14. Developmental Mechanisms for Chordate Evolution
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p
Fig. 1. Spatial expressioI) of deuterostome Brachyury genes. (A, B) The amphioxus
Am(Bb)Bral is expressed in the paraxial mesoderm (ms) and notochord (nc) of the neurula
(A), and in notochord (nc) and tail bud in the somitic-stage embryo (B). a, anterior; p,
posterior of the embryo; so, somite. (C) The ascidian As-T is expressed in the primordial
notochord cells of the llO-ceil embryo. bp, blastopore. The primordial notochord cells of the
11O-cell stage are diagrammatically shown in (C'). (D, E) The acorn worm PfBra expression in the archenteron invaginating region (ar) and stomodeum invaginating region (sci) of
the gastrula (D) and tomaria larva (E). pc, protocoel. (F, G) The sea urchin HpTa expression in cells of the vegetal plate (vp) in the blastula (F) and secondary mesenchyme cells
(smc) of the gastrula (G). ar, archenteron