考生舉行GRE瀏覽測驗的備考，恰當的演習是必弗成少的，本日給考生分享的是GRE瀏覽真題200篇LEVEL2: The two claws of the mature American...，願望須要的考生可如下載下來然後模仿演習。
39. The two claws of the mature American
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 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 the 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 burrowing 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 manipulatable environment or when they lost and regenerated one or both claws.
21. The passage is primarily concerned with
(A) drawing an analogy between asymmetry in lobsters and handedness in humans
(B) developing a method for predicting whether crusher claws in lobsters will appear on the left or right side
(C) explaining differences between lobsters’ crusher claws and cutter claws
(D) discussing a possible explanation for the way bilateral asymmetry is determined in lobsters
(E) summarizing the stages of development of the lobster
22. Each of the following statements about the development of a lobster’s crusher claw is supported by information in the passage EXCEPT:
(A) It can be stopped on one side and begun on the other after the juvenile sixth stage.
(B) It occurs gradually over a number of stages.
(C) It is initially apparent in the juvenile sixth stage.
(D) It can occur even when a prospective crusher claw is removed in the juvenile sixth stage.
(E) It is less likely in the absence of a manipulatable environment.
23. Which of the following experimental results, if observed, would most clearly contradict the findings of Victor Emmel?
(A) A left cutterlike claw is removed in the fifth stage and a crusher claw develops on the right side.
(B) A left cutterlike claw is removed in the fourth stage and a crusher claw develops on the left side.
(C) A left cutterlike claw is removed in the sixth stage and a crusher claw develops on the right side.
(D) Both cutterlike claws are removed in the fifth stage and a crusher claw develops on the left side.
(E) Both cutterlike claws are removed in the fourth stage and a crusher claw develops on the right side.
24. It can be inferred that of the two laboratory environments mentioned in the passage, the one with oyster chips was designed to
(A) prove that the presence of oyster chips was not necessary for the development of a crusher claw
(B) prove that the relative length of time that the lobsters were exposed to the oyster-chip environment had
little impact on the development of a crusher claw
(C) eliminate the environment as a possible influence in the development of a crusher claw
(D) control on which side the crusher claw develops
(E) simulate the conditions that lobsters encounter in their natural environment
25. It can be inferred from the passage that one difference between lobsters in the earlier stages of development and those in the juvenile fourth and fifth stages is that lobsters in the early stages are
(A) likely to be less active
(B) likely to be less symmetrical
(C) more likely to lose a claw
(D) more likely to replace a crusher claw with a cutter claw
(E) more likely to regenerate a lost claw
26. Which of the following conditions does the passage suggest is a possible cause for the failure of a lobster to develop a crusher claw?
(A) The loss of a claw during the third or earlier stage of development
(B) The loss of a claw during the fourth or fifth stage of development
(C) The loss of a claw during the sixth stage of development
(D) Development in an environment devoid of material that can be manipulated
(E) Development in an environment that changes frequently throughout the stages of development
27. The author regards the idea that differentiation is triggered randomly when paired claws remain intact as
(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
(E) unlikely because of apparent inconsistencies with theories on handedness in humans
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