Stages of oogenesis in Lytechinus varigetaus. (a) Oocytes attached to the somatic cells of the ovary. (B) previtellogenic primary oocytes. (C-E) Vitellogenic growth phase of the oogenesis. (E-I) Full grown oocytes complete meiosis with the large germinal vesicle moving asymmetrically to the cell periphery, where it breaks down and extrudes two polar bodies to produce a haploid mature egg that is now ready for fertilization. Scale bar = 50 microns. nc = nucleolus; gv = germinal vesicle; pbs = polar bodies; pn = pronucleus. Image courtesy of Gary Wessel, Brown Univ.

Oogenesis involves the progressive elaboration of the oocyte, and includes transport of yolk proteins into the oocyte, the localization of mRNAs, and the production and translocation of cortical granules, which lie immediately beneath the plasma membrane, and are crucial for events following fertilization.