The "organizer effect" refers to a phenomenon in developmental biology where certain tissues influence the development of nearby tissues. This effect is often observed during the early stages of embryonic development, where specific cells or structures, known as organizers, release signaling molecules that guide the formation and differentiation of surrounding cells. One well-known example of the organizer effect is the Spemann organizer, discovered by Hans Spemann in the early 20th century. This region of the embryo plays a crucial role in establishing the body plan and organizing the development of various structures, such as the nervous system and limbs.