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|A human ovum with corona radiata surrounding it|
Human and mammal ovaEdit
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.
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 animalsEdit
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
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