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me ascidkan tadpole larva is regard&d as aprototype of tbe ancestral cbordhte. Here we considkr recent stud&s on tbe devehpment of tbe tadpole hawa tbatprovidk new insights into cbordhte origins a& evokrtion a*be notocbord of ascidia~ larvae and vertebrates appear to be bomohgous structures based on their induction by endodhrm and expression of tbe Bracbyury (13 gene. Ebe muscle cells of ascidian larvae also appear bomobgous to those of vertebrates based on their expression of bIELH myogenic and muscle-type actin genes, altbougb they are spec@ed by cytopksmk determinants localked in tbe egg as well as embryonic itulucth Studies of tbe taiks larvae of anural ascidhas bave resutied in tbe idknt@ation of Manx, agene tbat may control tail deve@ment and evo&iot~ These and other resu&s * support tbe ascidian tadpole prototype fw tbe ancestral cbordhte. involute over the anterior lip of the blastopore during gastrulation and subsequently undergo differentiation, a change in cell shape and morphogenetic movements . leading to the extension of the tail. As the notochord also arises by induction in vertebratesl*, the mecha- nisms of notochord development may be conserved among the chordates. The development of the central nervous system and sensory organs is also achieved by in- duction in ascidian embryoSJ6. Understanding the molecular basis of notochord development in vertebrates has been advanced by cloning the mouse BEK~~UPJI (T) genen. Homologs of the mouse T gene have been isolated in X~opus and zebrafish. In these vertebrates, the T gene is expressed in the developing notochord and other mesodermal derivatives. Mutations in the mouse T gene lead to de- fects in the primitive streak, lack df notochord and pos- terior mesoderm differentiation, and failure of tail elon- gation. We have identified cDNA clones encoding an ascidian homolog (As-T) of the vertebrate T genel*J9. The N-terminal of the As-T protein has 78% an-r&o acid identity to the mouse T protein. The As-Ttranscripts are frost detected at the 64-cell stage, peak during the gas- trula stage, and decrease during the neurula and tailbud stages. The initiation of the expression of the As-Tgene is correlated with fate restriction vh the notochord lin- eage1*J9. As shown in Fig. 3b, the 32-cell embryo con- tains three pairs of presumptive notochord cells, ~6.2, A6.4 and B6.2. At this stage, these cells are also fated to produce spinal cord, muscle and mesenchyme cells. The As-Tgene is not expressed in the 32-cell embryo (Fig. 3f). The 64-ceil embryo also has three pairs of notochord-hqeage cells, A7.3, A7.7 and B7.3 (Fig. 3~). At this stage, the A7.3 and A7.7 pairs are destined to