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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

(5) 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

(10) 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

(15) 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

(20) 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 o show that,

in a sense, cell determination begins even before an egg

(25) is fertilized.

Studying sea urchins, biologist Paul Gross found

that an unfertilized egg contains substances that func-

tion as morphogenetic determinants. They are located

in the cytoplasm of the egg cell; i.e., in that part of the

(30) 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 fertilized,

the substances become active and, presumably, govern

(35) substances are unevenly 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

(40) 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

(45) histones move into the cell nucleus, where section of

DNA wrap around them to form a structure that resem-

bles 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

(50) DNA strings that guides the fate of the cells in which

they are located.

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

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

The second and third paragraphs of the passage indicate that morphogenetic determinants are substances in the embryo that are activated after the egg has been fertilized and that “tell a cell what to become” (lines 21-23). If, as the author asserts in the first paragraph, biologists have succeeded in dividing an embryo into two parts, each of which survives and develops into a normal embryo, it can be concluded that the quantity of morphogenetic determinants in the early embryo is greater than that required for the development of a single individual.

这道题挺好，正说明了ETS的细致之处。不过现在没时间，我有空一定回复，对不起古镯了，现在帮着顶一下吧。

2、The second and third paragraphs of the passage indicate that morphogenetic determinants are substances in the embryo that are activated after the egg has been fertilized and that “tell a cell what to become” (lines 21-23).

这句是说，先定位morphogenetic determinants，第二段，说他们一开始无活性，受精后，有活性了。从第一段知，有活性之后，形成的胚胎仍可以进行分割。也就是说受精和形成其最后定型的时间是不同的。所以选项C，inactive until the embryo cells become irreversibly committed to their final function这个有一定迷惑性的选项不对，因为时间点有误。

3、If, as the author asserts in the first paragraph, biologists have succeeded in dividing an embryo into two parts, each of which survives and develops into a normal embryo, it can be concluded that the quantity of morphogenetic determinants in the early embryo is greater than that required for the development of a single individual.

这是说，我们可以从生物学家分embryo的试验得知，morphogenetic determinants可用于多于一个的个体成长。这是一个简单的递进推理。

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

胚胎细胞发挥其作用，就是这些morphogenetic determinants活跃并发挥作用的结果，见原文第三、四段。好像还是我在上边对第2句解释的分析更容易明白些。

文中无。我补充：至少一个是激活，一个是未激活的。可能有此一点就够了，不能说他们完全一样。

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

这道题我也错了, 错在没有理解“tell a cell what to become” 就是 final function, 不知道我的理解对不对?

既然已经是early embryo, 当然已经是受精过得, 所以morphogenetic determinants有活性, 再去做final function----tell a cell what to become.

^_^，我觉得“这种特定embryo的个别性”确实存在，但我有点不同意见，我认为：问题中 the morphogenetic determinants present in the early embryo 是不受这种个别性所影响的，E选项所述内容是完全准确地正确的。

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.
这句说，无脊椎动物胚胎可以分成两个正常胚胎。

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.
这使他们相信胚胎中的每个细胞都有潜力以多种方式生长。

Later biologists found that the situation was not so simple. It matters in which plane the embryo is cut.
是否可以分成两个正常胚胎在于分割面。

If it is cut in a plane different from the one used by the early investigators, it will not form two whole embryos.
如果不以原先分割面进行切割，则不能形成两个完整的胚胎。

从上述来看，科学家们质疑第一句内容的焦点在于切割面，即是否能分成两个正常胚胎与分割面有关。而如果排除这种切割面的因素，象原来那样切割得当，就不能否定第一句中的命题了。当然，我同意楼上的观点，这是一个程度上的问题，而不是一个是与非的问题。E是最好的选项。

30. According to the passage, the morphogenetic determinants present in the unfertilized egg cell are which of the following?

由上可知，morphogenetic determinants存在于cytoplasm之中，而不是cytoplasm(细胞质)

Lines 26-28 in from us that in his study of sea urchins, Gross “found that an unfertilized egg contains substances that function as morphogetic determinants.”
只说明了sea urchins的特殊情况。

Lines 39-40 assert that the “substances that Gross studied are maternal messenger RNA’s,” and in lines 41-42 we learn that these maternal messenger RNA’s can be found in “ a wide variety of organisms.”
这里将这种物质扩大到多种生命有机体中。也就是递进了一下。更加对应问题的题目。

第二段最后一句，They have been able to show that, in a sense, cell determination begins even before an egg is fertilized.

这里的cell determination，我理解是指embryo cells become irreversibly committed to their fates。（不知道这个理解对不对？？）根据原文最后一句it is the structure of these beaded DNA strings that guide the fate of the cells in which they are located以及之前的描述，我认为embryo cells become irreversibly committed to their fates after an egg is fertilized，可是上面那句话说是before。本来我觉得这里可能原文写错了，可是后来发现它写的是cell determination begins even before ... ，多了个begins，把我弄糊涂了，按照我的理解，cell determination的最终完结是在the structure of these beaded DNA形成的时候，但是是什么时候开始的呢？？这个好像原文其他地方没有提到，特来请教大家，不然这句话看不明白

由上可知，morphogenetic determinants存在于cytoplasm之中，而不是cytoplasm(细胞质)??

以下是引用wangyu73cn在2004-11-10 23:08:00的发言：

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

文中无。我补充：至少一个是激活，一个是未激活的。可能有此一点就够了，不能说他们完全一样。

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

"这句是说，先定位morphogenetic determinants，第二段，说他们一开始无活性，受精后，有活性了。从第一段知，有活性之后，形成的胚胎仍可以进行分割。也就是说受精和形成其最后定型的时间是不同的。所以选项C，inactive until the embryo cells become irreversibly committed to their final function这个有一定迷惑性的选项不对，因为时间点有误。"

请问您这句话是不是说：胚胎（细胞受精后有了活性后形成的）仍然可以分割，并形成新的不同的胚胎，所以受精和定型的时间不同？

但是，我理解是这样：文中最后一段话“ And it is the structure of these beaded DNA strings that guides the fate of the cells in which they are located. ”指出了beaded DNA才是guide the fate，与选项中的“become irreversibly committed to their final function”说法一致，也就是说，我理解整个过程应该是"细胞受精-->活化-->RNA'S（morphogenetic determinants）指导h的合成-->h移至细胞核，绑定的DNA形成绳状-->决定其向最终命运发展" 因此，在活化和决定最终命运之间是有一个时间差的。

“文中无。我补充：至少一个是激活，一个是未激活的。可能有此一点就够了，不能说他们完全一样。”

我不太理解为什么说“一个是激活，一个是未激活？” early embryo是一定指受精后的egg cell吗？为什么早期的胚胎不能指未受精的卵细胞？

第二段开头则说：不知道决定胚胎命运的物质是什么？？（这跟如何切割有什么关系？？？？）

文章后面又说：MD在受精前，不active; 在受精后 active了，分裂后就会表现出不同的基因特质。（可是再也没提切割面的问题！！！！）

这么说later biologist说的话就是废话，在此处完全没有用，应该忽略不计？？？？

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