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   158748篇
  免费   4965篇
  国内免费   825篇
  2023年   381篇
  2022年   472篇
  2021年   1201篇
  2020年   910篇
  2019年   1134篇
  2018年   13110篇
  2017年   11773篇
  2016年   9407篇
  2015年   3576篇
  2014年   3459篇
  2013年   4433篇
  2012年   8849篇
  2011年   16831篇
  2010年   14513篇
  2009年   10468篇
  2008年   12923篇
  2007年   14361篇
  2006年   3565篇
  2005年   3279篇
  2004年   3716篇
  2003年   3456篇
  2002年   3084篇
  2001年   1998篇
  2000年   1805篇
  1999年   1368篇
  1998年   710篇
  1997年   544篇
  1996年   514篇
  1995年   499篇
  1994年   409篇
  1993年   406篇
  1992年   799篇
  1991年   761篇
  1990年   661篇
  1989年   657篇
  1988年   593篇
  1987年   596篇
  1986年   526篇
  1985年   500篇
  1984年   418篇
  1983年   392篇
  1982年   300篇
  1979年   401篇
  1978年   269篇
  1975年   261篇
  1974年   308篇
  1973年   309篇
  1972年   517篇
  1971年   524篇
  1970年   260篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
A formalin-fixed paraffin-embedded (FFPE) sample usually yields highly degraded DNA, which limits the use of techniques requiring high-quality DNA, such as Infinium Methylation microarrays. To overcome this restriction, we have applied an FFPE restoration procedure consisting of DNA repair and ligation processes in a set of paired fresh-frozen (FF) and FFPE samples. We validated the FFPE results in comparison with matched FF samples, enabling us to use FFPE samples on the Infinium HumanMethylation450 Methylation array.  相似文献   
13.
International Journal of Primatology - The selection of sleeping sites and sleeping trees in nonhuman primates is related to social and ecological factors. We investigate the role of body...  相似文献   
14.

Background  

Recent studies point to a great diversity of non-ribosomal peptide synthesis systems with major roles in amino acid and co-factor biosynthesis, secondary metabolism, and post-translational modifications of proteins by peptide tags. The least studied of these systems are those utilizing tRNAs or aminoacyl-tRNA synthetases (AAtRS) in non-ribosomal peptide ligation.  相似文献   
15.
16.
White‐sand forests are patchily distributed ecosystems covering just 5% of Amazonia that host many specialist species of birds not found elsewhere, and these forests are threatened due to their small size and human exploitation of sand for construction projects. As a result, many species of birds that are white‐sand specialists are at risk of extinction, and immediate conservation action is paramount for their survival. Our objective was to evaluate current survey methods and determine the relative effect of the size of patches of these forests on the presence or absence of white‐sand specialists. Using point counts and autonomous recorders, we surveyed avian assemblages occupying patches of white‐sand forest in the Peruvian Amazon in April 2018. Overall, we detected 126 species, including 21 white‐sand forest specialists. We detected significantly more species of birds per survey point with autonomous recorders than point counts. We also found a negative relationship between avian species richness and distance from the edge of patches of white‐sand forest, but a significant, positive relationship when only counting white‐sand specialists. Although we detected more species with autonomous recorders, point counts were more effective for detecting canopy‐dwelling passerines. Therefore, we recommend that investigators conducting surveys for rare and patchily distributed species in the tropics use a mixed‐method approach that incorporates both autonomous recorders and visual observation. Finally, our results suggest that conserving large, continuous patches of white‐sand forest may increase the likelihood of survival of species of birds that are white‐sand specialists.  相似文献   
17.

Background  

Substituted catechols are important precursors for large-scale synthesis of pharmaceuticals and other industrial products. Most of the reported chemical synthesis methods are expensive and insufficient at industrial level. However, biological processes for production of substituted catechols could be highly selective and suitable for industrial purposes.  相似文献   
18.
Glucose and ketone bodies production has been studied in hepatocytes isolated from fetuses at term of fed and fasted adrenalectomized mothers. Maternal adrenalectomy diminishes the fetal liver weight. This effect is increased when the adrenalectomized pregnant rat is fasted for the last 2 days of gestation. Maternal adrenalectomy diminishes glucose production in hepatocytes isolated from fetuses at term. This diminution is markedly greater when the adrenalectomized pregnant rat is fasted for the last 48 hr of gestation. Maternal adrenalectomy diminishes ketone bodies production in hepatocytes isolated from fetuses at term.  相似文献   
19.
20.
The relations between the saccus lacrimalis and different portions of the musculus orbicularis oculi were studied in orbital regions of human fetuses sectioned into numbered series. No insertions of the pars lacrimalis or Horner's muscle on the saccus were found. These muscular fibres pass along the dorsal wall of the saccus and are separated from it by the reflex tendon of the ligamentum palpebrale mediale. The only muscular fibres that insert on the saccus are those that approach the anterior face of the saccus and the fornix. The fibres that insert on the anterior face proceed from the deep bundles of the pars preseptalis of the lower eyelids, and those that insert on the fornix derive from the deep bundles of the pars preseptalis of the upper eyelid.  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