Abstract
Synopsis. 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 development ofthe dorsal axis, characteristic of chordates. Experimental treatment of embryos generates a variety of head abnormalities. Alteration of inductive patterns can produce large heads (macrocephaly), and com? parable alterations may yield new phenotypes naturally. Several paths lead to decreased head development, and one of these may mimic in reverse the path which led to the evolution of the vertebrate head.
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