A clash of traditions: the history of comparative and experimental embryology in Sweden as exemplified by the research of Gösta Jägersten and Sven Hörstadius.

Until the 1940s research traditions were often imported from Germany to Sweden, and young scientists went to German universities to learn new techniques and get in touch with the latest ideas. In developmental biology, the comparative, phylogenetic embryology advocated most forcefully by Ernst Haeckel co-existed with the "Entwickelungsmechanik" tradition developed by Wilhelm His, Wilheln Roux and others partly as a reaction to Haeckel's ideas. I use the zoology department at Uppsala University as a microcosmos to reflect the tensions between these traditions: Gösta Jägersten (1903-1993) and Sven Hörstadius (1898-1996) are used as exemples. Jägersten was a marine biologist who worked on the morphology and evolution of invertebrates, especiallly their larval forms. He developed a comprehensive theory describing the evolution of the life cycle in early metazoans. Recapitulation was an important ingredient, and Jägersten explicitly based his reasoning on Ernst Haeckel's "biogenetic law". Jägersten developed Haeckel's "Gastraea" theory into another hypothetical animal-Bilaterogastraea-that came into being when the holopelagic Blastaea settled on the ground as an adult and kept a pelagic, planktonic larval form. This was the birth of the pelago-benthic life cycle, which plays such an important role in Jägersten's speculations on the deep phylogeny of metazoans. Sven Hörstadius was a leading experimental embryologist in the mid-twentieth century. His most important work was on the determination and differentiation of the sea urchin embryo. Early work inspired by his teacher John Runnström's double gradient theory showed that gradients of animalness (ectodermal determination) and vegetalness (endodermal determination) existed in the 16- and 32- cell embryos. Hörstadius became famous for his elegant extirpation and transplantation experiments using glass needles, and for his microsugical skills. He also made important contributions to the study of cranial neural crest development in the Mexican axolotl, in collaboration with his student Sven Sellman. Hörstadius was the great experimentalist, but did not develop speculative hypotheses the way Jägersten did. The very different styles of scientific research might have played a role also in the development of the personal difficulties that existed for a long time between the two professors.