Ascidians are marine invertebrate chordates. Despite their adult appearance, which resembles little of a chordate, the larval have a tadpole body form with a notochord and an overlying dorsal neural tube. This neural tube develops from the rolling up of a neural plate, just like in vertebrates. Some of advantages of using ascidians to study developmental biology include the high numbers of eggs produced. Embryos develop rapidly and with a small number of cells, such that embryos begin to gastrulate with only 110 cells and the swimming larvae contain only in the region of 2600 cells. In addition, embryos have a fixed cell cleavage pattern and the embryonic lineages of all cells up to the gastrula stage have been well described. For some tissues such as the nervous system, the cell lineage has been described during later stages of development as well. The knowledge of the cell lineage in ascidians means that at each developmental time point, a cell can be identified and we can know exactly what this cell will give rise to. Much of our knowledge of the cell lineage dates back to 1905 when Edwin Conklin published a series of papers that precisely described the inheritance of different coloured cytoplasms into different cell lineages. More recently, works by Hiroki Nishida and Ian Meinertzhagen's labs have described cell-lineages in more detail.