The Spemann organizer is a group of cells in the developing embryo that plays a crucial role in organizing the body plan during early development. Discovered by German embryologist Hans Spemann in the early 20th century, this region is responsible for inducing the formation of various tissues and structures in the embryo, particularly in amphibians like frogs. The organizer is located in the dorsal lip of the blastopore, which is the opening that forms during gastrulation. It releases signaling molecules that guide surrounding cells to differentiate into specific types of tissues, helping to establish the overall layout of the organism. This process is essential for proper development and is a key concept in developmental biology.