The role of oocyte maturation in the ontogeny of the fertilization site in the hydrozoan Hydractinia echinata.

In hydrozoans the sperm will fuse with the egg only at the site of polar body formation. The primary oocyte and maturing oocytes which have produced the first polar body cannot be fertilized even though maturing oocytes which have produced the first polar body attract sperm. These eggs do not acquire the ability to be fertilized until after second polar body formation. If either first or second polar body formation is inhibited or if first and second polar body formation do not take place in close proximity to each other, the fertilization site is not set up. Under normal circumstances the site of polar body formation takes place at the region on the maturing oocyte surface nearest the site where the germinal vesicle resided in the primary oocyte. When maturing oocytes are centrifuged prior to polar body formation, the site of polar body formation is frequently shifted so that it does not correspond to the site where it would be given off under normal circumstances. Under these conditions the shifted site of polar body formation is the only site where the egg can be fertilized, indicating that the fertilization site is selected during oocyte maturation.Oocyte maturation in these hydrozoans is mediated by a hormone released by the somatic cells of gonophores as a consequence of bringing dark adapted gonophores into the light. The hormone acts directly on the oocyte to induce maturation. The oocyte only has to be exposed to the hormone for the first few minutes of the maturation process in order to complete the process of maturation.