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   97119篇
  免费   8737篇
  国内免费   42篇
  2023年   332篇
  2022年   417篇
  2021年   1715篇
  2020年   975篇
  2019年   1278篇
  2018年   1574篇
  2017年   1439篇
  2016年   2326篇
  2015年   3972篇
  2014年   4515篇
  2013年   5384篇
  2012年   7214篇
  2011年   7012篇
  2010年   4490篇
  2009年   4239篇
  2008年   5993篇
  2007年   6034篇
  2006年   5613篇
  2005年   5448篇
  2004年   5232篇
  2003年   5026篇
  2002年   4718篇
  2001年   1005篇
  2000年   762篇
  1999年   1048篇
  1998年   1326篇
  1997年   909篇
  1996年   830篇
  1995年   778篇
  1994年   670篇
  1993年   745篇
  1992年   683篇
  1991年   579篇
  1990年   583篇
  1989年   562篇
  1988年   528篇
  1987年   483篇
  1986年   445篇
  1985年   628篇
  1984年   689篇
  1983年   628篇
  1982年   727篇
  1981年   663篇
  1980年   668篇
  1979年   388篇
  1978年   469篇
  1977年   396篇
  1976年   398篇
  1974年   353篇
  1973年   347篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
52.
Voronoi polyhedra as structure probes in large molecular systems   总被引:2,自引:0,他引:2  
C W David 《Biopolymers》1988,27(2):339-344
  相似文献   
53.
TNF inhibitors are currently considered both effective and cost-effective in patients with active rheumatoid arthritis (RA), particularly in patients who have not responded fully to methotrexate. There is substantial doubt about the cost-effectiveness of TNF inhibitors as initial treatment for active RA. New data from the National Data Bank for Rheumatic Diseases now question the current consensus in methotrexate failures. The data suggest that in routine clinical practice TNF inhibitors provide only modest incremental benefits over best conventional therapy. If confirmed, these observational studies suggest that the economic argument underpinning the widespread use of TNF inhibitors in established RA is unsustainable.  相似文献   
54.
55.
56.
In the development of the horseshoe crab, Limulus polyphemus, the fertilized egg undergoes a complicated cleavage (Stages 1–3) resulting in blastoderm formation (Stage 4). Stage 1 involves intralecithal cleavage and consists of nine discrete surface modifications (events) which have been briefly described with light microscopy by Brown and Barnum ('83). Since in Stage 1 the cortical reaction (events 1–4) has already been examined with ultrastructural methods, the objectives of the present study were to examine with scanning electron microscopy: (1) the first two of three intermittent granulations (events 5 and 7), and (2) the associated events characterized by smooth surfaces (events 4, 6, and 8). The first granulation occurs 2 1/2 to 3 hours after fertilization (22°C) and lasts approximately 1 1/2 hours. The second granulation appears approximately 5 hours after fertilization and lasts about 3 hours. The dynamic changes that occur during the two granulations involve the transformation of a smooth appearing embryonic surface, liberally coated with microvilli, into a granule-dominated surface on which microvilli are greatly reduced in number. Also of considerable interest are the numerous projections which begin to appear on the surface near the end of the second granulation (event 7) and dominate the surface of the following smooth step stage (event 8). Hypotheses on the significance of these dynamic changes and surface modifications involve relationships to the cell cycle, possible mechanisms for membrane storage, and secretory function.  相似文献   
57.
58.
Dominant males of nonseasonally breeding species should have more opportunity than seasonal breeders to monopolize access to estrous females and thus enhance their reproductive success. They may also use other strategies for maximizing reproductive success such as producing offspring by relatively high ranking females. To test these hypotheses, the paternity of 35 (80%) of 44 Macaca fascicularis offspring, born over a 28-month period, was established using electrophoretically and serologically defined genetic markers. The social ranks of each of the seven potential fathers and 26 potential mothers were determined by recording outcomes of dyadio agonistic interactions. No evidence could be found to support either hypothesis. Further, in view of previously reported significant, positive correlations between male social rank and sexual activity rates in this study group, sexual activity rates do not appear to predict the number of offspring a male sires in this species.  相似文献   
59.
The genes encoding the six polypeptide components of the alkene monooxygenase from Xanthobacter Py2 have been sequenced. The predicted amino acid sequence of the first ORF shows homology with the iron binding subunits of binuclear non-haem iron containing monooxygenases including benzene monooxygenase, toluene 4-monooxygenase (>60% sequence similarity) and methane monooxygenase (>40% sequence similarity) and that the necessary sequence motifs associated with iron co-ordination are also present. Secondary structure prediction based on the amino acid sequence showed that the predominantly α-helical structure that surrounds the binuclear iron binding site was conserved allowing the sequence to be modelled on the co-ordinates of the methane monooxygenase α-subunit. Significant differences in the residues forming the hydrophobic cavity which forms the substrate binding site are discussed with reference to the differences in reaction specificity and stereospecificity of binuclear non-haem iron monooxygenases.  相似文献   
60.
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