developmental stage Medaka stage 2a OlatDv:0000040 Zoological Science 11: 825-839 (1994) - 30 min Stage 2a (30 min) Blastodisc stage a: The male and the female pronuclei migrate toward and associate with each other at the center of the thick cytoplasmic disc at the animal pole. Chromosomes then appear and divide into two groups at the poles of the spindle marking the end of this stage. Oscillatory contractions cause the peripheral cortical cytoplasm to migrate toward the animal pole where it forms a convex, lens-shaped blastodisc. Meanwhile, oil droplets migrate toward the vegetal pole and begin coalescing. - 30 min blastodisc stage a medaka_ontology