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   40577篇
  免费   4373篇
  国内免费   12篇
  2021年   495篇
  2020年   350篇
  2019年   437篇
  2018年   554篇
  2017年   518篇
  2016年   909篇
  2015年   1629篇
  2014年   1592篇
  2013年   2106篇
  2012年   2414篇
  2011年   2117篇
  2010年   1461篇
  2009年   1339篇
  2008年   1843篇
  2007年   1823篇
  2006年   1626篇
  2005年   1601篇
  2004年   1500篇
  2003年   1285篇
  2002年   1295篇
  2001年   1233篇
  2000年   1233篇
  1999年   1106篇
  1998年   597篇
  1997年   547篇
  1996年   542篇
  1995年   546篇
  1994年   497篇
  1993年   517篇
  1992年   1016篇
  1991年   760篇
  1990年   788篇
  1989年   763篇
  1988年   655篇
  1987年   600篇
  1986年   615篇
  1985年   716篇
  1984年   538篇
  1983年   414篇
  1982年   343篇
  1981年   314篇
  1980年   254篇
  1979年   384篇
  1978年   345篇
  1977年   241篇
  1976年   229篇
  1975年   196篇
  1974年   290篇
  1973年   251篇
  1972年   189篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
93.

Introduction  

We wanted to assess the importance of the levels of anti-citrullinated peptide antibody (anti-CCP) and immunoglobulin M (IgM) rheumatoid factor (RF) in predicting development of persistent arthritis from undifferentiated arthritis (UA), and to investigate whether there is an added predictive value for persistent arthritis in testing for both anti-CCP and IgM RF.  相似文献   
94.
95.
An expression vector containing the Rhizobium leguminosarum nodA promoter cloned in front of the Escherichia coli lacZ gene was used to characterize the properties of the R. leguminosarum nodA gene-inducing compound(s) present in sterile root exudate of the host plant Vicia sativa L. subsp. nigra (L.). The major inducing compound was flavonoid in nature, most likely a flavanone. The commercially available flavonoids naringenin (5,7,4'-trihydroxyflavanone), eriodictyol (5,7,3'4'-tetrahydroxyflavanone), apigenin (5,7,4'-trihydroxyflavone), and luteolin (5,7,3',4'-tetrahydroxyflavone) induced the nodA promoter to the same level as the root exudate. On the basis of chromatographic properties, it was concluded that none of these compounds is identical to the inducer that is present in root exudate. The induction of the nodA promoter by root exudate and by the most effective inducer naringenin was very similar, as judged from the genetic requirements and the kinetics of induction.  相似文献   
96.
97.
98.
Immunochemical study of nutritionally variant streptococci   总被引:11,自引:0,他引:11  
Nutritionally variant streptococci (NVS) have been characterized by their growth as satellite colonies around colonies of staphylococci or several other gram-positive or gram-negative bacterial strains. The majority of the NVS strains were isolated from patients with subacute bacterial endocarditis. Organisms identified as NVS were subdivided into three serotypes by rocket-line electrophoresis and hemagglutination inhibition assays. Ninety-nine of 103 strains expressed one or more of the three serotype antigens; however, a group antigen was not demonstrated in the various extracts of these streptococci. Surface protein studies confirmed the NVS differentiation into serotypes. Serotype I organisms expressed surface protein(s) specific for the serotype, whereas the serotype II and III NVS demonstrated common protein(s) on their surface. Furthermore, SDS extraction released a greater amount of radioiodinatable surface protein from serotypes I and III bacteria than serotype II. Finally, there was no correlation between the serotype or the disease of the patients from which the NVS strains were isolated.  相似文献   
99.
The possibility of somatic mobilisation of P elements in Drosophila melanogaster was investigated. Flies, trans-heterozygous for the genetic markers mwh and flr3, were obtained by crossing males containing transposition-competent P elements with females having M cytotype. The hybrid dysgenic flies were reared at 29 degrees C and their wings examined for mutant clones. The frequency of mutant spots found on the wings of the female flies was significantly higher than that of female control flies. We postulate that this increase in frequency may be due to P element mobilisation at high temperature in the wing cells of dysgenic hybrids. This is in direct contrast to the large body of research which indicates that P-transposition-mediated mutation is restricted to the germline.  相似文献   
100.
The microtopology of the motoneurons involved in protraction and retraction of the proboscis of the blowfly (Calliphora vicina) has been studied. In addition, taste input from the labellar hairs was investigated. As a result of this study it appears that protraction movements are controlled by two while retraction movements are guided by three motoneurons on each side. The neurons in each group apear to be in ipsicontralateral communication with each other. The musculi protractores fulcri (MPF) probably contain a proprioceptive cell group which projects to the MPF motoneurons. It is proposed that the proboscis motor system can be modulated by proprioception as well as by chemosensory labellar input. Neurosecretory cells may be involved in adjusting muscle power.  相似文献   
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