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   30631篇
  免费   67篇
  国内免费   107篇
系统科学   117篇
丛书文集   419篇
教育与普及   86篇
理论与方法论   115篇
现状及发展   14539篇
研究方法   1102篇
综合类   14114篇
自然研究   313篇
  2013年   148篇
  2012年   335篇
  2011年   631篇
  2008年   428篇
  2007年   468篇
  2006年   483篇
  2005年   491篇
  2004年   607篇
  2003年   493篇
  2002年   478篇
  2001年   802篇
  2000年   819篇
  1999年   553篇
  1994年   366篇
  1992年   493篇
  1991年   401篇
  1990年   459篇
  1989年   427篇
  1988年   415篇
  1987年   463篇
  1986年   512篇
  1985年   627篇
  1984年   420篇
  1983年   434篇
  1982年   376篇
  1981年   391篇
  1980年   413篇
  1979年   983篇
  1978年   820篇
  1977年   759篇
  1976年   597篇
  1975年   717篇
  1974年   953篇
  1973年   804篇
  1972年   837篇
  1971年   1046篇
  1970年   1231篇
  1969年   992篇
  1968年   1005篇
  1967年   927篇
  1966年   786篇
  1965年   613篇
  1964年   229篇
  1959年   340篇
  1958年   600篇
  1957年   454篇
  1956年   371篇
  1955年   362篇
  1954年   366篇
  1948年   273篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
261.
Three acidic polysaccharide (AP) fractions and the prostaglandin-like substances (PLS) isolated from P. acnes were investigated regarding their chemotactic activities on polymorphonuclear leukocytes. Both AP's and PLS induced a significant chemotatic response, which suggests their involvement in inflammatory acne vulgaris.  相似文献   
262.
It was shown that, in physiological concentrations, insulin enhances, in vitro, the immunological phagocytosis of sensitized sheep erythrocytes by cultured mouse peritoneal macrophages. Insulin seems to stimulate macrophage phagocytosis as a cholinomimetic agonist by increasing the intracellular levels of cyclic GMP.  相似文献   
263.
G Kaiser  J Dietz  G Wiemer  D Palm 《Experientia》1979,35(4):453-455
During the maturation of red blood cells from rats after stress erythropoiesis, adenyl cyclase activity and beta-adrenoceptor density (pmoles/mg protein) decrease at distinctly different rates suggesting a different turnover of these membrane units.  相似文献   
264.
(3H)-Ouabain binding to liver and kidney preparations was utilized to estimate the number of Na+, K+-ATPase enzyme units in livers and kidneys from rats fed 2% corn oil supplemented or fat-free diets. The specific (3H)-ouabain binding in liver and kidney preparations from fatty acid deficient rats was increased approximately 40%, but the affinity of the binding sites for ouabain (Kd-value) remained unchanged. The increased concentration of Na+, K+-ATPase enzyme units observed in the essential fatty acid deficient rats may contribute to the reduced body fat accumulation and elevated heat production observed in these animals.  相似文献   
265.
Summary An intermediate stage between the elementary and the initial bodies of theRickettsiella genus is defined as the beginning of an intracellular cycle. It is characterized by several structural changes in the dense elementary body: the cytoplasm becomes less electron-dense; thus, the nucleoid and the ribosomes are visible. The inner layer of the cell-wall becomes progressively clearer and the trilamellar structure of the inner and outer membranes appears distinctly. Preinitial body is proposed as name of this stage of development.  相似文献   
266.
Summary 2 new limonoid-type terpenes have been isolated from an aqueous extract of seeds produced by the Eastern Himalayan (India) plantAphanamixis grandifolia Bl. By interpreting principally mass spectral and nuclear magnetic resonance data, the structures of 12-hydroxyamoorastatin (2b) and amoorastatone (3) were elucidated. Unequivocal evidence for the 12-hydroxyamoorastatin structural assignment was obtained by chemical conversion to sendanin (4). Amoorastatin derivative2b was found to significantly inhibit growth of the murine P388 lymphocytic leukemia cell lines but amoorastatone in the same system was inactive. In a comparative biological study, sendanin (4) and anthothecol (7) were also found significantly to inhibit growth of the P388 cell line, while rohitukin (8) and limonin (9) were found to be inactive.Part 63 of the series Antineoplastic Agents. For the previous contribution refer to G.R. Pettit, T.S. Krupa and R.M. Reynolds, Int. J. Peptide Protein Res., in preparation. The present investigation was supported in part by Public Health Research Grant No. CA-16049-05 from the National Cancer Institute, the Fannie E. Rippel Foundation, Mrs Mary Dell Pritzlaff, the Spencer T. and Ann W. Olin Foundation, Mr John F. Schmidt, and the Phoenix Coca-Cola Bottling Company.  相似文献   
267.
Summary Viable cerebellar perikarya of mixed cell type obtained from 7–9-day-old rats were maintained in monolayer culture for up to 12 days. During this time extensive neurite formation and outgrowth occurred. The large majority of the cells developing in culture were tentatively identified as granule neurons. This identification was based on the large number isolated from the starting tissue, and the cells general morphological features in culture such as perikaryal and nuclear size, the bipolar nature of neurite extension, and their migratory behaviour.D.N. Currie was supported by a grant from Hoechst Pharmaceuticals, Ld, U.K.J.C. was supported by an S.R.C. Grant to S.P.R. RoseThe authors would like to thank Kathleen Tear for excellent technical assistance and members of the Brain Research Group for helpful discussions.  相似文献   
268.
Summary The insulinemic response to 1 g of tolbutamide i.v. is greatly enhanced (+145%) after a 60-min infusion of the -lytic dihydrogenated ergot alkaloid, dihydroergocristine (83.3 m/min, corresponding to a total dose of 5 mg) in 7 healthy subjects. No differences are observed in the glycemic responses.Acknowledgment. The authors are grateful to Mr Franco Della Sala, M.D., for his skillful technical assistance.  相似文献   
269.
270.
High-frequency generalised transduction by bacteriophage T4   总被引:30,自引:0,他引:30  
Wilson GG  Young KY  Edlin GJ  Konigsberg W 《Nature》1979,280(5717):80-82
  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