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|A human ovum with corona radiata surrounding it|
Human and mammal ova
In the viviparous animals (which include humans and all other placental mammals), the ovum is fertilized inside the female body, and the embryo then develops inside the uterus, receiving nutrition directly from the mother.
The human ova grow from primitive germ cells that are embedded in the substance of the ovaries. Each of them divides repeatedly to give rise to several smaller cells, the oogonia. The oogonia then develop into the ova, the primary oocytes.
Protist and plant ova
In protists, fungi and many plants, such as bryophytes, ferns, and gymnosperms, ova are produced inside archegonia. Since the archegonium is a haploid structure, egg cells are produced via mitosis. The typical bryophyte archegonium consists of a long neck with a wider base containing the egg cell. Upon maturation, the neck opens to allow sperm cells to swim into the archegonium and fertilize the egg. The resulting zygote then gives rise to an embryo, which will grow out of the archegonium as a young sporophyte.
In the flowering plants, the female gametophyte, which usually gives rise to the archegonium, has been reduced to just eight cells referred to as the embryo sac inside the ovule. The gametophyte cell closest to the micropyle opening of the embryo sac develops into the egg cell. Upon pollination, a pollen tube delivers sperm into the embryo sac and one sperm nucleus fuses with the egg nucleus. The resulting zygote develops into an embryo inside the ovule. The ovule in turn develops into a seed and in many cases the plant ovary develops into a fruit to facilitate the dispersal of the seeds. Upon germination, the embryo grows into a seedling.
In the moss Physcomitrella patens, the Polycomb protein FIE is expressed in the unfertilised egg cell (Figure, right) as the blue colour after GUS staining reveals. Soon after fertilisation the FIE gene is inactivated (the blue colour is no longer visible, left) in the young embryo. 
The ooplasm consists of the cytoplasm of the ordinary animal cell with its spongioplasm and hyaloplasm, often called the formative yolk; and the nutritive yolk or deutoplasm, made of rounded granules of fatty and albuminoid substances imbedded in the cytoplasm.
Mammalian ova contain only a tiny amount of the nutritive yolk, for nourishing the embryo in the early stages of its development only. In contrast, bird eggs contain enough to supply the chick with nutriment throughout the whole period of incubation.
Ova development in oviparous animals
In the oviparous animals (all birds, most fishes, amphibians and reptiles) the ova develop protective layers and pass through the oviduct to the outside of the body. They are fertilized by male sperm either inside the female body (as in birds), or outside (as in many fishes). After fertilization, an embryo develops, nourished by nutrients contained in the egg. It then hatches from the egg, outside the mother's body. See egg (biology) for a discussion of eggs of oviparous animals.
There is an intermediate form, the ovoviviparous animals: the embryo develops within and is nourished by an egg as in the oviparous case, but then it hatches inside the mother's body shortly before birth, or just after the egg leaves the mother's body. Some fish, reptiles and many invertebrates use this technique.
- The Ovarian Kaleidoscope Database description of 1800 genes involved in ovarian functions
- ↑ The Ovum in Gray's Anatomy. URL accessed on 2010-02-16..
- ↑ Search result of "120 micrometers" in Level O Biology - Google books
- ↑ Assaf Mosquna, Aviva Katz, Eva L. Decker, Stefan A. Rensing, Ralf Reski, Nir Ohad (2009): Regulation of stem cell maintenance by the Polycomb protein FIE has been conserved during land plant evolution. Development 136, 2433-2444.
- REDIRECT Template:Doi
- ↑ 4.0 4.1 4.2 The Ovum. Gray's Anatomy. URL accessed on 2010-10-18.
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