4. 青蛙抵御紫外线

　　主要思路：

　　某种酶对青蛙的孵化率hatching rate和免疫系统在UV的照射下的影响

　　主要内容：

　　第一段：两个科学家做蛙卵的孵化率的实地研究(field study)，发现孵化率在不断降低，于是就用青蛙卵做孵化实验。用了三个品种的卵拿来照紫外线，第一个品种在紫外线辐射下孵化率仍然很高，另外两种卵的孵化率则明显下降。第一个品种因为含有某种酶，此酶很活跃(activity)，因此抵挡紫外线，所以孵化率仍然很高;另外两个酶的活跃度较低，所以孵化率下降，必须减少紫外线照射孵化率才会回升。于是证明酶跟孵化率有关。又说青蛙和癞蛤蟆卵对太阳光中的紫外线辐射UV的忍耐度不太一样，有些忍耐力很强，有些则很容易就被杀死。三种蛙，有一类具有很高的耐受性，另外两种不太行，有了UV就死了，只有把UV移去才能恢复正常。而且紫外线照射不仅会影响卵的生长，还会影响青蛙的免疫系统，青蛙的免疫系统受到破坏就很容易受到一种叫S的真菌感染。

　　第二段：人类污染导致臭氧减少，紫外线升高，于是很多蛙类的孵化率降低。最后，因为缺乏以前的紫外线和青蛙孵化数据，所以仍然无法得出确切的结论。

　　题目

　　1. 主旨题

　　参考答案：1选阐述科学发现那个

　　2identify an explanation to some findings

　　参考文章

　　Blaustein and his colleagues tested whether or not UV-B could be a factor in lowering the hatching rate of amphibian(两栖动物) eggs. At two field sites, they divided the eggs of each of three amphibian species into three groups (Figure 3.6). The first group developed without any sun filter(过滤器，滤光器). The second group developed under a filter(滤波器，过滤器) that allowed UV-B to pass through. The third group developed under a filter that blocked UV-B from reaching the eggs. For Hylaregilla, the filters had no effect, and hatching success was excellent under all three conditions. For Ranacascadea and Bufoboreas, however, the UV-B blocking filter raised the percentage of eggs hatched from about 60% to close to 80%.

　　The environmental programs of experimental embryology(胚胎学) were a major part of the discipline when Entwicklungsmechanik was first established. However, it soon became obvious that experimental variables could be better controlled in the laboratory than in the field, and that a scientist could do many more experiments in the laboratory. Thus, field experimentation in embryology dwindled(变小) in the first decades of the twentieth century (see Nyhart 1995). However, with our increasing concern about the environment, this area of developmental biology has become increasingly important. Other recent work in this field will be detailed in Chapter 21.