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
2011-02-26
76
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 right 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 cutterlike. 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 Emmel. 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 cutter 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 Emmel’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
转载请注明原文地址:https://kaotiyun.com/show/6jBO777K
0
考博英语
相关试题推荐
YouwanttheseniorSecretaryintheGeneralOfficetoordersomeofficeequipment.Youneedaportablecomputer,acarryingcas
•Readthefollowingadvertisementforsalesmanager.VACANCYAleadinginternationaltradingcom
China’srapiddevelopmentinthetelecomindustryhasplayeda/an______roleonthew0orldtelecomindustry.______havecontr
Thereviewersuggeststhatoneadvantageofthebookisthat______.ThereviewercannotacceptStuart’sopinionsbecauseStua
TradingPlacesWendyLedgerlooksatjobswapping,astaffdevelopmentschemeinwhichemployeesexchangejobsforashort
•ReadthefollowingreviewofabookcalledNeverQuit!TheUpsandDownsofRunningaFamilyBusiness.•Foreachquestion
•Readtheatriclebelowabouttheneedofemployees.•ChoosethecorrectwordtofilleachgapfrOmA.BorContheopposit
Saleswerenotgoodbecausedistributionproblemsledtoaslightdropinprofits.Thecompanyhadproblemswith______.
Theexaminationhasbeencancelled.You______allthatreview,afterall.
______fromhercontract,DeHavillandsuedthestudioand,afteratwo-yearbattle,wonhercaseinalandmarkdecisionthatbe
随机试题
引起糖耐量降低的疾病有
传统的输精管吻合术的主要步骤为
根据合穴或下合穴的理论,其病最可能出现反应的穴位是:取太冲治疗薛某之疾,以疏肝理气止痛,主要是因为其穴属于:
李某,女,25岁。身体矮小,匀称,骨盆外测量结果如下:髂前上棘间径22era,髂嵴间径24cm,骶耻外径17cm,出口横径7.5cm,对角径11.5cm,此孕妇骨盆为
病区物理环境的要求中,不宜强求一致的是
在N=[*]这个公式中,K在容纳人数超过10000人的公共场所宜取值()
以“爱心”为陈述对象,以“泉水”或“歌谣”为喻体,依照下面的句式再造一个句子,使之与例句结构相似。例:爱心是一片照射在冬日的阳光,使贫困交迫的人感到人间的温暖;爱心是_____________________________
1978年,中国共产党重新确立马克思主义实事求是思想路线的会议是()。
A、什么时候都有时间去参加国际会议B、以前有时间去参加国际会议,现在没有时间C、以后一定有时间去参加国际会议D、一直没有时间去参加国际会议D
Largepartsoftheworldhavenotenjoyedtheremarkableglobalprogressinhealthconditionsthathavetakenplaceoverthepas
最新回复
(
0
)