全文获取类型
收费全文 | 2363篇 |
免费 | 21篇 |
国内免费 | 62篇 |
专业分类
系统科学 | 42篇 |
丛书文集 | 35篇 |
教育与普及 | 1篇 |
理论与方法论 | 7篇 |
现状及发展 | 876篇 |
研究方法 | 58篇 |
综合类 | 1423篇 |
自然研究 | 4篇 |
出版年
2015年 | 17篇 |
2014年 | 26篇 |
2013年 | 29篇 |
2012年 | 63篇 |
2011年 | 70篇 |
2010年 | 55篇 |
2009年 | 43篇 |
2008年 | 87篇 |
2007年 | 89篇 |
2006年 | 84篇 |
2005年 | 106篇 |
2004年 | 99篇 |
2003年 | 50篇 |
2002年 | 34篇 |
2001年 | 67篇 |
2000年 | 75篇 |
1999年 | 61篇 |
1994年 | 259篇 |
1992年 | 37篇 |
1991年 | 21篇 |
1990年 | 35篇 |
1989年 | 33篇 |
1988年 | 34篇 |
1987年 | 31篇 |
1986年 | 29篇 |
1985年 | 44篇 |
1984年 | 29篇 |
1983年 | 18篇 |
1982年 | 21篇 |
1981年 | 19篇 |
1980年 | 27篇 |
1979年 | 60篇 |
1978年 | 23篇 |
1977年 | 38篇 |
1976年 | 28篇 |
1975年 | 28篇 |
1974年 | 38篇 |
1973年 | 43篇 |
1972年 | 43篇 |
1971年 | 42篇 |
1970年 | 39篇 |
1969年 | 38篇 |
1968年 | 42篇 |
1967年 | 46篇 |
1966年 | 30篇 |
1965年 | 21篇 |
1964年 | 18篇 |
1959年 | 14篇 |
1958年 | 16篇 |
1957年 | 17篇 |
排序方式: 共有2446条查询结果,搜索用时 15 毫秒
991.
992.
993.
Aged murine killer T-cell clones acquire specific cytotoxicity for P815 mastocytoma cells 总被引:3,自引:0,他引:3
T-cell clones that grow continuously in tissue culture have become a major tool for studying the properties of T lymphocytes. It is therefore important to know to what extent such clones resemble their normal counterparts. Several reports have appeared recently which demonstrate that long-term T-cell lines may lose the specificity for which they were initially selected and acquire cytotoxic activity to a variety of targets, typical of the activity displayed by natural killer cells. We now report a number of instances in which murine cytotoxic T-cell clones have lost their original specific cytotoxic activity but have acquired strong specific cytotoxic activity for P815 mastocytoma target cells. Loss of the original specificity was usually observed after continuous in vitro cultivation for more than 6 months. We propose that this novel type of cytotoxicity should be called aged killer activity. 相似文献
994.
P T Lomedico U Gubler C P Hellmann M Dukovich J G Giri Y C Pan K Collier R Semionow A O Chua S B Mizel 《Nature》1984,312(5993):458-462
Interleukin-1 (IL-1), a peptide hormone produced by activated macrophages, possesses the ability to modulate the proliferation, maturation and functional activation of a broad spectrum of cell types and may play a major role in the initiation and amplification of immune and inflammatory responses through its action on these diverse cell populations. IL-1 exhibits microheterogeneity in terms of its relative molecular mass (Mr, 13,000-19,000) and charge properties, and although murine IL-1 has been purified and some of its basic structure-function relationships have been elucidated, it has proved difficult to prepare sufficient amounts of IL-1 for direct and detailed sequence and structural studies. Here we report the cloning, sequence analysis and expression of murine IL-1 cDNA in Escherichia coli. The IL-1 cDNA codes for a polypeptide precursor of 270 amino acids. Biologically active IL-1 was produced in E. coli by expressing the carboxy-terminal 156 amino acids of the IL-1 precursor. 相似文献
995.
996.
C. G. de Almeida T. U. Grafe M. Guttenbach M. Schmid 《Cellular and molecular life sciences : CMLS》1990,46(5):509-511
Summary C-banding and mithramycin staining were used to characterize the karyotypes of 10 specimens of the African reed frogHyperolius viridiflavus ommatostictus from Tanzania. The diploid chromosome number is 2n=24. Although no heteromorphic sex chromosomes were present in the mitotic karyotypes, in many diakineses of male meiosis one or two bivalents exhibited an end-to-end arrangement. In the laboratory 7 out of 24 females changed sex spontaneously. This indicates that an XY/XX system of sex determination operates inH. viridiflavus ommatostictus.This study was supported by the Deutsche Forschungsgemeinschaft (Schm 484/2-4). We thank K. E. Linsenmair and C. M. Richards for helpful comments on an earlier draft. 相似文献
997.
Detection of caprine arthritis-encephalitis- and maedi-visna viruses using the polymerase chain reaction 总被引:3,自引:0,他引:3
Summary The polymerase chain reaction (PCR) was used to demonstrate proviral DNA of lentiviruses of small ruminants in cultured cells. Primers for the Taq polymerase were selected in the GAG gene of Icelandic maedi-visna virus and POL gene of caprine arthritis-encephalitis (CAE) virus. Using PCR, proviral DNA of CAE virus was detected at 1 day post infection, 4 days beforeviral protein could be demonstrated using a sensitive immunoblotting protocol and 6 days before the appearance of syncytia. Primers derived from the published sequence of CAE virus successfully primed for the synthesis of homologous virus and Icelandic maedi-visna viruss but not for maedi-visna virus isolated in The Netherlands. In contrast, primers derived from the GAG region of Icelandic maedi-visna virus allowed the amplification of DNA of homologous virus, maedi-visna virus isolated in The Netherlands as well as CAE virus. 相似文献
998.
R Mechoulam J J Feigenbaum N Lander M Segal T U J?rbe A J Hiltunen P Consroe 《Experientia》1988,44(9):762-764
The 1,1-dimethylheptyl homolog of (-)-(3R,4R)-7-hydroxy-delta-6- tetrahydrocannabinol (compound II) is highly psychotropic in mice, rats and pigeons. The (+)-(3S,4S) enantiomer (III) was found to be psychotropically inactive at doses up to several thousand times those of the ED50 of (II). 相似文献
999.
Summary Myosin and actin were localized in the adrenal gland, using antibodies against these proteins which were isolated from chicken gizzard. Myosin and actin were preferentially located in vascular walls including endothelial cells and in the capsule. In rat and guinea-pig adrenal cortex, the amount of contractile elements in vascular walls corresponded well to the density of adrenergic nerves as revealed with the glyoxylic acid method.Supported by grants from Deutsche Forschungsgemeinschaft. 相似文献
1000.
Riassunto È stata studiata l'azione tossica di estratti di ghiandola sottomascellare di topo. La tossicità dell'estratto totale appare la risultante di varie componenti alcune delle quali di natura proteica. Mediante gel-filtrazione, è stata isolata e parzialmente purificata una frazione che a dosi subletali provoca una marcata atrofia del timo in topolini neonati ed adulti. 相似文献