动物科学专业英语文章..docVIP

Cells Through about three billions (3,000,000,000) years, scientists says, the developments of plants and animals on our earth have been going on. At first both were very small, much less than a pin point in size. We might say that the first plants or animals were too small for man’s eye to see. Such cells take food in（吸进） and let waste out （排出）through very small openings in their walls. They do not give birth to(生育) new cells，but, strange to say, divide to become two daughter cells. Life began, men think, in the salty sea. Near the land, where the waters were not deep, very small one-celled organisms came into being(形成、产生). The cells in our bodies all have water in them, though they are of different sorts. This water has in it about the same amount of salt, common table salt（食盐），as that in the water of sea in which the earliest organisms swam. That water was little less salty than sea water is today. In billions of years, the rivers have washed down more and more salt from the land into the sea. The sea has become more salty. The water in our bodies does not become more salty. Even if we eat a great amount of salt in our food，the body automatically controls how much it will keep in the blood stream. And the same is true if we let much salt out through the skin when we get overheated. Then we have to eat something that will give us new supplies of salt. The first living things, though they were too small to be seen, were eating, growing organism which divided to become like them. Under a microscope, a cell looks like a bit of clear jelly with a thin wall round it. In every cell there is a part like a little ball. This is the nucleus, which organizes the work of the cell. Though cells were discovered two and a half centuries ago, it is only in the last hundred years that knowledge of the work of the nucleus has developed. It is this nucleus which keeps the cell working at what has to be done. In short, all living things are developments from these one-cell o