such as hair,
feathers, shells, beaks or teeth, begin to differentiate (Fig. 7.3c). Vertebrate embryos
also display a wide variety of morphologies at the blastoderm stage, well before the
phylotypic stage (Fig. 7.3a). This early morphological diversity may be surprising
until one remembers that the egg is an adult cell type, produced by the mother for
the purpose of reproduction in a particular environment. Therefore evolutionary
pressures on the adult female shape the morphology of the egg and early embryo.
The diverse vertebrate morphologies of the eggs converge on a common body plan
at the phylotypic stage, in which diversity is constricted. After this stage, diversity
increases as the specialized features characteristic of each species begin to develop.
The constriction point has suggested an analogy with a sand-filled hourglass, which
modern developmental biologists refer to as the Developmental Hourglass (Slack
et al. 1993 ; Duboule 1994 ).
The phylotypic stage became a highly controversial concept over the years, in
part because it was enthusiastically adopted by Ernst Haeckel to promote his erro-
neous views on evolution and development, which he encapsulated with the
phrase “ontogeny recapitulates phylogeny” (Haeckel 1874 ; Sedgwick 1894 ).
Some have even accused Haeckel of making fraudulent drawings to support his
7 Establishment of the Vertebrate Germ Layers