In the early part of the 20th century, and prior to the organizer experiment, work by Hans Spemann and his colleagues had suggested that much of the amphibian embryo was regulative, such that if a piece of tissue was grafted from a donor embryo to a different location in a new host embryo, then the graft would develop according to its new surroundings. The organizer graft is a famous and influential experiment because it showed that one part of the embryo is endowed with special signaling properties that dictate the patterning of the neurulating embryo. In Xenopus, for example, it is known that the dorsal mesoderm is an organizing center, and that from the late blastula stage onwards, the tissues of the tadpole are elaborated by cell-to-cell signaling. But once these asymmetries are established, induction - a process by which a cell or tissue directs the development of a neighboring tissue or cell - takes over. According to our current understanding of amphibian embryogenesis, the egg starts with localized determinants, and after some cytoplasmic rearrangement, these determinants dictate the identities of the dorsal and ventral mesoderm, as well as the ectoderm and endoderm. How do vertebrate embryos generate a dorsal neural plate, notochord and somites on one side, and ventral blood and gut on the other? Two extreme possibilities are that: (1) the egg is endowed with determinants for each tissue that segregate into different regions during cleavage (a so-called mosaic mode of development) or (2) asymmetries build up progressively, and that devoted signaling centers organize the rest of the embryo by cell-cell interactions.
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