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   79365篇
  免费   300篇
  国内免费   596篇
系统科学   1304篇
丛书文集   1145篇
教育与普及   305篇
理论与方法论   581篇
现状及发展   39259篇
研究方法   1899篇
综合类   33575篇
自然研究   2193篇
  2013年   843篇
  2012年   931篇
  2011年   3119篇
  2008年   1213篇
  2007年   1385篇
  2006年   1443篇
  2005年   1600篇
  2004年   2690篇
  2003年   2326篇
  2002年   2066篇
  2001年   2218篇
  2000年   1680篇
  1999年   1402篇
  1997年   785篇
  1994年   691篇
  1993年   699篇
  1992年   1357篇
  1991年   1121篇
  1990年   1236篇
  1989年   1070篇
  1988年   1016篇
  1987年   1065篇
  1986年   1079篇
  1985年   1280篇
  1984年   1057篇
  1983年   917篇
  1982年   956篇
  1981年   979篇
  1980年   1151篇
  1979年   2115篇
  1978年   1874篇
  1977年   1837篇
  1976年   1481篇
  1975年   1561篇
  1974年   1832篇
  1973年   1825篇
  1972年   1906篇
  1971年   2129篇
  1970年   2567篇
  1969年   2081篇
  1968年   2008篇
  1967年   1961篇
  1966年   1663篇
  1965年   1259篇
  1959年   740篇
  1958年   1097篇
  1957年   867篇
  1956年   748篇
  1955年   668篇
  1954年   714篇
排序方式: 共有10000条查询结果,搜索用时 6 毫秒
641.
J Rocca-Serra  C Tonnelle  M Fougereau 《Nature》1983,304(5924):353-355
Immunoglobulin diversity seems to arise largely by three mechanisms: (1) the existence of several germ-line genes, which must be rearranged before expression--that is, V and J for the light (L) chains, V, D and J for the heavy (H) chains; (2) somatic events, including mutations and gene conversion; and (3) combinatorial association of heavy and light chains, leading to the proposal that random pairing of p X H and q X L chains might generate p X q antibody molecules expressing discrete specificities. As heavy and light chains derived from the same immunoglobulin molecule would frequently reassociate preferentially, it is likely that only a fraction of potential heavy--light pairs actually provides "valid' antibodies. As a consequence of combinatorial heavy--light chain pairing, antibodies of discrete specificities sharing the same VH region, associated with distinct light chains (or vice versa) should be encountered. We report here that two heavy chains, derived from the same VH germ-line gene, may be present in anti-NP or anti--GAT antibodies, depending on their association with a specific lambda or kappa light chain, respectively.  相似文献   
642.
643.
644.
Summary On the basis of comparative spectral data, the structures of 3 novel steroidal glycosides from the Mediterranean starfishHacelia attenuata have been elucidated as3, 4 and5. These are further examples of a novel group of 24-O-glycosidated steroids recently encountered in the same species and in the Pacific speciesProtoreaster nodosus.Part 9. L. Minale, C. Pizza, R. Riccio and F. Zollo, Experientia39 (1983) 567. This contribution is part of the Progetto Finalizzato Chimica fine e secondaria del C.N.R., Roma.Acknowledgments. We thank Prof. K. Nakanishi, Columbia University, New York, for FD-mass spectral analyses, the Centro Interfacoltà di Metodologie Chimico-Fisiche for 270 MHz NMR facilities, and Miss R. Aquino for part of the experimental work.  相似文献   
645.
Summary A dissociation between inhibition of RNA synthesis and cell lysis was observed when thymocytes of adrenalectomized rats were incubated with high concentrations of pregn-4-ene-11-ol-3, 20-dione and pregna-1,4-diene-11-ol-3,20-dione. In contrast, no dissociation of these effects was found with the typical glucocorticoids cortisol and corticosterone, nor with their 1,4-diene analogs under the same conditions.Acknowledgments. This work was supported by grants from the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina to the Instituto de Biologia y Medicina Experimental and the Programa de Regulación Hormonal y Metabólica as well as by financial help from the Secretaría de Ciencia y Tecnología.  相似文献   
646.
Summary Normal and chloroform-extracted human sera, fractionated by Sephadex column chromatography, have been tested for inhibitory activity on granulocyte-macrophage (GM) colony formation. It was found that this activity is connected with lipoproteins (inhibitors of granulocyte-macrophage colony stimulating factor) and low molecular weight substances (7000; 13,000) which can act as specific polypeptide chalones.  相似文献   
647.
Summary Daily evening injections of isoproterenol extended the period of high pineal N-acetyltransferase activity and retarded the growth of testes, seminal vesicles and prostate in rats treated neonatally with testosterone propionate.The authors are grateful to Mrs Marie Svobodová for her skillful technical assistance.  相似文献   
648.
Summary Cell-free extracts ofTrypanosoma cruzi contain proteolytic activity able to degrade endogenous substrates. The activity was maximal at pH 3 to 4, had an optimal temperature of 65°C, and was strongly inhibited by N--p-tosyl-L-lysine chloromethyl ketone.This investigation received financial support from the UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases, the Fundación Julio Cherny, and the Ministerio de Salud Pública y Medio Ambiente de la República Argentina. The authors gratefully acknowledge the technical assistance of Mr Esteban Bontempi.  相似文献   
649.
The ocean as a supplier of food and energy   总被引:1,自引:0,他引:1  
Summary This paper presents the concept of raising seaweeds and other valuable organisms with the aid of huge structures (ocean farms) emplaced in the surface waters of the open oceans. Potential advantages from and difficulties to be expected in realizing the associated technologies are briefly set forth. Much of the published literature pertaining to the concept is referenced and summarized. Wave-powered upwelling of cool, nutrient-rich waters through vertical pipes extending to depths of 100–300 m is indicated as desirable. Technologies are outlined for using the harvested seaweeds to create foods and other valuable products such as animal feeds, fertilizers, fibers, plastics, synthetic natural gas (methane), and alcohol and gasoline fuels. Results from site selection studies and economic analyses are given. It appears that dynamically positioned farms orbiting with the surface current patterns typically found on the ocean will be most cost-effective. The general conclusion is stated that open ocean farming will become economically more feasible as the cheaper fossil fuels and food producing lands of the earth become increasingly consumed in the course of the next century.  相似文献   
650.
Summary Sodium diethyldithiocarbamate, 2,6-di-tert-butylphenol and N,N-diphenyl-p-phenylenediamine inhibited the generation of medullary prostaglandin E, but 1,2-dimethoxyethane (OH scavenger) and 2,5-dimethylfuran (1O2 scavenger) stimulated basal prostaglandin E production 1.2–1.3-fold. These results suggest that the balance between formation and removal of cellular lipid peroxides sets a peroxide level that can regulate the rate of prostaglandin formation in cells.  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