Homeosis and atavistic regeneration: the 'biogenetic law' in Entwicklungsmechanik.

Homeosis is a developmental abnormality corresponding to the transformation of a part of the body into another one. This term was introduced in 1894 by William Bateson, who aimed to make an inventory of all kinds of biological variation in order to understand how evolution proceeds. But, immediately afterwards experimental embryology, or Entwicklungsmechanik in Germany, adopted and redefined this term to refer to abnormal regenerations in which the newly developed organ was not identical to the initial one but rather resembled another part of the body. At that time, many experimental embryologists, such as Wilhelm Roux, were calling for the elimination of any phylogenetic explanation of development and were attempting to promote more mechanistic, proximate explanations. Despite these recommendations, several biologists continued to account for developmental processes by turning to phylogeny instead of mechanical forces. The case of homeosis is representative. Indeed, abnormal regenerations were often seen as examples of atavisms, or recurrence of ancestral characteristics, and many embryologists appealed to Ernst Haeckel's 'fundamental biogenetic law' to explain these strange phenomena. The break between Haeckelian tradition and Entwicklungsmechanik is thus less radical than often assumed, and the homeosis concept represents one of the factors of this continuity.