首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   911811篇
  免费   89505篇
  国内免费   1358篇
  2018年   19768篇
  2017年   18280篇
  2016年   18213篇
  2015年   15105篇
  2014年   17243篇
  2013年   24435篇
  2012年   31419篇
  2011年   40828篇
  2010年   31040篇
  2009年   26027篇
  2008年   34453篇
  2007年   36934篇
  2006年   23664篇
  2005年   23720篇
  2004年   23443篇
  2003年   22807篇
  2002年   21640篇
  2001年   38729篇
  2000年   38870篇
  1999年   30717篇
  1998年   10760篇
  1997年   11235篇
  1996年   10535篇
  1995年   9835篇
  1994年   9570篇
  1993年   9371篇
  1992年   24807篇
  1991年   24079篇
  1990年   23431篇
  1989年   22810篇
  1988年   21180篇
  1987年   19774篇
  1986年   18321篇
  1985年   18150篇
  1984年   15066篇
  1983年   12574篇
  1982年   9538篇
  1981年   8493篇
  1980年   7951篇
  1979年   13498篇
  1978年   10501篇
  1977年   9438篇
  1976年   8530篇
  1975年   9507篇
  1974年   10128篇
  1973年   10026篇
  1972年   9517篇
  1971年   8703篇
  1970年   6974篇
  1969年   6734篇
排序方式: 共有10000条查询结果,搜索用时 453 毫秒
111.
112.
The effect of the chromosomal ends of Tetrahymena thermophila on the stability of linear transforming molecules in the filamentous fungus Podospora anserina was tested. A derivative of an integrative vector for this fungus has been constructed, so that after linearization, the ends of the plasmid are the telomeric sequences of T. thermophila. After transformation, this linear molecule was maintained as an extrachromosomal plasmid with no integrated copies in about 50% of the transformants. Under selective conditions, there was approximately one linear molecule per 5 to 10 nuclei, and these extrachromosomal molecules were rapidly lost under nonselective conditions. The circular plasmid carrying an inverted repeat of T. thermophila telomeres could be linearized and processed in vivo.  相似文献   
113.
Experiments were made on rabbit fed an atherogenic diet (0.5 g/kg cholesterol) singly for 15 and 24 h and repeatedly for 3, 9 and 30 days. At early stages of lipid metabolism distress the interrelationship was established between blood rheological disorders and microcirculatory abnormalities. The dependence of the initial reaction of some rheological characteristics on their initial level was marked.  相似文献   
114.
115.
With DNA sequencing now getting cheaper more quickly than data storage or computation, the time may have come for genome informatics to migrate to the cloud.  相似文献   
116.
OBJECTIVE--To test the hypothesis that farmers are at high risk of hip osteoarthritis and to investigate possible causes for such a hazard. DESIGN--Cross sectional survey. SETTING--Five rural general practices. SUBJECTS--167 male farmers aged 60-76 and 83 controls from mainly sedentary jobs. All those without previous hip replacement underwent radiography of the hip. MAIN OUTCOME MEASURES--Hip replacement for osteoarthritis or radiological evidence of hip osteoarthritis. RESULTS--Prevalence of hip osteoarthritis was higher in farmers than controls and especially in those who had farmed for over 10 years (odds ratio 9.3, 95% confidence interval 1.9 to 44.5). The excess could not be attributed to any one type of farming, and heavy lifting seems the likely explanation. CONCLUSIONS--Manual handling in agriculture should be limited where possible. Consideration should be given to making hip osteoarthritis a prescribed industrial disease in farmers. There may be wider implications for the prevention of hip osteoarthritis in the general population.  相似文献   
117.
The metabolism of the cholestatic triterpene acid reduced lantadene A has been studied in susceptible and resistant rats and in sheep which are susceptible to intoxication. Sheep and susceptible female rats produced a similar major metabolite and rats produced a second metabolite which was a glucuronide. These metabolites were also observed in extracts of bile canalicular membranes prepared from intoxicated rats. Resistant male and female rats produced a similar major metabolite which was different to those synthesized by susceptible animals. It is concluded that in rats and sheep there is a correlation between the type of metabolites produced in the liver and the susceptibility to intoxication by reduced lantadene A.  相似文献   
118.
Growth of fathead minnow yolk sac larvae was characterized from changes in dry weight and total content and concentrations of RNA, DNA and protein in fish exposed to a sublethal level of HCN (58 micrograms/l) and in age matched controls. Cyanide toxicosis occurred within 24 hr of exposure as evidenced by significant reductions in protein and RNA content and RNA/DNA ratio of larvae. After 96 hr exposure to HCN, larvae exhibited the same growth rate and protein synthetic rate (RNA/DNA) as control fish. HCN toxicosis and recovery is rapid and at least partial tolerance to HCN develops within 96 hr of exposure in larval fathead minnows.  相似文献   
119.
120.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号