全文获取类型
收费全文 | 5650篇 |
免费 | 480篇 |
国内免费 | 4篇 |
专业分类
6134篇 |
出版年
2023年 | 31篇 |
2022年 | 86篇 |
2021年 | 130篇 |
2020年 | 58篇 |
2019年 | 71篇 |
2018年 | 92篇 |
2017年 | 80篇 |
2016年 | 141篇 |
2015年 | 218篇 |
2014年 | 232篇 |
2013年 | 295篇 |
2012年 | 404篇 |
2011年 | 327篇 |
2010年 | 227篇 |
2009年 | 190篇 |
2008年 | 297篇 |
2007年 | 302篇 |
2006年 | 262篇 |
2005年 | 234篇 |
2004年 | 226篇 |
2003年 | 184篇 |
2002年 | 170篇 |
2001年 | 184篇 |
2000年 | 162篇 |
1999年 | 130篇 |
1998年 | 60篇 |
1997年 | 44篇 |
1996年 | 28篇 |
1995年 | 40篇 |
1994年 | 37篇 |
1993年 | 36篇 |
1992年 | 93篇 |
1991年 | 77篇 |
1990年 | 81篇 |
1989年 | 86篇 |
1988年 | 61篇 |
1987年 | 63篇 |
1986年 | 68篇 |
1985年 | 68篇 |
1984年 | 55篇 |
1983年 | 49篇 |
1982年 | 41篇 |
1981年 | 33篇 |
1980年 | 33篇 |
1979年 | 43篇 |
1978年 | 31篇 |
1976年 | 28篇 |
1975年 | 28篇 |
1974年 | 34篇 |
1973年 | 36篇 |
排序方式: 共有6134条查询结果,搜索用时 0 毫秒
61.
62.
The predictability of leucocyte typing in kidney transplantation was assessed by an analysis of 37 kidney transplants from sibling donors. Recipients who were identical for the HL-A antigens with their donors gave highly predictable results. In comparison with those siblings who were incompatible or compatible but not identical their grafts functioned earlier, they required less immunosuppression, and had never had any rejections. They also appeared to have less postoperative morbidity. These results indicate that less immunosuppression than is current in many transplant centres could well be used with benefit in HL-A identical sibling transplants. This could reduce the risk of infection and possibly minimize the adverse effects of steroids on wound healing in these patients. 相似文献
63.
Coleoptile and root tips ofTriticum aestivum cv. Arnej 624 and those ofAvena sativa cv. Victory (Svalöf) as well as dry excised embryos ofTriticum aestivum cv. Rival (Svalöf) and those ofArachis hypogaea cv. 34 3A. H. were cultivated in media containing various concentrations of sucrose and growth regulators, like ascorbic acid, indole-3-acetic acid and gibberellin. Growth, differentiation and water uptake of the various explants were determined at regular time intervals. Further, the concentration of the endogenous ascorbic acid in mg./g. fresh weight, as well as the amount of this growth regulator utilized as per cent of the total were determined. Although all the three growth regulators promote growth in the explants, their effect is best felt when sucrose of a higher concentration (1.0 per cent) is added to the medium. In fact, the response to 1.0 per cent sucrose is sometimes as good as a combination of a growth regulator with sucrose, especially in the case of root explants. The results clearly indicate that the biosynthesis of ascorbic acid in the explants is catalyzed by the addition of indole-3-acetic acid as well as gibberellin. Simultaneously, the utilization of ascorbic acid is also appreciably increased by the presence of these growth regulators. Addition of 1.0 per cent sucrose to the medium containing the above mentioned growth regulators augments to a considerable extent not only the concentration of ascorbic acid, but also steps up its utilization. Enhancement of ascorbic acid as well as its increased utilization are correlated with rapid imbibition of water, growth and differentiation. The role of ascorbic acid in growth is discussed; and on the basis of the data presented here it is postulated that: (1) auxin and gibberellin function in the growth process by catalyzing the biosynthesis of ascorbic acid; and (2) that ascorbic acid not only participates in activation of various enzyme systems, but also stimulates the production of adenosine triphosphate by acting as an electron donor in photosynthetic phosphorylation as well as oxidative phosphorylation; (3) that the above action of ascorbic acid creates a favourable redox balance for synthesis of nucleic acids, proteins, enzymeproteins, and cell-wall constituents, thus enabling the processes of cell division and enlargement to proceed at a fast rate; and (4) that the relative rates of cell division and cell enlargement as well as “ageing” will determine the pattern of plant development. 相似文献
64.
65.
66.
