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   855331篇
  免费   87432篇
  国内免费   277篇
  943040篇
  2018年   8516篇
  2017年   8108篇
  2016年   11403篇
  2015年   14450篇
  2014年   17237篇
  2013年   24902篇
  2012年   27810篇
  2011年   28560篇
  2010年   19608篇
  2009年   18050篇
  2008年   25575篇
  2007年   26550篇
  2006年   24784篇
  2005年   23823篇
  2004年   23601篇
  2003年   22721篇
  2002年   22152篇
  2001年   37093篇
  2000年   36457篇
  1999年   29334篇
  1998年   10840篇
  1997年   10935篇
  1996年   10419篇
  1995年   9586篇
  1994年   9241篇
  1993年   9289篇
  1992年   23784篇
  1991年   23348篇
  1990年   22813篇
  1989年   22200篇
  1988年   20352篇
  1987年   19525篇
  1986年   18269篇
  1985年   18095篇
  1984年   14916篇
  1983年   13031篇
  1982年   9856篇
  1981年   8952篇
  1980年   8305篇
  1979年   13921篇
  1978年   10994篇
  1977年   9908篇
  1976年   9482篇
  1975年   10648篇
  1974年   11379篇
  1973年   11322篇
  1972年   10339篇
  1971年   9224篇
  1970年   8092篇
  1969年   7976篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
The ECF™ lateral flow assay test is marketed to detect non-pregnancy in mares. The objectives of the present study were to determine the accuracy of the ECF test, the accuracy of the electronic reader accompanying the ECF test, and agreement between two human readers and the electronic reader. Serum samples were collected from anestrus, cycling but not inseminated, and inseminated mares, and were evaluated with the ECF™ test (EDP Biotech Company, Knoxville, TN, USA) at The Ohio State University and at the EDP Biotech Laboratory. Specificity ranged from 0.07 to 0.16, the negative predictive value ranged from 0.15 to 0.33, and accuracy ranged from 0.43 to 0.52. The electronic reader did not add improve the accuracy or predictive values of the test. Based on the electronic reader, 80.0% of the serum samples collected from the anestrus mares were false positives; Readers 1 and 2 had 60.0 and 33.3% false positives, respectively. For samples collected during the estrous cycle, 83.9% were false positives by the electronic reader, whereas Readers 1 and 2 had 43.7 and 26.4% false positives. We concluded that, regardless of whether the test strips were evaluated by a human or electronic reader, this assay was not accurate for determination of the non-pregnant mare.  相似文献   
942.
943.
944.
945.
Abstract The parasitoid wasp Trichogramma dendrolimi Matsumura sets the number of progeny allocated to its insect egg hosts according to the duration of its initial transit walk across the host surface. Although cooling the wasp reduces its walking speed, reduced temperature does not affect progeny allocation. By locally heating and cooling the wasps, the initial transit can be thermally uncoupled from the subsequent oviposition. Using this technique we show that the timing of initial transit duration is temperature-dependent. These findings suggest that short interval timing by Trichogramma differs physiologically from the temperature compensated clocks that have been described for other insects.  相似文献   
946.
The combination of alpha interferon and vinblastine has been reported to yield a response rate of 30-40% in previously untreated patients with metastatic renal cell carcinoma. This combination was given to nine patients with advanced metastatic renal carcinoma after they failed or relapsed on alpha-interferon alone, to attempt to evaluate the role of vinblastine in this combination. Neither complete nor partial response was observed. Two patients had disease stabilization for two and seven months. Our preliminary results suggest that vinblastine did not add to the efficacy of interferon in this group of patients.  相似文献   
947.
It is shown that both intracerebral and intraperitoneal neurotropin administration resulted in a decrease of seizure susceptibility of preliminary picrotoxin--kindled rats. On the other hand, neurotropin did not change the course of kindling development. Under conditions of acute picrotoxin--induced seizures it was observed that preliminary cycloheximide (protein-synthesis blocker) administration abolished anticonvulsant properties of neurotropin. It is concluded that anticonvulsant effects of neurotropin are realized via modulation of endogenous peptides synthesis and, in particular, cerulein.  相似文献   
948.
A cytophysiological study was carried out of the functional status of a halo as a response of the host plant to contact with a powdery mildew pathogen. Interactions of the powdery mildew causative agents with barley, wheat, wheat–wheat-grass hybrids, wheat-aegilops lines, and aegilops with different genotypic resistance lead to the expression of haloes during pathogens, which are induced by infection pegs of the primary growth tubes, appressoria, and hyphal lobes. Haloes are visualized using cytochemical reactions to proteins and scanning electron microscopy. The observed differences in the size of haloes and intensity of their staining (uniform or zonal) are related, to a great extent, to individual reactions of the plant cell at the penetration site and, to a lesser extent, to the level of genotypic resistance. An analysis of electron microscopy and cytochemistry studies suggests that the halo as a physiologically active zone is localized at the level of the plant cell plasmalemma. Active taxis of the cell organelles to the site of infection during the formation of a halo suggests that some kind of informational signals to changes in the cell metabolism are spread from the halo zone, which lead to compatible or incompatible interactions.  相似文献   
949.
950.
New material ofTrischizolagus dumitrescuae from Moldova and Ukraine is described. The variation of p3 inTrischizolagus shows the gradual shift of morphotype frequencies from the ‘Hypolagus’ pattern in Turolian through the mixture of three patterns (including ‘Nekrolagus’ morphotype) in Early Ruscinian to the dominant ‘Alilepus’ pattern in the Late Ruscinian samples. These transformations took place parallel to that of the North AmericanNekrolagus. Probably North AmericanSylvilagus, Brachylagus, andRomerolagus had an North American origin fromNekrolagus, whereas Eurasiatic and AfricanOryctolagus, Caprolagus, Nesolagus, andPoelagus could have originated in the Old World fromTrischizolagus.  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