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A recently described experimental system for analyzing the mode of action of a morphogen gradient involves the in situ hybridization of sectioned tissue constructs. In these constructs, a source of activin signaling induces the transcription of several mesodermal genes in blastula animal caps, according to the position of cells in a concentration gradient. New experiments show that activin-loaded beads emit a signal for only 2 hr and that the same cell can be induced to express different genes. We determine the position in the gradient and the time after the start of activin signaling at which early genes, including Mix1, Xpo, Xwnt8, Xchd, and Xlim1, are activated, relative to the previously tested genes Xbra and Xgsc.
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