The ovary of teleosts is also often hollow, but in this case, the eggs are shed into the cavity, which opens into the oviduct .  Certain nematodes of the genus Philometra are parasitic in the ovary of marine fishes and can be spectacular, with females as long as 40 cm, coiled in the ovary of a fish half this length.  Although most normal female vertebrates have two ovaries, this is not the case in all species. In most birds and in platypuses , the right ovary never matures, so that only the left is functional. (Exceptions include the kiwi and some, but not all raptors , in which both ovaries persist.   ) In some elasmobranchs, only the right ovary develops fully. In the primitive jawless fish , and some teleosts, there is only one ovary, formed by the fusion of the paired organs in the embryo. 
While cycles may be irregular in the beginning, they will eventually become more regular with about 28 days between the first days of each period. Each month, approximately 10 to 12 egg follicles will begin to develop. One will continue on to produce a mature egg. The rest will be reabsorbed into the ovarian tissue. About 14 days into a woman's cycle, that mature egg will be released in a process known as ovulation. After ovulation occurs, the empty follicle is known as a corpus luteum. It will produce progesterone and other hormones crucial for pregnancy for about 14 days.
On another note, 2 studies conducted at the Mayo Clinic published in August 2007 (data for the 2 studies was derived from the Rochester Epidemiology Project, one of the largest long-term integrated databases of patient records in the world) showed that women who underwent oophorectomy before menopause had almost double the risk of developing dementia or parkinsonism. The younger the women at the time of surgery, the greater the risk. Another surprising but distressing finding of these studies is that even removal of one ovary seems to have the same adverse effect neurologically as removal of both ovaries. However, a reassuring finding in this study is that women who had their ovaries removed but received estrogen replacement therapy returned to normal risk. I think this is yet another piece of evidence supporting the benefit of estrogen therapy started at the time of surgical menopause.