2012年12月8日雅思阅读考题回顾
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考试日期： |
2012年12月8日 |
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Reading Passage 1 |
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Title: |
对于鸟类发展的研究 |
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Question types: |
Table Completion; True/False/Not Given |
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文章内容回顾 |
用一个正例和一个反例讲达尔文的进化论。较后结论归到要*护环境也要*护动物进化的历程不被人打扰。开头先讲了达尔文在一个岛上观察一种鸟被选择的过程。然后是两个科学家在自己生活的地方也观察到同样的情况，三种鸟，不同的喙可以咬开不同大小的seed, 分别是一场干旱一场大雨和另一场干旱影响了土地上种子的大小，进而影响三种鸟的数量。然后两位科学家就在一个岛上观察到动物的进化被人类影响的状况。在这个岛中部，没有人的影响，鸟类自由繁衍，别的地方因为有人的影响，本应该灭绝的鸟活得很好。 |
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英文原文阅读 |
The major structural characteristic of avian, reptilian, and mammalian gastrulation is the primitive streak. This streak is first visible as a thickening of the epiblast at the posterior region of the embryo, just anterior to Koller''s sickle. This thickening is caused by the ingression of endodermal precursors from the epiblast into the blastocoel and by the migration of cells from the lateral region of the posterior epiblast toward the center. As these cells enter the primitive streak, the streak elongates toward the future head region. At the same time, the secondary hypoblast cells continue to migrate anteriorly from the posterior margin of the blastoderm. The elongation of the primitive streak appears to be coextensive with the anterior migration of these secondary hypoblast cells. The streak eventually extends 60-75% of the length of the area pellucida. The primitive streak defines the axes of the embryo. It extends from posterior to anterior; migrating cells enter through its dorsal side and move to its ventral side; and it separates the left portion of the embryo from the right. Those elements close to the streak will be the medial (central) structures, while those farther from it will be the distal (lateral) structures. As cells converge to form the primitive streak, a depression forms within the streak. This depression is called the primitive groove, and it serves as an opening through which migrating cells pass into the blastocoel. Thus, the primitive groove is analogous to the amphibian blastopore. At the anterior end of the primitive streak is a regional thickening of cells called the primitive knot orHensen''s node. The center of this node contains a funnel-shaped depression (sometimes called the primitive pit) through which cells can pass into the blastocoel. Hensen''s node is the functional equivalent of the dorsal lip of the amphibian blastopore (i.e., the organizer) and the fish embryonic shield. As soon as the primitive streak has formed, epiblast cells begin to migrate through it and into the blastocoel). The primitive streak has a continually changing cell population. Cells migrating through Hensen''s node pass down into the blastocoel and migrate anteriorly, forming foregut, head mesoderm, and notochord; cells passing through the lateral portions of the primitive streak give rise to the majority of endodermal and mesodermal tissues. Unlike the Xenopus mesoderm, which migrates as sheets of cells into the blastocoel, cells entering the inside of the avian embryo ingress as individuals after undergoing an epithelial-to-mesenchymal transformation. At Hensen''s node and throughout the primitive streak, the breakdown of the basal lamina and the release of these cells into the embryo is thought to be accomplished by scatter factor, a 190-kDa protein secreted by the cells as they enter the strea. Scatter factor can convert epithelial sheets into mesenchymal cells in several ways, and it is probably involved both in downregulating E-cadherin expression and in preventing E-cadherin from functioning
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