ABSTRACT
Most changes during development originate from inductive signals transmitted between closely aligned cells. However, late in the development of vertebrate embryos, some changes in cell composition are induced by hormones that are signal molecules carried in the blood. It appeared that signal molecules released by mesoderm might be easily identified by these simple techniques, and, in the 1930s, an outburst of experiments followed throughout the world. Adjacent subgroups differ in having variations in two kinds of molecule on the cell surface. When increasing amounts of unheated tissue were mixed with heated tissue, there was a progressive shift toward induction of the types of cells found toward the rear of the embryo. It appeared that two kinds of inducing molecules are active: a heat-sensitive molecule, which induces spinal cord cells when acting alone, and another molecule stable to moderate heating, which induces forebrain cells.
