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   81069篇
  免费   6982篇
  国内免费   39篇
  2022年   335篇
  2021年   994篇
  2020年   611篇
  2019年   771篇
  2018年   979篇
  2017年   944篇
  2016年   1531篇
  2015年   2537篇
  2014年   2913篇
  2013年   3944篇
  2012年   4864篇
  2011年   4995篇
  2010年   3297篇
  2009年   2984篇
  2008年   4399篇
  2007年   4507篇
  2006年   4292篇
  2005年   4283篇
  2004年   4390篇
  2003年   3965篇
  2002年   3938篇
  2001年   1048篇
  2000年   754篇
  1999年   1030篇
  1998年   1264篇
  1997年   933篇
  1996年   842篇
  1995年   803篇
  1994年   802篇
  1993年   729篇
  1992年   707篇
  1991年   673篇
  1990年   651篇
  1989年   683篇
  1988年   584篇
  1987年   550篇
  1986年   498篇
  1985年   659篇
  1984年   786篇
  1983年   673篇
  1982年   798篇
  1981年   821篇
  1980年   745篇
  1979年   541篇
  1978年   580篇
  1977年   550篇
  1976年   549篇
  1975年   419篇
  1974年   524篇
  1973年   471篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
22.
S P Sutton  R Dam  M K Nielsen 《Life sciences》1974,15(12):2097-2108
Levels of 7.8, 18.5 and 26 mole % deuterium oxide were administered sequentially to Coturnix japonica (Japanese quail) via the drinking water. The primary effect observed was on egg frequency, which decreased from a normal level of 0.89 for 7.8 mole % D2O to a low of 0.38 during the administration of 26 mole % D2O. Adverse symptoms, such as hyperexcitability, convulsions, skin ulcerations, comatosity, weight loss, or death, which have been associated with deuterium toxicity in other animals, were not observed in these experiments. The amount of deuterium deposited in the water of the egg was 6.9, 13.98, and 19.83 mole % when 7.8, 18.5 and 26 mole % deuterium respectively was administered. For each period, the deuterium content of egg water rapidly reached a maximum concentration after which the concentration decreased slightly. This dilution effect has not been noted previously in body fluids from other animals.  相似文献   
23.
24.
25.
The gene of Cytotoxic T Lymphocyte-associated Antigen 4 (CTLA4), a negative regulator of T lymphocytes, contains a single-nucleotide polymorphism (SNP) at position +6230A->G (ct60A->G), which has been found associated with several autoimmune diseases and appears to reduce T-cell inhibitory activity. In Ghana, West Africa, we compared the frequencies of CTLA4 +6230 A/G and 6 haplotype-tagging SNPs in 2010 smear-positive, HIV-negative patients with pulmonary tuberculosis (TB) and 2346 controls matched for age, gender and ethnicity. We found no difference in allele frequencies between cases and controls. However, +6230A and a distinct CTLA4 haplotype and a diplotype comprising the +6230A allele were significantly less frequent among cases with large opacities in chest radiographs compared to those with small ones (Pcorrected [cor] = 0.002, Pcor = 0.00045, P = 0.0005, respectively). This finding suggests that an increased T-cell activity associated with the CTLA4 +6230G allele contributes to pathology rather than to protection in pulmonary TB.  相似文献   
26.
27.
28.
Microbial diversity and distribution are topics of intensive research. In two companion papers in this issue, we describe the results of the Cariaco Microbial Observatory (Caribbean Sea, Venezuela). The Basin contains the largest body of marine anoxic water, and presents an opportunity to study protistan communities across biogeochemical gradients. In the first paper, we survey 18S ribosomal RNA (rRNA) gene sequence diversity using both Sanger- and pyrosequencing-based approaches, employing multiple PCR primers, and state-of-the-art statistical analyses to estimate microbial richness missed by the survey. Sampling the Basin at three stations, in two seasons, and at four depths with distinct biogeochemical regimes, we obtained the largest, and arguably the least biased collection of over 6000 nearly full-length protistan rRNA gene sequences from a given oceanographic regime to date, and over 80 000 pyrosequencing tags. These represent all major and many minor protistan taxa, at frequencies globally similar between the two sequence collections. This large data set provided, via the recently developed parametric modeling, the first statistically sound prediction of the total size of protistan richness in a large and varied environment, such as the Cariaco Basin: over 36 000 species, defined as almost full-length 18S rRNA gene sequence clusters sharing over 99% sequence homology. This richness is a small fraction of the grand total of known protists (over 100 000–500 000 species), suggesting a degree of protistan endemism.  相似文献   
29.
30.
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