Csillag A. Bourne R. C. Patel Sanjay N. Stewart Michael G. Tömböl Teréz 《Brain Cell Biology》1989,18(3):369-379
Brain Cell Biology - The distribution of GABA-like immunoreactivity (GABA-LI) in the ectostriatal core (Ec) of domestic chicks (one to two days old) was investigated using (1) preembedding GABA... 相似文献
67.
4-Carboxymethylamino-4-oxo-3-(4'-aminophenylamino) butanoic acid (25), its ethyl ester (26) and the corresponding unsubstituted-aryl analogues (17) and (16) are fairly potent inhibitors of enkephalinase (neutral endopeptidase; EC 3.4.24.11), Ki = 0.14-0.39 microM, with weak inhibitory potency, Ki = 15-75 microM, towards aminopeptidase MII. In the mouse abdominal constriction test, the esters (26) and (16) showed systemic inhibitory (antinociceptive) activity with ED50 values 62 +/- 3.05 and 81 +/- 1.74 mg/kg respectively. In the mouse tail immersion test, both (26) and (16) exhibited antinociceptive activity when administered intracerebroventricularly and (26) also exhibited a systemic effect which was only partially reversed by naltrexone. The antinociceptive effect seen with (26) reflects its ranking in vitro as an inhibitor of enkephalinase (Ki = 0.14 microM) but it is possible that this effect is not totally opioid-mediated. Compounds (26) and (16) represent the first combined inhibitors of enkephalinase and aminopeptidase MII. 相似文献
68.
Lineage, migration, and morphogenesis of longitudinal glia in the Drosophila CNS as revealed by a molecular lineage marker 总被引:4,自引:0,他引:4
Previous studies described three different classes of glial cells in the developing CNS of the early Drosophila embryo that prefigure and ensheath the major CNS axon tracts. Among these are 6 longitudinal glial cells on each side of each segment that overlie the longitudinal axon tracts. Here we use transformant lines carrying a P element containing a 130 bp sequence from the fushi tarazu gene in front of the lacZ reporter gene to direct beta-galactosidase expression in the longitudinal glia. Using this molecular lineage marker, we show that 1 of the "neuroblasts" in each hemisegment is actually a glioblast, which divides once symmetrically, in contrast to the typical asymmetric neuroblast divisions, producing 2 glial cells, which migrate medially and divide to generate the 6 longitudinal glial cells. As with neuroblasts, mutations in Notch and other neurogenic genes lead to supernumerary glioblasts. The results indicate that the glioblast is similar to other neuroblasts; however, the positionally specified fate of this blast cell is to generate a specific lineage of glia rather than a specific family of neurons. 相似文献
69.
Insulin receptor monoclonal antibodies that mimic insulin action without activating tyrosine kinase 总被引:9,自引:0,他引:9
D M Hawley B A Maddux R G Patel K Y Wong P W Mamula G L Firestone A Brunetti E Verspohl I D Goldfine 《The Journal of biological chemistry》1989,264(5):2438-2444
HTC rat hepatoma cells were transfected with human insulin receptor cDNA to a level of 40,000 receptors/cell. In these cells, as well as in nontransfected cells, insulin stimulated the uptake of alpha-aminoisobutyric acid. Two monoclonal antibodies directed against the human insulin receptor alpha subunit, like insulin, stimulated amino acid uptake in transfected HTC cells, but not in nontransfected HTC cells. The antibodies, in contrast to insulin, failed to stimulate insulin receptor tyrosine kinase activity, both in intact transfected cells and in cell free extracts prepared from them. These data suggest, therefore, that activation of insulin receptor tyrosine kinase may not be an obligatory step in all of the transmembrane signaling mechanisms of the insulin receptor. 相似文献
70.
Physical and genetic mapping of the protein A gene in the chromosome of Staphylococcus aureus 8325-4 总被引:13,自引:0,他引:13
The gene coding for protein A (spa) has been mapped close to nov on the genetic map of the chromosome of Staphylococcus aureus 8325-4. A rapid mapping procedure has been developed which first allowed the region of the chromosome carrying the spa gene to be identified by blot +hybridization of large DNA fragments which had been separated by pulsed-field gel electrophoresis. Restriction endonuclease SmaI fragment G was shown to carry the spa gene. An insertion mutation in spa was constructed by in vitro insertion of a fragment of DNA expressing resistance to kanamycin and neomycin. A spa::Kan(r)Neo(r) mutation was isolated in S. aureus 8325-4 by allele replacement. This provided a selectable marker which allowed the spa gene to be mapped by transformation analysis. 相似文献