HEAD ECTODERMAL PATTERNING AND AXIAL DEVELOPMENT IN FROGS

Two transient glands, the hatching and cement glands, define critical boundaries on the head of the frog embryo. They can be used to monitor formation of the head, which in turn is a sensitive indicator of deve lopment of the dorsal axis, characteristic of chordates. Experimental treatment of embryos generates a variety of head abnormalities. Altera tion of inductive patterns can produce large heads (macrocephaly), and comparable alterations may yield new phenotyeps naturally. Several pa ths lead to decreased head development, and one of these may mimic in reverse the path which lead to the evolution of the vertebrate head.