问杨继63篇的第14篇（就是关于胚胎细胞分离的那篇）中的几个题

原文（我把我阅读时，读结构的关键词和生词标出来了）

Passage 14 (14/63)

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[生]细胞质 of the egg cell; i.e., in that part of the cell’s protoplasm[生]原生质
            that lies outside of the nucleus. In the unfertilized egg, the substances are inactive and are not distributed homogeneously(Of the same or similar nature or kind).
                When the egg is fertilized, the substances become active and, presumably, govern the behavior of the genes they interact with. Since the substances are unevenly(参差不齐的; 不规则的) distributed in the egg, when the fertilized egg divides, the resulting cells are different from the start and so can be qualitatively different in their own gene activity.

The substances that Gross studied are maternal messenger
                    RNA’s—products of certain of the maternal genes. He and other biologists studying a wide variety of organisms have found that these particular RNA’s direct, in large part, the synthesis of histones, a class of proteins that bind to DNA. Once synthesized, the histones move into the cell nucleus, where section of DNA wrap around them to form a structure that resembles beads, or knots, on a string. The beads are DNA segments wrapped around the histones; the string is the intervening DNA. And it is the structure of these beaded DNA strings that guide the fate of the cells in which they are located.

2.    It can be inferred from the passage that the morphogenetic determinants present in the early embryo are

(A) located in the nucleus of the embryo cells

(B) evenly distributed unless the embryo is not developing normally

(C) inactive until the embryo cells become irreversibly committed to their final function

(D) identical to those that were already present in the unfertilized egg

(E) present in larger quantities than is necessary for the development of a single individual

答案选E

A.原文是outside the the nucleus. 所以错

B.原文是are not distributed homogeneously(Of the same or similar nature or kind). 和 the substances are unevenly(参差不齐的; 不规则的) distributed。所以错

C.inactive until the embryo cells 是第三段的叙述，而irreversibly committed to their final function是第二段的叙述，这种结构通常是错误的。但是原文"When the egg is fertilized, the substances become active and, presumably, govern the behavior of the genes they interact with"是不是可以理解为第二段叙述的irreversibly committed to their final function，然后选C呢？ 但是C不是正确答案，望NN帮忙解答。

D.identical 是个关于比较结果的词，原文没有叙述，所以错。

E.正确答案，不知道为什么？请NN帮忙！杨的解释是：本文一开始提到，在胚胎发育早期，切割胚胎为两个部分将产社会那个两个独立个体。若干哦词结论为非（一个胚胎中形态发射还能够决定体的数目只够它自身发育），此实验结果就不可能出现。 好像是逻辑推理，但是还是不太明白，请NN帮忙阿！

怎么没有人理我阿？

帮你顶一下！

关于.    According to the passage, when biologists believed that the cells in the early embryo were undetermined, they made which of the following mistakes?

(A) They did not attempt to replicate the original experiment of separating an embryo into two parts.

(B) They did not realize that there was a connection between the issue of cell determination and the outcome of the separation experiment.

(C) They assumed that the results of experiments on embryos did not depend on the particular animal species used for such experiments.

(D) They assumed that it was crucial to perform the separation experiment at an early stage in the embryo’s life.

(E) They assumed that different ways of separating an embryo into two parts would be equivalent as far as the fate of the two parts was concerned.

为什么选E

请看到文中：If it is cut in a plane different from the one used by the early investigators, it will not form two whole embryos

(A) in the cytoplasm

(B) in the maternal genes

(C) throughout the protoplasm

(D) in the beaded portions of the DNA strings

(E) in certain sections of the cell nucleus

为什么选A

请mm再读读我下面highlight的地方：-）

看第三段：biologist Paul Gross found that an unfertilized egg contains substances that function as morphogenetic determinants. They are located in the cytoplasm[生]细胞质 of the egg cell; i.e., in that part of the cell’s protoplasm[生]原生质
that lies outside of the nucleus. In the unfertilized egg, the substances are inactive and are not distributed homogeneously(Of the same or similar nature or kind). When the egg is fertilized, the substances become active and, presumably, govern the behavior of the genes they interact with.

然后第四段：

The substances that Gross studied are maternal messenger
      RNA’s—products of certain of the maternal genes. He and other biologists studying a wide variety of organisms have found that these particular RNA’s direct, in large part, the synthesis of histones, a class of proteins that bind to DNA. Once synthesized, the histones move into the cell nucleus,