In the phylum Chordata, a genome duplication event took place twice in the process of vertebrate evolution. This suggests that ascidian is a good model system to investigate functions of genomes of chodates, espacially when considering that functional analysis of genes is easily accessible by gene introduction to the ascidian eggs.
Fertilized eggs cleave many times to give rise to multicellular organisms.
Within embryos, embryonic blastomeres develop into various types of tissues such as epidermis, muscles and nervous systems. In the processes of early embryogenesis, maternal factors stored in the egg cytoplasms are known to play various significant roles. But the functions of the maternal factors in chordate eggs remain elusive compared with those of fruitfly and nematode. Since the last century, ascidian egg has been well known as a mosaic egg in which many blastomeres in the early embryo differentiate autonomously. Recent works have revealed that there exist cytoplasmic determinants that direct formation of epidermis, muscle and endoderm as well as cytolasmic factors involved in axis specification of the embryo and in gastrulation.