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   33648篇
  免费   142篇
  国内免费   266篇
系统科学   491篇
丛书文集   624篇
教育与普及   94篇
理论与方法论   162篇
现状及发展   14215篇
研究方法   1322篇
综合类   16442篇
自然研究   706篇
  2013年   376篇
  2012年   681篇
  2011年   1402篇
  2010年   461篇
  2009年   410篇
  2008年   756篇
  2007年   860篇
  2006年   883篇
  2005年   818篇
  2004年   753篇
  2003年   560篇
  2002年   566篇
  2001年   866篇
  2000年   888篇
  1999年   640篇
  1992年   520篇
  1991年   433篇
  1990年   456篇
  1989年   425篇
  1988年   389篇
  1987年   419篇
  1986年   427篇
  1985年   541篇
  1984年   424篇
  1983年   358篇
  1982年   352篇
  1981年   352篇
  1980年   402篇
  1979年   982篇
  1978年   787篇
  1977年   784篇
  1976年   571篇
  1975年   688篇
  1974年   992篇
  1973年   813篇
  1972年   835篇
  1971年   949篇
  1970年   1147篇
  1969年   943篇
  1968年   909篇
  1967年   953篇
  1966年   794篇
  1965年   643篇
  1964年   211篇
  1959年   329篇
  1958年   528篇
  1957年   398篇
  1956年   312篇
  1955年   328篇
  1954年   309篇
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
201.
K R Dumbell  L C Archard 《Nature》1980,286(5768):29-32
Monkeypox mutants arising spontaneously or after serial, high multiplicity passage were characterized phenotypically and by restriction endonuclease mapping. Some resemble "whitepox" and variola viruses in several of the markers tested but all are distinguishable phenotypically from these. None resembles "whitepox" viruses in genome structure although near-terminal deletions or symmetrical, terminal rearrangements, relative to parental monkeypox, occurred. "Whitepox" viruses isolated from animals closely resemble variola in both phenotype and genome structure.  相似文献   
202.
203.
Summary Furocaulerpin, an acetylenic sesquiterpenoid possessing a furan ring, has been isolated from the marine algaCaulerpa prolifera, and its stereostructure elucidated mainly on the basis of physico-chemical data.This work was carried out in the frame work of the Progetto Finalizzato per l'Oceanografia e i Fondi Marini, CNR, Rome. Thanks are due to the Centro di Metodologie Chimico-Fisiche of the University of Naples (Mr I. Giudicianni) for the determination of PMR-spectra.  相似文献   
204.
Summary After 3 weeks of continuous ethanol intoxication by inhalation, the maximal number and affinity of the -and -receptors of rat heart were unchanged. These data indicate that adrenergic receptor disturbances are not involved in the mechanism of chronic ethanol-induced triglyceride deposition in the heart.Acknowledgments. This work was supported by grants from the Institut National de la Santé et de la Recherche Médicale. We wish to thank Mr M. Clément for skilled technical assistance in performing chronic ethanol inhalation treatments.  相似文献   
205.
Summary A novel sesquiterpene lactone, 9--hydroxyparthenolide (1) was isolated from a chloroform extract ofAnvillea garcini that was collected in Iran. The structure of1 was assigned on the basis of high field NMR decoupling experiments and other spectroscopic evidence. This -methylene lactone has shown significant activity in both the 9KB cell culture and P388 mouse leukemia test systems.The authors acknowledge the support of contract No. N01-CM-97296 from the National Cancer Institute and the use of the Purdue University Biochemical Magnetic Resonance Laboratory (NIH grant No. RR01077). This is paper 16 in the series Potential Antitumor Agents.  相似文献   
206.
Summary A needle type of colloid osmometer has been devised with special attachments which made it useable with 3-l samples. It has been used in oncometry of fresh water teleosts to give the following results: 5.1±1.0 mm Hg for goldfish, 5.2±0.71 mm Hg for carp, and 14.3±0.75 mm Hg for eel.This study was supported in part by Research Grant 387036 from the Ministry of Education of Japan.invited to National Institute of Physiological Sciences, Okazaki, Japan.  相似文献   
207.
208.
209.
210.
Summary Autoradiographic, stereological and histological studies have been carried out to determine the origin of muscle fibre splitting which supposedly occurs during muscle hypertrophy. The results obtained clearly indicate that the supposedly split fibres are a transient response probably derived from satellite cells and are not derived from pre-existing fibres by true splitting. Similarly, increases in muscle fibre size are not achieved by recruitment of satellite structures as indicated by lack of myonuclear recruitment.Acknowledgment. This work was carried out with the aid of a grant from the Medical Research Council of Great Britain. The authors are grateful for the excellent technical assistance of Miss H. Caulton, M.J. Wild and M. Fenner.  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