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7.4.5 Conclusion
A comparison of the fate maps of the frog, fish, bird and mammalian blastoderm
reveals a striking conservation of organization, despite the dramatic differences in
morphology. In all species, ectodermal tissues arise from cells in the future anterior
of the embryo, while endoderm arises from the future posterior, where gastrulation
initiates. Mesodermal precursors are distributed in between. The deep conservation
of the vertebrate pre-gastrula fate map suggests that the conserved body plan
observed at the phylotypic stage originates with a common mechanism of germ
layer specification.
The intermingling of germ layer territories observed in the fish, avian and mam-
malian fate maps has largely been overlooked due to the prevalence of oversimpli-
fied fate map cartoons in textbooks and review articles. Precursors of the germ
layers are highly segregated in the fate map of the Xenopus blastula, but there is
considerable intermixing of germ layer precursors in other amphibian species
(Smith and Malacinski 1983 ; Purcell and Keller 1993 ; Delarue et al. 1994 ). The
intermingling of germ layer precursors reflects the fact that cells are constantly
changing their positions, due to both passive movements in response to cell division
and division and by active, but random movements apparent in some species. The
directed movements of gastrulation result in even more dramatic changes in cell
position. Cells move between 20–70 μm in random directions in the zebrafish blas-
toderm, which is approximately 500 μm in diameter (Trinkaus 1973 ; Bensch et al.
2013 ). Cells migrate up to 150 μm in the killifish blastoderm, which is about 1.8 mm
in diameter (Trinkaus 1973 ; Bensch et al. 2013 ). In the amniote epibilast, cells also
move great distances before the primitive streak forms, and cell proliferation in the
mouse is coupled to a dramatic increase in embryonic volume (Snow and Bennett
1978 ; Gardner and Cockroft 1998 ; Chuai et al. 2006 ). These movements have pro-
found consequences for the molecular mechanisms that induce the germ layers.
7.5 Germ Layer Induction
Although Pander understood that the three germ layers “work mutually together
although destined for different ends,” von Baer’s Germ Layer Theory took a more
rigid deterministic view of the germ layers as it developed over the century. At the
turn of the twentieth century, tissue grafting and explant culturing methods were
developed that finally permitted scientists to functionally test the roles of different
embryonic tissues (Oppenheimer 1940 ). Vogt’s fate map made it possible to cor-
rectly interpret these experiments, since it is necessary to understand how tissues
behave in their normal locations before trying to understand how they behave when
placed in an abnormal environment. These embryological experiments demon-
strated the source of signals that induce the germ layers, and showed that these
signals act before gastrulation.
W. Tseng et al.