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An Introduction to Zoology:  Chapter 6
An Introduction to Zoology: Chapter 6
Published by The Lone Ranger
Default Mammalian Development

Mammalian Development:
The placental mammals (Phylum Chordata, Class Mammalia, Subclass Theria, Infraclass Eutheria) or eutherians, embryos are retained within the motherís body until they reach a relatively advanced stage of development. In many eutherians, newborns can walk and run within minutes of birth. This is in contrast to the monotremes, which lay eggs, and the marsupials, which give birth to underdeveloped young that then crawl into a pouch (marsupium) on the motherís body, where they continue their development.

Eutherians can give birth to such highly-developed young because the developing embryo/fetus is nourished by a placenta. The placenta develops from both embryonic and maternal tissues, and is connected to the developing embryo/fetus by the umbilical cord.

The umbilical cord develops from the yolk sac and the allantois and contains blood vessels that transport fetal blood to and from the placenta. In the placenta, blood vessels from the mother and fetus come into close proximity, but there is normally no mixing of fetal and maternal blood. In the placenta, metabolic wastes diffuse from the blood of the fetus into the motherís blood and are transported away. Meanwhile, oxygen and nutrients diffuse from the motherís blood into the blood of the fetus. In this way, the fetus is completely supported by its mother until it is sufficiently developed that it can survive outside her body.

The placenta, which allows eutherian mammals to give birth to highly-developed young, is one of the major reasons that eutherians can be found in such a wide variety of habitats. Eutherian mammals can be found on all continents and in the oceans. Eutherian mammals give birth in deserts, on Arctic ice floes where temperatures are well below freezing, and even under water.

In eutherian mammals, the placenta lines the uterus of a pregnant
female and is connected to the developing fetus by an umbilical cord.
The placenta allows the motherís blood to supply oxygen and nutrients
to the fetusí blood and also removes metabolic wastes from the fetusí blood.


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By monruw on 03-30-2011, 01:55 PM
Default Re: An Introduction to Zoology: Chapter 6

why it's called animal-vegetal axis? sound like kinda food, meat and vegetable or what else~ any story behind this?
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By The Lone Ranger on 05-22-2011, 01:35 AM
Default Re: An Introduction to Zoology: Chapter 6

"Vegetal," is related to "vegetable." Many plants can reproduce asexually, whereas virtually all animals reproduce sexually.

Probably for this reason, "vegetal" came to refer to processes in living things that are "plant-like," especially processes that do not occur through sexual reproduction. More to the point, perhaps, plants generally grow much more slowly than do animals. So the "vegetal" pole of an egg gets its name for the fact that the cells in this region grow and divide much more slowly than do the cells in the "animal" pole.


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