Characteristics of the morphological changes in the nuclear structures in oogenesis in holothurians.

Chromosomes and nucleoli were studied at the meiosis diplotene stage in oocytes of Stichopus japonicus and Cucumaria fraudatrix. The S. japonicus nuclear structures were found to change in the same way as in other holothurioidea (chromosomes turn into lampbrushes, there is a single rapidly increasing nucleolus). The C. fraudatrix oocytes are notable for their lampbrushes located in immediate vicinity of the nucleus membrane. The same modes of behavior of nuclear structures are known for three more types of Cucumaria (no more species have been studied). The specific features of behavior of the nuclear structures are discussed with regard to phylogeny of holothurioidea oogenesis.