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

The notion of preformation was strongly advocated by most seventeenth- and eighteenth-century naturalist-philosophers. Some, however, thought it more likely that development began not with an already-formed individual, but with undifferentiated material that gradually developed into an embryo. This theory is known as the theory of epigenesis.

Preformationist notions have a number of obvious problems, which is why no less a thinker than Leonardo da Vinci had argued that the “seed of the female was as potent as that of [the] male in generation” back in the 15th century. As da Vinci hinted, preformationist notions fail to provide an adequate explanation for the obvious fact that both parents contribute more or less equally to a child’s makeup. Another problem with preformationist notions is that they lead to an “infinite regress” problem. If each sperm (or ovum) contains a tiny, fully-formed individual, then its gonads must contain sperm (or ova) with even tinier, fully-formed individuals inside. And inside those individuals, there must be even tinier fully-formed individuals. And so on, and so on. Pretty soon, the whole notion begins to sound awfully silly.

As microscopes became better, it became increasingly clear that neither sperm nor ova contained tiny, fully-formed individuals. In 1759, the German embryologist Kaspar Friedrich Wolff demonstrated that the earliest developmental stages in a chicken involved no tiny, pre-formed chickens. Instead, as he showed, an undifferentiated zygote divided, producing cells that organized themselves into layers. These layers of cells thickened in some areas while thinning in others, then began to organize themselves into distinct segments. These cell layers folded in on themselves and continued to segment until a recognizable embryo was formed. Thus, Wolff conclusively demonstrated that animal development occurs through epigenesis, and not through preformation.

According to modern epigenetic theory, development involves two important and closely-related processes, differentiation and morphogenesis. Differentiation is the process by which cells form specific tissues and structures as the embryo develops. Morphogenesis is the process by which cells differentiate (become different), through changes in size and shape, and also the movement of differentiated cells to different parts of the developing embryo as the cells organize themselves into tissues and then organs.


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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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