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In our era of computers and computer models, the importance of physical or graphical models for both research and education in developmental biology (embryology) is often forgotten or at least underappreciated. Still, one important aspect of embryology is the (evolutionary) developmental anatomy of both human and animal embryos. Here, we present a short history of the visualization of Ernst Haeckel's "biogenetic law" and his "gastraea theory" in biology textbooks from the German Democratic Republic (GDR) between 1951 and 1988. Our analysis of GDR textbooks showed embryology was integrated into different disciplines and remained an educational constant within the school textbooks throughout the GDR despite various educational reforms. While the majority of these textbooks failed to reference either Ernst Haeckel or his contributions to embryology, they often did mention Haeckel in sections dedicated to the theory of evolution and the promotion of Soviet ideals such as materialism.

150 years ago, in 1866, Ernst Haeckel published a book in two volumes called "Generelle Morphologie der Organismen" (General Morphology of Organisms) in which he formulated his biogenetic law, famously stating that ontogeny recapitulates phylogeny. Here we describe Haeckel's original idea and follow its development in the thinking of two scientists inspired by Haeckel, Alexei Sewertzoff and Adolf Naef. Sewertzoff and Naef initially approached the problem of reformulating Haeckel's law in similar ways, and formulated comparable hypotheses at a purely descriptive level. But their theoretical viewpoints were crucially different. While Sewertzoff laid the foundations for a Darwinian evolutionary morphology and is regarded as a forerunner of the Modern Synthesis, Naef was one of the most important figures in 'idealistic morphology', usually seen as a type of anti-Darwinism. Both Naef and Sewertzoff aimed to revise Haeckel's biogenetic law and came to comparable conclusions at the empirical level. We end our review with a brief look at the present situation in which molecular data are used to test the "hour-glass model", which can be seen as a modern version of the biogenetic law.

The importance of embryonic development for evolutionary biology has been an issue ever since Charles Darwin and Ernst Haeckel, however, Modern Synthesis approaches to evolution have often neglected certain aspects of developmental biology or treated development as a black box (Mayr and Provine 1980; Breidbach and Ghiselin 2007a, b). Although Wallace’s statement that ‘‘problems concerned with the orderly development of the individual are unrelated to those of the evolution of organisms through time’’ (Wallace 1986, p. 149) is extreme, midtwentieth century mainstream evolutionary biology did not feel much need for an integration of developmental biology into its theoretical foundations. The fact that evolutionary questions have been of interest to some developmental biologists between the era of Darwin and Haeckel and the 1980s, i.e., that modern evolutionary developmental biology, or Evo-devo, as the field is often called by its practitioners, in fact has a history, has received relatively little attention. It has even been claimed that ‘‘Following a quiescent period of almost a century, present-day Evo-devo erupted out of the discovery of the homeobox on the 1980s’’ (Arthur 2002, p. 757). Thus, there is a need for more research into the period ‘‘between Ernst Haeckel and the homeobox’’ in order to gain a more complete understanding of the roots of modern Evo-devo. We are helped by the recent upsurge in interest in the history of Evodevo, which has produced several edited volumes (e.g., Laubichler and Maienschein 2007) and thematic issues of journals. Also this journal has contributed to this increased publication activity on the pre-history and history of Evodevo, having had thematic issues (Theory in Biosciences 124:259–420; 126:115–175) on different aspects of the subject. The history of Evo-devo in the Anglo-American world has received renewed attention recently, as exemplified, e.g., by the work of Alan Love (e.g., Love 2003, 2006; Love and Raff 2003), whose scheme of the historical development of the relationship between evolution and development is here reproduced as Fig. 1. We have ourselves concentrated on the history of Evo-devo in the Germanand Russian-speaking countries (e.g., Hossfeld and Olsson 2003; Levit et al. 2004, 2006; Olsson et al. 2006; Breidbach and Ghiselin 2007a, b; Reis et al. 2007). In Love’s scheme (Fig. 1), he contrasts the ‘‘textbook version’’ (left) with an improved, updated version (right). In the left diagram, evolutionary biology is split from developmental biology, which was dominated by ‘‘Entwickelungsmechanik’’ (Developmental Mechanics) in the first third of the twentieth century. The developmental biologist Thomas H. Morgan (1866–1945) is seen as an example of the split between experimental embryology and genetics, which he helped to found and develop into molecular genetics. Another part of genetics, population genetics, became an important part of the Modern Synthesis of evolutionary biology (Mayr and Provine 1980; L. Olsson (&) Institut fur Spezielle Zoologie und Evolutionsbiologie mit Phyletischem Museum, Friedrich-Schiller-Universitat Jena, Erbertstr, 1, 07743 Jena, Germany e-mail: Lennart.Olsson@uni-jena.de

