The Evolution and Development of Comparative Embryology : Oscar Hertwig and the Modern Age (2007.4.2[Mon])


Oscar Wilhelm August Hertwig (1849-1922) was an eminent scholar of anatomy, the elder of the renowned Hertwig brothers of German biology. The younger brother, Richard (1850-1937), was, sLehrbuch der Zoologie (Textbook of Zoology; Gustav Fischer, Jena). Both of the brothers were students under Carl Gegenbau and Ernst Haeckel at the University of Jena, where Oscar later served as lecturer before attaining a professorship in Anatomy. The Hertwig brothers' comparative studies of mesodermal coelom development were undeniably in line with the Haeckelian dogma of recapitulation, which demonstrates only their loyalty to the teachings of their mentor, while their true loyalty to the spirit of scientific inquiry became evident thereafter, as described below.

 In 1885, Richard moved to Munich to become Professor of Zoology, three years later in 1888, Oscar took a position as Professor of Anatomy in Berlin. For Oscar, embryonic development served as a mechanistic element capable of explaining the process of evolution, an influence doubtless inherited from his former instructor. But the work itself was firmly rooted in advanced cell biological concepts of the day, as evidenced in his Elemente der Entwicklungsgeschichte der Menschen und der Wirbeltiere (1900) in which his ideas on chemical and physical recapitulation in embryogenesis are already evident. In the Handbuch der vergleichenden und experimentellen Entwickelungslehre der Wirbeltiere (Gustav Fischer, Jena, 1906) he also maintains his mechanistic approach to development, stating, There are limited means by which a single cell can give rise to a complex morphology, and it appears that the development of the individual recapitulates that of its phylogeny. But this work was a compilation edited by Hertwig that included his own writing as well as chapters contributed by other eminent biologists, the overall style is perhaps inevitably redolent of the prevailing comparative embryological concepts of the day. Oscar was also the first to elucidate a number of phenomena that are now commonplaces of biological thought, the most extreme example being that the sperm penetrates the egg at the time of fertilization. He also observed the loss of chromosomes during development and hypothesized that the nucleus is the conveyor of genetic information.

 I would like to take a moment here to clarify the term evolution, which originally was used to mean ontogenetic developmental processes. Like the German word 'Entwicklung' or its English translation 'development', these words can carry the connotation of something being unveiled, like a scroll unspooling to reveal a previously written text. In this sense, both evolution and embryogenesis are foreordained and deterministic processes of differentiation and emergent complexity. Thus, there was at first no clear distinction between the definitions of evolution and embryonic development. Herein lies also the germinal source of Haeckel's theory recapitulation, an intimation that the same fundamental process is at work in both ontogeny (Entwicklung) and phylogeny (Entwicklungsgeschichte). Behind all of this, stand our own prejudices about developmental mechanisms, namely the idea of 'progenesis' (preformation) as opposed to epigenesis.

  If we exclude Aristotle, who was fascinated by observing the wondrous emergence of a chick within the egg, embryology finds its classical roots in preformation, the idea that a perfectly formed template of the adult slumbers inside the ovum (or, in some versions, as a sperm-borne homunculus), and then simply develops into its ultimate form. Epigenesis, on the other hand, asserts that there are patterns not yet present in the early embryo, which can only emerge as the result of further development. Preformation has a certain hermeneutic appeal, but falls short of explaining a number of actualities, while epigenesis more accurately describes the biological reality as we now know it, but nonetheless fails to address the mechanisms. To do justice to this rivalry would require a review of the entire history of the field and more space than I can commit to it in this essay, suffice it to say that the history of developmental biology is nothing less than a long disputation between proponents of these competing theories, with each age engendering new versions of this age-old debate and giving birth to new questions (even as our own age of genomics and cloning has done). Researchers who now puzzle over just how far the genetic information encoded in the nucleus can be relied on to guide development are essentially struggling with the same sorts of questions that scientists in previous generations confronted. It seems we have yet to put this long argument to rest.

 Oscar Hertwig is currently seen as an advocate of the theory of epigenesis; his main rival was August Weismann, of germ plasm fame. After a tortuous academic course, Weismann finally arrived at a theory of recapitulation that was essentially based in epigenesis, writing, nuclear factors are the sole determinants of the direction of differentiation, and changes are determined by the amount and content of those factors (resulting in mosaic developmental patterns), a clear indication of a deterministic image dependent solely on cell lineage. In contrast, Oscar Hertwig stressed that, The direction of development is influenced by interactions between cells, and the overall state of the embryo, and does not proceed in a mosaic fashion determined by intracellular factors. His was a view that emphasized cellular interactions, such as those now studied as epigenetic phenomena, and the importance of extracellular signals (in the first half of the 19th century, Geoffroy St. Hilaire had espoused a similar view based on his anatomical studies.) You don't have to consult your Waddington to see that this is unmistakable preformationism. We can easily trace the evolution of the epigenesis vs. preformation conflict to the cell biological level from the late 19th to the 20th century, but we should also recognize that the same fundamental debate continues unabated even today. In fact, we can see clearly that both types of mechanisms are at work in the embryonic development of animals such as sea squirts and nematodes (although Weismann's determinant factors is in need of revision). But if we look back on what the biologists of previous eras actually observed and thought, despite our possession of answers to the questions they contended with and mysteries they pondered, the marvelous essence that embryologists since Aristotle have sought remains yet out of our reach….

  I think that the reason that the 1906 Handbuch I referred to above remains in high regard even now can be found in the very first chapter of the first volume, where Oscar Hertwig asked the question of how cellular embryology, which grew from comparative studies of morphology and embryology, related to the evolutionary process in a sweeping historical context. Although a student of Haeckel's, Hertwig remains a biological thinker of undiminished relevance today.

Reference : "Handbuch der vergleichenden und experimentellen Entwickelungslehre der Wirbeltiere"