全文获取类型
收费全文 | 71921篇 |
免费 | 294篇 |
国内免费 | 594篇 |
专业分类
系统科学 | 1277篇 |
丛书文集 | 788篇 |
教育与普及 | 291篇 |
理论与方法论 | 565篇 |
现状及发展 | 37461篇 |
研究方法 | 1554篇 |
综合类 | 28678篇 |
自然研究 | 2195篇 |
出版年
2013年 | 825篇 |
2012年 | 805篇 |
2011年 | 2934篇 |
2009年 | 636篇 |
2008年 | 1063篇 |
2007年 | 1207篇 |
2006年 | 1277篇 |
2005年 | 1418篇 |
2004年 | 2650篇 |
2003年 | 2167篇 |
2002年 | 1874篇 |
2001年 | 1963篇 |
2000年 | 1452篇 |
1999年 | 1300篇 |
1998年 | 655篇 |
1997年 | 778篇 |
1994年 | 690篇 |
1993年 | 692篇 |
1992年 | 1234篇 |
1991年 | 994篇 |
1990年 | 1123篇 |
1989年 | 960篇 |
1988年 | 883篇 |
1987年 | 969篇 |
1986年 | 982篇 |
1985年 | 1194篇 |
1984年 | 976篇 |
1983年 | 813篇 |
1982年 | 911篇 |
1981年 | 953篇 |
1980年 | 1074篇 |
1979年 | 1908篇 |
1978年 | 1689篇 |
1977年 | 1649篇 |
1976年 | 1397篇 |
1975年 | 1446篇 |
1974年 | 1602篇 |
1973年 | 1658篇 |
1972年 | 1727篇 |
1971年 | 1863篇 |
1970年 | 2156篇 |
1969年 | 1817篇 |
1968年 | 1782篇 |
1967年 | 1663篇 |
1966年 | 1401篇 |
1965年 | 1088篇 |
1959年 | 631篇 |
1958年 | 952篇 |
1957年 | 727篇 |
1956年 | 614篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
641.
642.
643.
644.
S. Y. Dike Meera Kamath J. R. Merchant 《Cellular and molecular life sciences : CMLS》1977,33(8):985-985
Summary A one-step synthesis of hexahydrocannabinoid analogs (HHC) is described making use of the condensation of phenolic ketones and aldehydes with citronellal in the presence of pyridine.Grateful thanks are accorded to Drs L. Chopard and S. Selvavinayakam for spectral measurements and Dr I. Fleming for useful discussion.R. Mechoulam and Y. Gaoni, Recent advances in the chemistry of Hashish, in: Progress in the Chemistry of Organic natural products, vol. 25, p. 175. Ed. L. Zechmeister. Springer Verlag, Wien 1967; R. Mechoulam, Marijuana Chemistry and Pharmacology, metabolism and clinical effects. Academic Press, New York 1973; R. Mechoulam, N. K. McCallum and S. Burstein, Chem. Rev.76, 75 (1976). 相似文献
645.
Proton pump coupled to cytochrome c oxidase in mitochondria. 总被引:1,自引:0,他引:1
M K Wikstrom 《Nature》1977,266(5599):271-273
646.
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
647.
L. Minale C. Pizza R. Riccio F. Zollo 《Cellular and molecular life sciences : CMLS》1983,39(6):569-571
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
648.
M. B. Castillo M. D. Gonzalez G. Burton C. P. Lantos A. Roldán 《Cellular and molecular life sciences : CMLS》1983,39(6):617-618
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
649.
M. Szmitkowski 《Cellular and molecular life sciences : CMLS》1983,39(7):771-772
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
650.
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