Starfish: Echinoderms having bodies of usually five radially disposed arms coalescing at the center.Asterina: A genus of STARFISH in the family Asterinidae. They externally hold developing embryos (EMBRYO, NON-MAMMALIAN) among the spines below the oral surface.Asterias: A genus of STARFISH in the family Asteriidae. One species, Asterias rubens, is the most common in the north-east Atlantic region.Oocytes: Female germ cells derived from OOGONIA and termed OOCYTES when they enter MEIOSIS. The primary oocytes begin meiosis but are arrested at the diplotene state until OVULATION at PUBERTY to give rise to haploid secondary oocytes or ova (OVUM).Ovum: A mature haploid female germ cell extruded from the OVARY at OVULATION.Echinodermata: A phylum of the most familiar marine invertebrates. Its class Stelleroidea contains two subclasses, the Asteroidea (the STARFISH or sea stars) and the Ophiuroidea (the brittle stars, also called basket stars and serpent stars). There are 1500 described species of STARFISH found throughout the world. The second class, Echinoidea, contains about 950 species of SEA URCHINS, heart urchins, and sand dollars. A third class, Holothuroidea, comprises about 900 echinoderms known as SEA CUCUMBERS. Echinoderms are used extensively in biological research. (From Barnes, Invertebrate Zoology, 5th ed, pp773-826)Fertilization: The fusion of a spermatozoon (SPERMATOZOA) with an OVUM thus resulting in the formation of a ZYGOTE.Adenine: A purine base and a fundamental unit of ADENINE NUCLEOTIDES.Invertebrate Hormones: Hormones produced by invertebrates, usually insects, mollusks, annelids, and helminths.Maturation-Promoting Factor: Protein kinase that drives both the mitotic and meiotic cycles in all eukaryotic organisms. In meiosis it induces immature oocytes to undergo meiotic maturation. In mitosis it has a role in the G2/M phase transition. Once activated by CYCLINS; MPF directly phosphorylates some of the proteins involved in nuclear envelope breakdown, chromosome condensation, spindle assembly, and the degradation of cyclins. The catalytic subunit of MPF is PROTEIN P34CDC2.