Working in Germany in the early 1930s, Johannes Holtfreter conducted a series of innovative experiments on how the Spemann-Mangold organizer signals in gastrulating urodele embryos. Holtfreter found that developing embryos soaked in a hypertonic salt solution failed to form a blastocoel, a hollow cavity that develops inside the spherical embryo. During normal development, the blastocoel is the space into which surface cells migrate as they pass through the organizer, which is located on the dorsal lip of the embryo. Holtfreter called his blastocoel-less embryos “exogastrulas” because the cells that would normally turn into the blastocoel instead turned outward. This experimental manipulation allowed him to uncover exactly how the organizer signals to surrounding tissues during gastrulation.
Holtfreter visualized the directional movements of exogastrula tissues by staining the early gastrula with vital dyes in certain locations and then following where the dye ended up as gastrulation proceeded. All along the way, Holtfreter meticulously documented the path of the dyes with numerous drawings of each embryo. In these drawings, he used colors to show the location of each dye and arrows to indicate the movements of tissues. Holfreter also took micrographs of his exogastrulas, but they primarily served to provide a snapshot of what he was seeing through the microscope. Most his observations were synthesized and documented through drawing.