Cyclostome embryology and early evolutionary history of vertebrates

Modern agnathans include only two groups, the lampreys and thehagfish, that collectively comprise the group Cyclostomata.Although accumulating molecular data support the cyclostomesas a monophyletic group, there remain some unsettled questionsregarding the evolutionary relationships of these animals inthat they differ greatly in anatomical and developmental patternsand in their life histories. In this review, we summarize recentdevelopmental data on the lamprey and discuss some questionsrelated to vertebrate evolutionary development raised by thelimited information available on hagfish embryos. Comparisonof the lamprey and gnathostome developmental patterns suggestssome plesiomorphic traits of vertebrates that would have alreadybeen established in the most recent common ancestor of the vertebrates.Understanding hagfish development will further clarify the,as yet, unrecognized ancestral characters that either the lampreysor hagfishes may have lost. We stress the immediate importanceof hagfish embryology in the determination of the most plausiblescenario for the early history of vertebrate evolution, by addressingquestions about the origins of the neural crest, thyroid, andadenohypophysis as examples.     

The history of scientific endeavors towards understanding hagfish embryology

Due to their curious phylogenetic position and anatomy, hagfishes have attracted the interest of zoologists, especially in the context of vertebrate evolution. Embryological information on these animals is now also needed in the field of evolutionary developmental biology (Evo-Devo), as it is expected to provide hints about the origin of vertebrate traits, whether the hagfishes are an in-or outgroup of vertebrates. This review summarizes the importance of hagfish embryology from a phylogenetic perspective, and the history of attempts to obtain hagfish eggs and embryos. Clearly, the main difficulty associated with these animals is their deep-sea habitat. To circumvent this problem, this review also discusses the future prospects for obtaining embryological material, both from the wild and in the laboratory.     

A history of normal plates, tables and stages in vertebrate embryology

Developmental biology is today unimaginable without the normal stages that define standard divisions of development. This history of normal stages, and the related normal plates and normal tables, shows how these standards have shaped and been shaped by disciplinary change in vertebrate embryology. The article highlights the Normal Plates of the Development of the Vertebrates edited by the German anatomist Franz Keibel (16 volumes, 1897-1938). These were a major response to problems in the relations between ontogeny and phylogeny that amounted in practical terms to a crisis in staging embryos, not just between, but (for some) also within species. Keibel's design adapted a plate by Wilhelm His and tables by Albert Oppel in order to go beyond the already controversial comparative plates of the Darwinist propagandist Ernst Haeckel. The project responded to local pressures, including intense concern with individual variation, but recruited internationally and mapped an embryological empire. Though theoretically inconclusive, the plates became standard laboratory tools and forged a network within which the Institut International d'Embryologie (today the International Society of Developmental Biologists) was founded in 1911. After World War I, experimentalists, led by Ross Harrison and Viktor Hamburger, and human embryologists, especially George Streeter at the Carnegie Department of Embryology, transformed Keibel's complex, bulky tomes to suit their own contrasting demands. In developmental biology after World War II, normal stages-reduced to a few journal pages-helped domesticate model organisms. Staging systems had emerged from discussions that questioned the very possibility of assigning an embryo to a stage. The historical issues resonate today as developmental biologists work to improve and extend stage series, to make results from different laboratories easier to compare and to take individual variation into account.     

The early history of the polarizing region: from classical embryology to molecular biology

The polarizing region of the developing limb bud is one of the best known examples of a cell-cell signalling centre that mediates patterning in vertebrate embryos. This article traces some highlights in the history of the polarizing region from its discovery by John Saunders and early work that defined polarizing activity through a period in which modelling was pre-eminent, right up to the discovery of defined molecules with polarizing activity. There is a particular focus on the discovery that retinoic acid could mimic signalling of the polarizing activity and this finding is then set in the context of more recent work which implicates Shh and BMPs in mediating polarizing activity.     

Comparative embryology of basal angiosperms

Recent phylogenetic analyses of basal angiosperms have identified those lineages central to the study of the origin and early diversification of flowering plants. As we begin to understand the early evolution of endosperm developmental patterns in flowering plants, it is apparent that we know little about the other basic embryological features of basal angiosperms, such as the nature of the female gametophyte and even whether a process of double fertilization occurs.