Abstract
The populations of A. mentula from the Gulf of Naples, the Plymouth Sound and the west coasts of Scotland possess the same number of chromosomes, n=9. Like other species of the family Ascidiidae, A. mentula is characterized by pale and sticky oocyte bivalents, haploid number n=9, metacentric and submetacentric chromosomes only and embryonic mitoses where an anatelophase stage can be described. In common with all ascidians, A. mentula shows somatic pairing of homologous chromosomes and precocious distancing of daughter kinetochores in embryonic chromosomes. The only karyological character which differentiates A. mentula from other ascidians is the banding pattern revealed in stretched prophase chromosomes of early embryos.
