Fig. 1. Asymmetric divisions directed by the centrosome attracting body (CAB) in the posterior of the ascidian embryo. (A,B) In the 8-cell stage embryo, the two posterior vegetal blastomeres (B4.1 cells) contain both the CAB (cER-mRNA-rich domain in red) and the myoplasm (mitochondria-rich domain in green). (A) B4.1 divides asymmetrically to form a large (B5.1) and a small (B5.2) daughter cell which inherits the CAB. The small daughter cell divides asymmetrically again, generating B6.4 (large cell) and B6.3, the small cell, which contains the CAB and undergoes a final asymmetric division to yield a very small cell (B7.6) which ceases to divide. Blue bars indicate the daughters formed by each CAB-directed asymmetric division. (C) Final position of myoplasm and cER-mRNA domains in the tadpole larva. The myoplasm-containing cells give rise to tail muscle. The B7.6 cells containing the cER-mRNA domain form part of the endodermal strand and are thought to give rise to the germline after metamorphosis (Takamura et al., 2002; Tomioka et al., 2002).