GMAT考试考前必练阅读模拟题4.docxVIP

Word文档下载后（可任意编辑） 第 第 PAGE 1 页 共 NUMPAGES 1 页 GMAT考试考前必练阅读模拟题4 阅读对于我们来说并不陌生，在我们从小到大的考试中都已经有很多经验了，阅读也是我们很容易提分的一个考察科目，所以今天带来了这篇阅读练习题，在GAMT考试还有很多相关的内容和GMAT考试时间，现在我们就先来看看这篇文章吧！ Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of its life, it would survive and develop as two normal embryos. This led them to believe that the cells in the early embryo are undetermined in the sense that each cell has the potential to develop in a variety of different ways. Later biologists found that the situation was not so simple. It matters in which plane the embryo is cut. If it is cut in a plane different from the one used by the early investigators, it will not form two whole embryos.  A debate arose over what exactly was happening. Which embryo cells are determined, just when do they become irreversibly committed to their fates, and what are the “morphogenetic determinants” that tell a cell what to become? But the debate could not be resolved because no one was able to ask the crucial questions in a form in which they could be pursued productively. Recent discoveries in molecular biology, however, have opened up prospects for a resolution of the debate. Now investigators think they know at least some of the molecules that act as morphogenetic determinants in early development. They have been able to show that, in a sense, cell determination begins even before an egg is fertilized.  Studying sea urchins (any of numerous echinoderms (class Echinoidea) that are usually enclosed in thin brittle globular tests covered with movable spines海胆), biologist Paul Gross found that an unfertilized egg contains substances that function as morphogenetic determinants. They are located in the cytoplasm of the egg cell; i.e., in that part of the cell’s protoplasm that lies outside of the nucleus. In the unfertilized egg, the substances are inactive and are not distributed homogeneously. When the egg is fert