The homology concept has had a long and varied history, starting out as a geometrical term in ancient Greece. Here we describe briefly how a typological use of homology to designate organs and body parts in the same position anatomically in different organisms was changed by Darwin’s theory of evolution into a phylogenetic concept. We try to indicate the diversity of opinions on how to define and test for homology that has prevailed historically, before the important books by Hennig (1950. Grundzuge einer Theorie der Phylogenetischen Systematik. Deutscher Zentralverlag, Berlin) and Remane (1952. Die Grundlagen des Naturlichen Systems, der Vergleichenden Anatomie und der Phylogenetik. Geest & Portig, Leipzig) brought more rigor into both the debate on homology and into the usage of the term homology among systematists. Homology as a theme has recurred repeatedly throughout the history of the “Phylogenetisches Symposium” and we give a very brief overview of the different aspects of homology that have been discussed at specific symposia over the last 48 years. We also honour the fact that the 2004 symposium was held in Jena by pointing to the roles played by biologists active in Jena, such as Ernst Haeckel and Carl Gegenbaur, in starting the development towards a homology concept concordant with an evolutionary world view. As historians of biology, we emphasize the importance of major treatises on homology and its history that may be little read by systematists active today, and have sometimes also received less attention by historians of biology than they deserve. Prominent among these are the works of Dietrich Starck, who also happened to be both a student, and later a benefactor, of systematics at Jena University.

In the evolution of land-living vertebrates, the transition from spending the entire life cycle in the water to first a biphasic (adult on land, eggs and larvae in water) and later a terrestrial life-history mode was achieved by changes in developmental processes and regulatory mechanisms. Lungfishes, salamanders and frogs are studied as examples of species which span this transition. The migration and fate of the embryonic cells that form the head is studied, using experimental embryology (extirpation and transplantation of cells), molecular markers and novel microscopy techniques — such as confocal microscopy. Knowing the migratory routes and fates of the cells that form head structures is important for an elucidation of the changes that took place e.g. when gill arches transformed into head cartilages, and when the specialised larval mouth structures present in today’s frogs and toads arose as an evolutionary innovation. Results so far indicate that the early migration and pattern formation of neural crest cells in the head region is surprisingly conserved. Both the amphibians investigated and the Australian lungfish have the same number of migrating neural crest streams, and the identity of the streams is preserved. The major difference lies in the timing of migration, where there has been a heterochronic shift such that cell migration starts much later in the Australian lungfish than in the amphibians. The molecular mechanisms regulating the formation of streams of cranial neural crest cells seem, at least in part, to be differential expression of ephrins and ephrin receptors, which mediate cell sorting. Our understanding of the behaviour of migrating cells (primarily the more well characterised neural crest cells) could be enhanced by a modelling approach. I present preliminary ideas on how this could be achieved, inspired by recent work on Dictyostelium development and our own previous work on pigment cells and their pattern formation during salamander embryogenesis.

Comparative anatomy and zoology both have long academic traditions in Jena. At first, the two subjects developed in parallel and had many similarites in research topics. This development is covered in the first part of the paper. The close relationship between the two subjects started to break apart when Carl Gegenbaur and Ernst Haeckel were active at Jena University. In 1865 Haeckel became the first full professor of zoology in Jena, and zoology became more independent from comparative anatomy. In the second part of our paper, we follow the developments in comparative anatomy in Jena from Gegenbaur’s immediate students up until the end of Hans Boker’s tenure in Jena in the mid-1930s. Certain subjects are in focus throughout this period, for example vertebrate head morphology and development, (the “head problem”), the relationship between anatomy and biology, and evolutionary questions. Some of these subjects have remained important research topics in zoology and comparative anatomy in Jena until the present day.