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   1920篇
  免费   121篇
  2041篇
  2023年   11篇
  2022年   16篇
  2021年   33篇
  2020年   39篇
  2019年   29篇
  2018年   29篇
  2017年   39篇
  2016年   77篇
  2015年   92篇
  2014年   113篇
  2013年   138篇
  2012年   136篇
  2011年   149篇
  2010年   108篇
  2009年   88篇
  2008年   110篇
  2007年   108篇
  2006年   81篇
  2005年   92篇
  2004年   86篇
  2003年   75篇
  2002年   83篇
  2001年   22篇
  2000年   11篇
  1999年   24篇
  1998年   15篇
  1997年   18篇
  1996年   19篇
  1995年   12篇
  1994年   13篇
  1993年   15篇
  1992年   8篇
  1991年   7篇
  1990年   13篇
  1988年   4篇
  1987年   7篇
  1985年   4篇
  1984年   4篇
  1983年   9篇
  1982年   11篇
  1981年   12篇
  1980年   6篇
  1978年   11篇
  1977年   6篇
  1976年   6篇
  1973年   5篇
  1970年   4篇
  1969年   3篇
  1959年   4篇
  1955年   4篇
排序方式: 共有2041条查询结果,搜索用时 0 毫秒
991.
The aim was to discriminate the countries with relatively comprehensive inventories of freshwater fishes from those with insufficiently prospected inventories. We used a data set of 16,734 freshwater fish species with a total of 1,373,449 occurrence records. Accumulation curves relating the increase in the number of species to the number of records, and completeness values obtained after extrapolating these curves to estimate the total number of predicted species were calculated for each country using the RWizard application KnowBR. Using the final slope values of the accumulation curves, the obtained completeness values, and the ratio between the number of records and the observed species, maps and plots representing the location of good, fair and poor quality inventories at country level were obtained. Inventory completeness ranged from 5.3% (Guinea-Bissau) to 108.4% (United Kingdom), with a pooled mean of 65.9%. We observed that a completeness higher than 90%, a slope lower than 0.02 and a ratio of records/species observed greater than 15 were good thresholds for identifying countries with good quality inventories; only 26 countries met these requirements, mainly located in Europe and North America. However, more than 71% of countries worldwide have inventories that can be categorised as of poor quality. Furthermore, even those countries with relatively accurate national inventories possess a high variability in the completeness of their provincial or regional inventories.  相似文献   
992.
Parallel evolution of similar phenotypes provides strong evidence for the operation of natural selection. Where these phenotypes contribute to reproductive isolation, they further support a role for divergent, habitat‐associated selection in speciation. However, the observation of pairs of divergent ecotypes currently occupying contrasting habitats in distinct geographical regions is not sufficient to infer parallel origins. Here we show striking parallel phenotypic divergence between populations of the rocky‐shore gastropod, Littorina saxatilis, occupying contrasting habitats exposed to either wave action or crab predation. This divergence is associated with barriers to gene exchange but, nevertheless, genetic variation is more strongly structured by geography than by ecotype. Using approximate Bayesian analysis of sequence data and amplified fragment length polymorphism markers, we show that the ecotypes are likely to have arisen in the face of continuous gene flow and that the demographic separation of ecotypes has occurred in parallel at both regional and local scales. Parameter estimates suggest a long delay between colonization of a locality and ecotype formation, perhaps because the postglacial spread of crab populations was slower than the spread of snails. Adaptive differentiation may not be fully genetically independent despite being demographically parallel. These results provide new insight into a major model of ecologically driven speciation.  相似文献   
993.
Obesity has become a serious health problem worldwide because it is closely related to the leading causes of morbidity and mortality, including diabetes mellitus, hypertension, atherosclerosis, and dyslipidemia. It is therefore imperative for the scientific community to understand the main environmental and social-cultural factors, as well as organic disorders arising from breakdown of the physiological mechanisms that control energy balance in our body, all of which are ultimately responsible for development of obesity. Adequate understanding of the mechanisms involved in regulation of energy balance is therefore essential to understand the pathogenesis and pathophysiology of the growing pandemic of obesity. This study was intended to review the main factors involved in development of obesity and advances in understanding of the mechanisms that regulate body weight and appetite and their pathophysiology.  相似文献   
994.
995.
996.
Summary

The Authors describe in Sambucus nigra L the occurrence of the » apohomeotypic teiosis « followed by a »mitosis with diplounivalents « (Battaglia 1945, 1947) in ae somatic tissue of the style. The AA. observed also the occasional formation f four haploid nuclei and then the occurence of the »eumeiosis« (Battaglia 1945).  相似文献   
997.
998.
Human embryonic stem cells (hESCs) are self-renewing pluripotent cells that can differentiate into specialized cells and hold great promise as models for human development and disease studies, cell-replacement therapies, drug discovery and in vitro cytotoxicity tests. The culture and differentiation of these cells are both complex and expensive, so it is essential to extreme aseptic conditions. hESCs are susceptible to Mycoplasma sp. infection, which is hard to detect and alters stem cell-associated properties. The purpose of this work was to evaluate the efficacy and cytotoxic effect of PlasmocinTM and ciprofloxacin (specific antibiotics used for Mycoplasma sp. eradication) on hESCs. Mycoplasma sp. infected HUES-5 884 (H5 884, stable hESCs H5-brachyury promoter-GFP line) cells were effectively cured with a 14 days PlasmocinTM 25 µg/ml treatment (curative treatment) while maintaining stemness characteristic features. Furthermore, cured H5 884 cells exhibit the same karyotype as the parental H5 line and expressed GFP, through up-regulation of brachyury promoter, at day 4 of differentiation onset. Moreover, H5 cells treated with ciprofloxacin 10 µg/ml for 14 days (mimic of curative treatment) and H5 and WA09 (H9) hESCs treated with PlasmocinTM 5 µg/ml (prophylactic treatment) for 5 passages retained hESCs features, as judged by the expression of stemness-related genes (TRA1-60, TRA1-81, SSEA-4, Oct-4, Nanog) at mRNA and protein levels. In addition, the presence of specific markers of the three germ layers (brachyury, Nkx2.5 and cTnT: mesoderm; AFP: endoderm; nestin and Pax-6: ectoderm) was verified in in vitro differentiated antibiotic-treated hESCs. In conclusion, we found that PlasmocinTM and ciprofloxacin do not affect hESCs stemness and pluripotency nor cell viability. However, curative treatments slightly diminished cell growth rate. This cytotoxic effect was reversible as cells regained normal growth rate upon antibiotic withdrawal.  相似文献   
999.
1000.
A new class of 4-(aminoheterocycle)piperidine derived 1,3,4 trisubstituted pyrrolidine CCR5 antagonists is reported. Compound 4a is shown to have good binding affinity (1.8 nM) and antiviral activity in PBMC's (IC(95)=50 nM). Compound 4a also has improved PK properties relative to 1.  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