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mesoderm (Fig. 7.8b, small arrows). Cells only become committed to these fates,
however, after they invaginate through the primitive streak (Kimura et al. 2006 ).
The node gains the ability to induce neural tissue when the streak has reached its full
extension (Fig. 7.8b, white arrows) (Storey et al. 1992 ).

7.5.4 Conclusion

The discovery of the germ layers by Pander and von Baer raised the question of how
the three layers cooperate to ensure the proper placement of organs relative to each
other and with respect to the body axis. Some mechanism must act to guarantee that
the foregut, for instance, always forms in the head rather than in some other loca-
tion. The extent of communication between the germ layers only became apparent
in the early twentieth century, when experimental embryologists began dissecting
embryos and transplanting tissues to different locations, or developing them in

+

Quail PMZ, no KS, or

mesoderm inducing signal

a b

PMZ

KS

AO

Fig. 7.8 Experiments to find the mesoderm inducing signals in chicken. (a) The PMZ can induce
an ectopic primitive streak and Hensen’s Node when grafted at a 90° angle with respect to the
original axis. The PMZ from a quail embryo can induce a primitive streak to form when grafted to
the anterior portion of a chicken epiblast. (b) Signals from the PMZ induce formation of the primi-
tive streak in the posterior area pellucida (top embryo, black arrows). At a later stage, the primitive
streak and Hensen’s Node express signals that induce mesoderm and endoderm, and signals from
Hensen’s Node pattern the neural tissue (white arrows)

7 Establishment of the Vertebrate Germ Layers