ROLES OF THE POLAR CYTOPLASMIC REGION IN MEIOTIC DIVISIONS IN OOCYTES OF THE SEA-CUCUMBER, HOLOTHURIA-LEUCOSPILOTA

The sea cucumber oocyte has a marked cytoplasmic protrusion at its presumptive animal pole. The role of this cytoplasmic region (called the "pole" in this paper) in meiotic divisions was investigated. With maturation, the germinal vesicle (GV) migrates to the pole and breaks down. When the migration of the GV was impeded by compression, a pair of asters developed in the cytoplasmic region at the pole. A meiotic spindle formed when these two asters united with the nucleoplasmic area after breakdown of the GV. The origin of these asters was then examined by transecting oocytes microsurgically. Upon maturation, the fragments containing the pole, but lacking the GV, developed a pair of asters in the cytoplasmic region at the pole. Fragments containing both the pole and the GV formed a meiotic spindle. However, no asters formed in fragments lacking the pole (either containing or lacking the GV). The results demonstrate that the pair of asters are the organizing centers of the spindle, and that they are derived from the pole, indicating that the centrosome(s) resides in the pole of the oocyte.