According to the sources, oogenesis is a staggered process. The first meiotic division is completed within the tertiary follicle just prior to ovulation . The resulting secondary oocyte is released from the Graafian follicle during ovulation . The second meiotic division is only triggered when a sperm penetrates the ovum's plasma membrane . This division is also unequal, resulting in a second polar body and a haploid ovum (ootid), which then proceeds to fuse with the sperm nucleus to form a zygote .