Fig. 8. Illustrations showing different modes of axial patterning of the Drosophila and spider embryo. The Drosophila cellular blastoderm stage (A) and the spider germ disc stage (B) are compared. a, anterior; p, posterior; v, ventral; d, dorsal. In each illustration, the otd- or At.otd-expressing anterior region of the embryo is indicated by yellow, and the posterior end of the embryo by purple. The area shown in gray indicate yolk (B). (A) In the Drosophila embryo, dpp is expressed in blastoderm cells at the 40% most dorsal region (green). This expression initiates simultaneously along the AP axis. In the same cell population that dpp is expressed, phosphorylation of Mad is induced. (B) In the spider embryo, future anterior cells (yellow) are located at the rim, whereas future posterior cells (purple) that form the caudal lobe exist around the center of the germ disc. The CM cells, which express At.dpp (green), migrate from the center to the rim of the germ disc (black arrow), progressively inducing phosphorylation of Mad in epithelial cells (light green). The area that once expressed pMad is shaded in green (right panel). It develops into the dorsal tissues. The site diametrically opposite to the cumulus is defined as the anterior pole (a, right panel). Broken line indicates the ventral midline; blue arrows indicate the directions of cell movement during stage 6.

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