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Pander’s work addressed this criticism directly. He wrote:
We must remind our readers that when folds of the membranes are mentioned, they should
not imagine lifeless membranes, whose folds would necessarily extend over the entire sur-
face if they were mechanically formed, and would not be restricted to a particular zone;
because this view would inevitably lead to erroneous ideas. The folds that cause the meta-
morphosis of the membranes have a spontaneous organic origin, and form at the proper
place, whether by expansion of already existing vesicles or by the emergence of new vesi-
cles with no alteration in the remainder of the blastoderm. (p. 40) (Pander 1817a)To scientists of the time, the ability to move was one of the defining characteris-
tics of animal life. Pander realized that life itself was the driving force of morpho-
genesis, eliminating the mysticism of previous incarnations of epigenesis. After
Pander, any lingering support for preformation evaporated. In a deeper sense,
though, only now, we are only now beginning to get a satisfactory answer to von
Haller’s mechanistic critique of epigenesis by using hte modern molecular genetic
and imaging techniques currently at our disposal.
7.2.1 The Germ Layer Theory
Pander quickly abandoned embryos to study fossils, but his work inspired others to
study the germ layers in both vertebrate and invertebrate embryos, including his
friend and fellow student of Döllinger’s, Karl Ernst von Baer (1792–1876). von
Baer is perhaps better known today for his discovery of the mammalian egg in 1826,
but he was one of the first individuals to perform a comparative analysis of embryos
from different species (Baer and Stieda 1828 ). von Baer extended Pander’s observa-
tions of germ layers in the chicken embryo to all vertebrate embryos, while other
scientists inspired by Pander found the germ layers in invertebrates (Oppenheimer
1940 ). By the middle of the nineteenth century it was recognized that germ layers
are a universal feature of animal development. This was a major conceptual break-
through because it suggested for the first time that all animals share a common
mechanism of development. von Baer formalized this concept into the Germ Layer
Theory, which stated that all gastrula stage embryos consist of three visible layers,
each dedicated to play a specific role in the development of the organism (Baer and
Stieda 1828 ). This theory provided an objective basis for comparing embryonic
development in different species. Embryogenesis could be divided into three stages
(at least) that are common to all animals. Before gastrulation, the embryo consists
of an undifferentiated blastoderm. During gastrulation, the blastoderm divides into
three germ layers, which make distinct contributions to the body. Finally, after gas-
trulation, the cells begin to differentiate into specific tissues.
The Germ Layer Theory also posited that homologous tissues in different spe-
cies were derived from the same germ layers. For example, if the heart was derived
from the mesoderm in chicken, it was also mesodermal tissue in amphibians and
mammals. von Baer applied these fundamental insights into his systematic com-
parison of embryonic development in different species to develop his famous laws
W. Tseng et al.