This article is about how the famous organizer experiment has been perceived since it was first published in 1924. The experiment involves the production of a secondary embryo under the influence of a graft of a dorsal lip from an amphibian gastrula to a host embryo. The early experiments of Spemann and his school gave rise to a view that the whole early amphibian embryo was \"indifferent\" in terms of determination, except for a special region called \"the organizer\". This was viewed mainly as an agent of neural induction, also having the ability to generate an anteroposterior body pattern. Early biochemical efforts to isolate a factor emitted by the organizer were not successful but culminated in the definition of \"neuralizing (N)\" and \"mesodermalizing (M)\" factors present in a wide variety of animal tissues. By the 1950s this view became crystallized as a \"two gradient\" model involving the N and M factors, which explained the anteroposterior patterning effect. In the 1970s, the phenomenon of mesoderm induction was characterized as a process occurring before the commencement of gastrulation. Reinvestigation of the organizer effect using lineage labels gave rise to a more precise definition of the sequence of events. Since the 1980s, modern research using the tools of molecular biology, combined with microsurgery, has explained most of the processes involved. The organizer graft should now be seen as an experiment which involves multiple interactions: dorsoventral polarization following fertilization, mesoderm induction, the dorsalizing signal responsible for neuralization and dorsoventral patterning of the mesoderm, and additional factors responsible for anteroposterior patterning.

For 70 years from the very beginning of developmental biology, the salamander embryo was the pre-eminent model for these studies. Here I review the major discoveries that were made using salamander embryos including regionalization of the mesoderm; patterning of the neural plate; limb development, with the pinnacle being Spemann\'s Nobel Prize for the discovery of the organizer; and the phenomenon of induction. Salamanders have also been the major organism for elucidating discoveries in organ regeneration, and these are described here too beginning with Spallanzani\'s experiments in 1768. These include the neurotrophic hypothesis of regeneration, studies of aneurogenic limbs, the concept of dedifferentiation and transdifferentiation, and advances in understanding pattern formation via molecules located on the cell surface. Also described is the prodigious power of brain and spinal cord regeneration and discoveries from lens regeneration, all of which reveal how important salamanders have been as research models.

Organizers, which comprise groups of cells with the ability to instruct adjacent cells into specific states, represent a key principle in developmental biology. The concept was first introduced by Spemann and Mangold, who showed that there is a cellular population in the newt embryo that elicits the development of a secondary axis from adjacent cells. Similar experiments in chicken and rabbit embryos subsequently revealed groups of cells with similar instructive potential. In birds and mammals, organizer activity is often associated with a structure known as the node, which has thus been considered a functional homologue of Spemann\'s organizer. Here, we take an in-depth look at the structure and function of organizers across species and note that, whereas the amphibian organizer is a contingent collection of elements, each performing a specific function, the elements of organizers in other species are dispersed in time and space. This observation urges us to reconsider the universality and meaning of the organizer concept.