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verify their observations. d’Alton documented their findings with a series of draw-
ings that stood out for their remarkable detail and three-dimensional complexity
(Fig. 7.2A and B). To clarify the spatial relationship between the germ layers,
remarkably, they examined the embryos edge-on and produced what may be the first
optical cross sections of embryos (Fig. 7.2C, figs. 8 and 9). They complained that
the images under the microscope were blurry, not surprising given the quality of
lenses available to them. So to convince themselves that the folds in the germ layers
were not merely optical illusions created by poor optics, they used needles and
bristles to touch and physically move the tissue (Fig. 7.2C, figs. 6 and 7). Such cau-
tion was warranted because Pander and Döllinger realized the significance of their
findings for a debate that had been raging for centuries.

7.2 Historical Context

The full impact of Pander’s studies can only be understood in the broader context of
the work of Albrecht von Haller (1708–1777) and Friedrich Caspar Wolff (1734–
1794), two eighteenth century biologists who stood on opposing sides of the ancient
question about “generation.” von Haller was the main proponent of the theory of
preformation of his time (Roe 1981 ). He differed from other preformationists, how-
ever, in that he emphasized the primacy of the egg, rather than the semen, as the resi-
dence of the preformed body. In his view, the egg contained a fully formed body that

Ectoderm Mesoderm Endoderm

Epidermis Bones, cartilage, tendon Digestive System

Central Nervous System Muscle (smooth,

striated)

Respiratory System

Neural Crest Derivatives:

Peripheral Nervous

System

Melanocytes

Facial cartilage

Dentin of teeth

Skull bones

Circulatory System,

including heart, vessels

Pancreas

Lymphatic system Liver

Gonads Thyroid Epithelium

Kidney Parathyroid Epithelium

Adipose Tissue Thymus

Notochord Bladder

Fig. 7.1 Diagram of the germ layers in a gastrulating rabbit embryo, after Kölliker ( 1882 ). Blue =
ectoderm; Red = mesoderm; Green = endoderm. The table describes the derivatives of the three
germ layers in vertebrates

7 Establishment of the Vertebrate Germ Layers