Using an extensive series of experiments on the developing salamander eggs carried over a period of almost a quarter of a century, Spemann discovered the organizer. The experiments can be traced back to his first experiments early this century, in which the blastomeres of the two-celled stage were separated by tightening a human baby’s hair around the first cleavage furrow. In some eggs, each blastomere developed into an entire embryo small in size (twins), whereas in the majority of eggs one of the blastomeres developed into a perfect embryo and the other into an unorganized mass of living
cells. These
differences in results were
explained on the basis of
variable position of the first
cleavage plane with reference to
the median plane of the future
embryo. In the cases where two
normal dwarf embryos developed,
the first cleavage plane happened
to coincide with the median plane
whereas in those cases where only
one blastomere gave a whole
embryo the first cleavage plane
lies at right angles to the
median plane, so dividing the egg
into dorsal and ventral halves.
By constricting eggs during early
gastrulation this explanation was
proved to be correct. Thus
Spemann concluded that as early
as the two-celled stage, the
dorsal half differs qualitatively
from the ventral half. The dorsal
posses a quality which enables it
to form an embryo whereas the
ventral lacks such a quality.In 1918, he reported that small pieces of ectoderm (prospective neural plate and epidermis) exchanged between embryos of early gastrula stages developed in accord with their new position whereas a piece of the dorsal lip behaved in a different manner, i.e., it developed into a embryo-like body with a neural tube, notochord and somites when transplanted to new positions. From these contrasting results Spemann made the assumption that the dorsal lip was already determined and he suggested that it might represent a "center of differentiation" from which a process of determination gradually spreads forward to the undetermined ectoderm of the gastrula.
It is of historical interest to note that Lewis(1907) using frog embryos had previously reported that a piece of the dorsal lip grafted to a host embryo develops into a nerve tube, chorda and muscle. This he interpreted as indicating that the dorsal lip is determined and undergoes self-differentiation.
Although Spemann had noted as early as 1918 that a graft of the dorsal lip to a host embryo tended to result in the formation of an embryo, the question arose in his mind as how much of its development was due to self-differentiation, and how much to induction. Lewis had assumed that the transplanted dorsal lip self-differentiated. Concepts of induction had already been developing in Spemann’s mind as a result of his earlier studies (1901-1912) on the relationship between optic cup and overlying lens ectoderm, and he recognized that the "organizing" effect of the dorasl lip might be related to inductive action by the archenteron roof on the ectoderm overlying it.
The answer to the question was made possible by grafting a dorsal lip of one species differing in amount of dark pigment. In this way the source of cells which contribute to the secondary embryo could be followed in a precise manner. In a postscript to his 1921 paper on heteroplastic transplantation, Spemann, in referring to experiments made by his student, Hilde Mangold, gave the first description of the dorsal lip as an organizing centre which named "organizer" (Organisator in German) for short. The experiment described in the 1924 article was a crucial one. It showed that both the dorsal lip and the host embryo participate in the formation of a secondary embryo but that the graft contributes the major portion of the chord-mesoderm and hosts the major part of the nervous system. The discovery of the organizer and the analysis of its action was one of the most significant events in experimental embryology; it gave a reality to the epigentic concepts of embryologists, demonstrating experimentally that one step in development is a necessary condition for the next, and for this discovery, Hans Spemann won the Nobel Prize for Physiology and Medicine in 1935.
