Passage 14
　　Nearly a century ago， biologists found that if theyseparated an invertebrate¹ animal embryo² into two partsat an early stage of its life， it would survive and developas two normal embryos³. This led them to believe that the（5）　cells in the early embryo are undetermined in the sensethat each cell has the potential to develop in a variety ofdifferent ways. Later biologists found that the situationwas not so simple. It matters in which plane the embryois cut. If it is cut in a plane different from the one used（10） by the early investigators⁵， it will not form two wholeembryos.
　　A debate arose over what exactly was happening.Which embryo cells are determined⁴， just when do they-become irreversibly committed to their fates， and what（15） are the “morphogenetic determinants” that tell a cellwhat to become？ But the debate could not be resolvedbecause no one was able to ask the crucial questionsin a form in which they could be pursued productively.Recent discoveries in molecular⁷ biology， however， have（20）　opened up prospects⁸ for a resolution of the debate.Now investigators think they know at least some of themolecules that act as morphogenetic determinants inearly development. They have been able o show that，　 in a sense， cell determination begins even before an egg（25） is fertilized⁹.　
　　Studying sea urchins¹⁰， biologist Paul Gross foundthat an unfertilized egg contains substances that func-tion as morphogenetic determinants. They are locatedin the cytoplasm of the egg cell； i.e.， in that part of the（30） cell‘s protoplasm that lies outside of the nucleus¹¹. In theunfertilized egg， the substances are inactive and are notdistributed homogeneously. When the egg is fertilized，the substances become active and， presumably， governthe behavior of the genes¹² they interact with. Since the（35） substances are unevenly¹³ distributed in the egg， when thefertilized egg divides， the resulting cells are differentfrom the start and so can be qualitatively¹⁴ different intheir own gene⁶ activity.
　　The substances that Gross studied are maternal¹⁵（40） messenger RNA‘s ——products of certain of the maternalgenes. He and other biologists studying a wide varietyof organisms have found that these particular RNA’sdirect， in large part， the synthesis of histones， a classof proteins that bind¹⁶ to DNA¹⁷. Once synthesized， the（45） histones move into the cell nucleus， where section ofDNA wrap around them to form a structure that resem-bles beads¹⁸， or knots， on a string. The beads are DNAsegments wrapped around the histones； the string is theintervening DNA. And it is the structure of these beaded（50） DNA strings¹⁹ that guides the fate of the cells in which　 they are located.

　　1. The passage is most probably directed at which kind of　audience？
　　　（A） State legislators deciding about funding levels for a  state-funded biological laboratory
　　　（B） Scientists specializing in molecular genetics
　　　（C） Readers of an alumni newsletter published by the college that Paul Gross attended
　　　（D） Marine²⁰ biologists studying the processes that give rise to new species
　　　（E） Undergraduate biology majors in a molecular biology course

　　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

　　3. The main topic of the passage is
　　　（A） the early development of embryos of lower marine organisms
　　　（B） the main contribution of modern embryology to molecular biology
　　　（C） the role of molecular biology in disproving older  theories of embryonic²¹ development
　　　（D） cell determination as an issue in the study of embryonic development
　　　（E） scientific dogma as a factor in the recent debate over the value of molecular biology

　　4. 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 replicate²² 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.e fate of the two parts was concerned.

　　5. It can be inferred from the passage that the initial　production of histones after an egg is fertilized takes　place
　　　（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

　　6. It can be inferred from the passage that which of the　following is dependent on the fertilization of an egg？
　　　（A） Copying of maternal genes to produce maternal messenger RNA‘s
　　　（B） Sythesis of proteins called histones
　　　（C） Division of a cell into its nucleus and the cytoplasm
　　　（D） Determination of the egg cell‘s potential for division
　　　（E） Generation of all of a cell‘s morphogenetic determinants

　　7. According to the passage， the morphogenetic　determinants present in the unfertilized egg cell are　which of the following？
　　　（A） Proteins bound to the nucleus
　　　（B） Histones
　　　（C） Maternal messenger RNA‘s
　　　（D） Cytoplasm
　　　（E） Nonbeaded intervening DNA

　　8. The passage suggests that which of the following plays a　role in determining whether an embryo separated into　two parts will two parts will develop as two normal　embryos？
　　Ⅰ.The stage in the embryo‘s life at which the separation occurs
　　Ⅱ. The instrument with which the separations is accomplished²³
　　Ⅲ. The plane in which the cut is made that separates the embryo
　　　（A） Ⅰonly
　　　（B） Ⅱ only
　　　（C） Ⅰ and Ⅱ。only
　　　（D） Ⅰ and Ⅲ。only
　　　（E） Ⅰ，Ⅱ， and Ⅲ

　　9. Which of the following circumstances is most　comparable to the impasse²⁴ biologists encountered in　trying to resolve the debate about cell determination　（lines 12-18）？
　　　（A） The problems faced by a literary scholar who wishes to use original source materials that are written in an unfamiliar foreign language
　　　（B） The situation of a mathematician who in preparing a proof of a theorem for publication detects a reasoning error in the proof
　　　（C） The difficulties of a space engineer who has to design equipment to function in an environment in which it cannot first be tested
　　　（D） The predicament of a linguist trying to develop a　theory of language acquisition when knowledge of the structure of language itself is rudimentary at best
　　　（E） The dilemma confronting a foundation when the funds available to it are sufficient to support one of two equally deserving scientific projects but not both