Homology, Recapitulation, and Experimental Embryology: Paths and Meaning of Current Evo-Devo

Evolutionary morphology, including comparative embryology, flourished in the 19th century, epitomized in Haeckel's recapitulation theory. However, it subsequently declined, largely due to accumulating evidence of caenogenesis, while embryology's mainstream gradually shifted toward developmental mechanics-experimental embryology-which laid the foundation for modern developmental biology grounded in reductionist and mechanistic principles. A closer examination of Haeckel's scientific trajectory reveals that, even before formulating the Gastrea theory and the Biogenetisches Grundgesetz (recapitulation theory), he employed siphonophores in what may be regarded as one of the earliest examples of authentic experimental embryology, anticipating many conceptual foundations of contemporary evolutionary developmental biology (Evo-Devo). Despite its innovative contributions, Evo-Devo is not without methodological limitations. One such limitation is highlighted in Sewertzoff's theory of "secondary Archallaxis," logically coherent within the framework of heterochrony, which presupposes an evolutionary process characterized by a persistent search for stable creodes culminating in novel phenotypes-an idea resonant with Waddington's genetic assimilation and Schmalhausen's stabilizing selection. At the same time, the theory underscores the intrinsic difficulty, if not impossibility, of precisely identifying the timing of shifts in developmental programs, a challenge that may constitute a critical vulnerability of the Evo-Devo paradigm. The future advancement of Evo-Devo will depend on the development of methodologies capable of visualizing embryonic developmental pathways and their dynamic transformations.