IV.

Embryonic Induction

One of the outstanding achievements in embryology in the 20th century has been the elucidation of some of the reasons for morphogenesis, that is, the development of pattern and form, and for differentiation, that is, the development of a diversity of cell and tissue types. Observation and experiment, especially on amphibian embryos, have shown that a stimulus emanates from some of the material that invaginates during the process of gastrulation. Those cells that invaginate on what will be the future dorsal side of the embryo have the capacity to induce overlying cells to differentiate into the primary axial organs and associated structures, such as the nervous system, notochord, and muscle segments.

If the potentially inducing cells making up the so-called dorsal lip of the blastopore, which is the opening of the cavity of the gastrula, are prevented from invaginating, the embryo remains alive but will not undergo further differentiation. Conversely, grafting a second dorsal lip to the flank of an embryo induces the formation of a secondary embryo out of tissues that normally would have formed something altogether different. Studies show that various chemical substances can imitate in part the stimulus or stimuli that derive from the inducing embryonic tissue.