Mechanisms that regulate the number of cells that constitute the body have remained largely elusive. We approached this issue in the ascidian, Halocynthia roretzi, which develops into tadpole larva with small number of cells. Embryonic cells divide 11 times on average from fertilization to hatching. The number of cell division rounds varies between tissue types. For example, notochord cells divide 9 times and give rise to large postmitotic cells in the tadpole. The number of cell division rounds in the partial embryos that were derived from tissue-precursor blastomeres isolated at the 64-cell stage also varied between tissues, and coincided with their counterparts in the intact whole embryos to some extent, suggesting tissueautonomous regulation of cell division. Manipulation of cell fates in notochord, nerve cord, muscle, and mesenchyme lineage cells by inhibition or ectopic activation of the inductive FGF signal changed the number of cell division according to the altered fate. Knockdown and missexpression of Brachyury (Bra), an FGF-induced notochord-specific key transcription factor for notochord differentiation, indicated that Bra is responsible not only for notochord differentiation but also regulates the number of cell division rounds in the notochord lineage cells, suggesting that Bra activates a putative machinery to stop cell division at the specific stage. Results of precocious expression of Bra suggested that the machinery refers the developmental clock that is likely shared in other blastomeres than notochord, and functions to terminate cell division at three rounds after the 64-cell stage. Bra does nothing about the progression of developmental clock itself.