首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
The two claws of the mature American lobster are decidedly different from each other. The crusher claw is short and stout; the c
The two claws of the mature American lobster are decidedly different from each other. The crusher claw is short and stout; the c
admin
2014-03-02
52
问题
The two claws of the mature American lobster are decidedly different from each other. The crusher claw is short and stout; the cutter claw is long and slender. Such bilateral asymmetry, in which the fight side of the body is, in all other respects, a mirror image of the left side, is not unlike handedness in humans. But where the majority of humans are right-handed, in lobsters the crusher claw appears with equal probability on either the right side or left side of the body.
Bilateral asymmetry of the claws comes about gradually. In the juvenile fourth and fifth stages of development, the paired claws are symmetrical and cutter like. Asymmetry begins to appear in the juvenile sixth stage of development, and the paired claws further diverge toward well-defined cutter and crasher claws during succeeding stages. An intriguing aspect of this development was discovered by Victor Emmer. He found that if one of the paired claws is removed during the fourth or fifth stage, the intact claw invariably becomes a crusher, while the regenerated claw becomes a cutter. Removal of a claw during a later juvenile stage or during adulthood, when asymmetry is present, does not alter the asymmetry; the intact and regenerate claws retain their original structures.
These observations indicate that the conditions that trigger differentiation must operate in a random manner when the paired claws are intact, but in a nonrandom manner when one of claws is lost. One possible explanation is that differential use of the claws determines their asymmetry. Perhaps the claw that is used more becomes the crusher. This would explain why, when one of the claws is missing during the fourth or fifth stage, the intact claw always becomes a crusher. With two intact claws, initial use of one claw might prompt the animal to use it more than the other throughout the juvenile fourth and fifth stages, causing it to become a crusher.
To test this hypothesis, researchers raised lobsters in the juvenile fourth and fifth stages of development in a laboratory environment in which the lobsters could manipulate oyster chips. (Not coincidentally, at this stage of development lobsters typically change from a habitat where they drift passively, to the ocean floor where they have the opportunity to be more active by borrowing in the substrate.) Under these conditions, the lobsters developed asymmetric claws, haft with crusher claws on the left, and half with crusher claws on the right. In contrast, when juvenile lobsters were reared in a smooth tank without the oyster chips, the majority developed two cutter claws. This unusual configuration of symmetrical cutter claws did not change when the lobsters were subsequently placed in a manipulatable environment or when they lost and regenerated one or both claws.
The author regards the idea that differentiation is triggered randomly when paired claws remain intact as ______.
选项
A、irrefutable considering the authoritative nature of Emma’s observations
B、likely in view of present evidence
C、contradictory to conventional thinking on lobster-claw differentiation
D、purely speculative bemuse it is based on scattered research and experimentation
答案
B
解析
第三段的第一句话已经说明:这个观点是有证据支持的,但这些证据只能说明它的可能性,但不是无可反驳的。
转载请注明原文地址:https://kaotiyun.com/show/tDQO777K
0
考博英语
相关试题推荐
Oldpeoplearealwayssayingthattheyoungarenotwhattheywere.Thesamecommentismadefromgenerationtogenerationandi
Sleepisanimportantpartofourlives.Wespendaboutone-thirdofourlivessleeping.Formillionsofpeople,【51】,gettingen
InmostearthquakestheEarth’scrustcrackslikeporcelain.Stressbuildsupuntilafractureformsatthedepthofafewkilom
Thepreviouspassageistobecontinuedbythefollowingpassageaboutasbesto-relateddiseases,inwhichfourphraseshavebeen
GordonShawthephysicist,66,andcolleagueshavediscoveredwhat’sknownasthe"Mozarteffect,"theabilityofaMozartsonat
positiondefineforgetfulfancykeybalanceimportanceunhealthylacksubjectopenple
Thereisanincorrectassumptionamongscientistsandmedicalpeoplethateveryoneagrees______whatconstitutesabenefittoa
Inthepast,Americancollegesanduniversitieswerecreatedtoserveadualpurposetoadvancelearningandtoofferachancet
Advertisingispaid,nonpersonalcommunicationthatisdesignedtocommunicateinacreativemanner,throughtheuseofmassor
CampaigningontheIndianfrontierisanexperiencebyitself.Neitherthelandscapenorthepeoplefindtheircounterpartsina
随机试题
在重量分析中能使沉淀溶解度减小的因素是()。
属抗抑郁症药物的是属抗精神分裂症药物的是
属于神经氨酸酶抑制剂的抗流感痫毒药是
煤气站的水煤气和半水煤气的含氧量达到()时必须停炉。
下列有关建设工程材料设备采购合同特点的说法中,正确的有()。
美国当地时间2015年6月18日,由清华大学、美国华盛顿大学和微软公司合作创建的()在美国华盛顿州西雅图正式启动。这是中国高校第一次到美国办学,也标志着中国高校在美国设立的第一个实体校区和综合性教育科研平台正式建立。
下列关于“三农”问题表述有错误的一项是()。
考虑系统开发战略化策略的根本出发点是()。Ⅰ.是企业的重大建设项目Ⅱ.应是一项技术和社会性工程Ⅲ.应涉及到企业高层管理人员、业务技术管理人员、计算机技术员以及用户Ⅳ.应涉及多学科技术Ⅴ.强调投资后的经济效益和社会效益
协议是最常用的建立电话线或ISDN拨号连接的协议。
窗体中有命令按钮Commandl和文本框Textl,事件过程如下:Functionresult(ByValxAsInteger)AsBooleanIfXMod2=0Thenresult=TrueEl
最新回复
(
0
)