This inference is supported by the fact that all the three above structures first originate in vertebrates only. In this interpretation, adenohypophysis and the much earlier established PCM may be considered as derivatives of Spemann's organizer. The adenohypophysis seems to arise directly from the PCM, or cells of the ectoderm influenced by the PCM may be subsequently transformed into humoral cells of adenohypophysis. This conclusion is drawn not only from the same topology of both these structures, but also from the similarities of a set of specific genetical markers expressed in these, that makes it possible to suppose the existence of deep connections and succession between them. After the main body plan formation the PCM is replaced by adenohypophysis. It is supposed that spreading of differentiating signals from the PCM occurs along the main body axis in both caudal and rostral directions. The PCM seems to play an exclusive role in the formation of a head in vertebrate, because some mutations in genes expressed in the PCM result in the entire head deletion. during gastrulation Spemann's organizer transfers from the lip of blastopore to the prechordal zone. It is supposed that the early dorsal lip and the prechordal mesoderm (PCM) are one and the same cell population, i.e. When the dorsal-ventral orientation of the embryo is established and the organizer is switched on the very early invaginating cells of the dorsal blastopore lip (in the case of amphibia) move in advance of the entire invaginating mesoderm and by the end of gastrulation occupy the place just in front of the notochord. These population, determined to become the head of the trunk organizers still at the blastula stage, may be located either in the single continuous cell layer (as in amphibia and birds) or separated among different tissue germs (as in mammals). Analysis of data on inductive abilities of the organizer cells, on the use of markers, and on the observation of expression of specific genes allowed to conclude that Spemann's organizer in amphibia and its homologues in other vertebrates too are heterogeneous: they are composed of distinct cell populations able to induce primarity the development of either the head or trunk parts of the embryo. Using these data, I have attempted to trace the fate of Spemann's organizer after the early gastrula stage. Within the last 15 years, genetic and molecular techniques have been vastly improved, to help in tracing the fate of many cell lineages, and in compiling more exactly the fate maps for different parts of the embryo.
However, no real advance was achieved in their understanding. Since then, for as long as 75 years, attempts have been made to establish the intimate mechanisms of the organizer activity. The dorsal lip was termed "the organizer". Mangold discovered that a piece of the dorsal lip of a blastopore from Triturus cristatus, after transplantation to the ventral side of another embryo, was able to cause the neighbouring tissues to change their fate and participate in the formation of a new embryo. A role for the dorsal lip of the blastopore as the organizer is discussed in relation to the origin of the notochord.In 1924 H. The meaning of DORSAL LIP is the margin of the fold of blastula wall that delineates the dorsal limit of the blastopore, constitutes the primary organizer. These observations suggest organizing or guiding roles for the notochord in the formation of germ layers. From a comparison of the relative locations of tissues in embryos at different stages of development, it was shown that the notochord elongates by a remodeling of the mass of the primitive notochord, and that, as the anteriorly directed translocation of the neural area and the invagination of endoderm occur, these processes keep pace with the elongation of the notochord. Mesodermal cells other than notochord cells were mesenchymal until the neurula stage, when primitive somites appeared on both sides of the notochord. Based on grafting experiments, Mangold and Spemann showed the dorsal blastopore lip of an amphibian gastrula to be able to induce a secondary body axis 1. The primitive notochord was also linked to endoderm at its right and left margins, facing the archenteron. Only after the formation of the yolk plug, a narrow strip of primitive notochord, which consisted of columnar cells, established a close contact with the central part of the overlaying presumptive neural plate. In gastrulae, the inner postinvolution layer was not in direct contact with the outer preinvolution layer as a result of the presence of an intervening layer of cells. Three-dimensional relationships between tissues during the formation of germ layers were studied in sections of normally developing embryos of the newt, Cynops pyrrhogaster.
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