- During the early 19th century embryologists were concerned mainly with describing and comparing the embryos of various species at different stages in their development. In 1828 the Russian zoologist Karl Ernst von Baer observed that in their earlier phases the embryos of higher species resemble those of lower species but that the embryos of the higher species are never like the adults of the lower species. In 1867 the German biologist Ernst Heinrich Haeckel used Von Baer’s findings to formulate his “biogenetic law”—ontogeny recapitulates phylogeny. According to Haeckel, the development of an embryo (ontogeny) retraces the evolution of its species (phylogeny).
- Toward the close of the 19th century the German anatomist Wilhelm Roux founded experimental embryology, the study of the causes of embryogeny. In 1924 the German experimental embryologist Hans Spemann discovered “embryonic induction.” He found that the mesoderm of an undifferentiated embryo was the “organizer” of brain and spinal cord formation from the overlying ectoderm. In the 1950s the Chinese-born American embryologist Man Chiang Niu discovered that a substance containing a nucleic acid passes from the “organizer” mesoderm to the ectoderm. Later work was aimed at finding the specific ways in which nucleic acids induce differentiation in the early embryo.
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