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
2017-03-25
37
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 crasher 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 crusher 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 the 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, half 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 ma-nipulable 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 because it is based on scattered research and experimentation
答案
B
解析
本题的四个选项中,只有B项为正确答案。这可从文中第三段的第句话“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 the claws is lost.”推知,即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://jikaoti.com/ti/JQyYFFFM
0
考博英语
相关试题推荐
IfCatlinwasthepainteroftheAmericanIndian,andBierstadttheportrayeroftheRockyMountains,theartistoftheWestern
Recentresearchhasclaimedthatanexcessofpositiveionsintheaircanhaveanilleffectonpeople’sphysicalorpsychologi
Wearemovingtowardsamore______andcooperativesociety,whichisgettingbetterandbetter.
InChicago,acomputerizedsystemhasbeendevelopedthatcontrolstrafficinthecity’ssevenonexpresswaysnow,oneman—a
InChicago,acomputerizedsystemhasbeendevelopedthatcontrolstrafficinthecity’ssevenonexpresswaysnow,oneman—a
Researchershavefoundthatmigratinganimalsuseavarietyofinnercompassestohelpthemnavigate.Somedirectthemselvesby
Culturalnormssocompletelysurroundpeople,sopermeatethoughtandaction,thatweneverrecognizetheassumptionsonwhicht
HerearetwoelementsinlifethatAmericansdosavecarefully:timeandlabor.Americansare"slavestonothingbutthecl
TheFrencheducationsystemisverydifferentfromtheEnglishoneinitsaims,itsorganizationanditsresults.TheFrenchchi
Earlysignsof_____seenintheherbalmedicinestudyareextremelyencouragingandbaseduponthesedata,wearenowplanningar
随机试题
下列关于心室处于减慢射血期的叙述,正确的是
乳幼儿大便呈果酱色,伴阵发性哭闹,常为
按照规定,B房地产开发公司的注册资本最低应为()万元。如果B房地产开发公司与中国建设银行A市分行签订在建工程抵押贷款合同后,应在()日内,持有关证件,到A市房地产管理部门办理在建工程抵押登记。
建设项目需要配套建设的环境保护设施,必须与主体工程实行“三同时”。“三同时”是指()。
国家预算的作用是国家预算职能在经济生活中的具体体现,主要包括()。
假设某投资项目需购买一台设备。支付现金500000元,设备寿命期5年,税法规定的残值率为10%,采用直线折旧法。项目的资本成本是10%,所得税税率是25%。其他信息如下表所示。要求:确定该方案在三种情景下的净现值,计算该方案净现值的标准差和变化系数
某组织在申请质量管理体系认证前,编制了质量手册,确定了企业产品标准和有关程序,收集了相关法规,明确了认证的依据,对文件记录实行了电子化管理并对某些有关ISO9000族标准的问题进行了学习讨论。关于标准之间的相互关系,正确的有()。
一般来说美术又被称为“造型艺术”,它主要包括绘画、_______、_______和雕塑等几大门类。雕塑按形态可分为_______、_______和镂空雕三种。
下列关于VisualFoxPro数据库的描述,正确的是()。
有以下程序intadd(inta,intb){return(a+b);}main(){intk,(*f)(),a=5,b=10;f=add;…}则以下函数调用语句错误的是
最新回复
(
0
)