In females, meiosis occurs in cells known as oocytes (singular&colon; oocyte). Primordial follicles are immature primary oocytes surrounded by a single layer of granulosa cells. Fertilization of two different oocytes by two different spermatozoa results in fraternal twins. This fluid contains nutrients for spermatozoa, oocytes, and zygotes. PGD is defined as the testing of pre-implantation stage embryos or oocytes for genetic defects. Morphology is a predictor of success in fertilizing oocytes during in vitro fertilization. A degenerative process called follicular atresia reabsorbs vitellogenic oocytes not spawned. For this reason, many centers will freeze all oocytes or embryos following agonist trigger. Both hormones were comparable in the number of mature oocytes produced and fertilization rates. These cells are connected by cytoplasmic bridges with oocytes. Estradiol appears necessary to maintain oocytes in the ovary. The zona pellucida first appears in unilaminar primary oocytes. These results indicate that BMP4 may aid in survival and prevention of apoptosis in oocytes. Ookinete forms oocytes from which sporozoites are formed. Oogonia are the cells that turn into primary oocytes in oogenesis.