全文获取类型
收费全文 | 9461篇 |
免费 | 665篇 |
国内免费 | 3篇 |
专业分类
10129篇 |
出版年
2022年 | 48篇 |
2021年 | 102篇 |
2020年 | 68篇 |
2019年 | 84篇 |
2018年 | 109篇 |
2017年 | 95篇 |
2016年 | 190篇 |
2015年 | 299篇 |
2014年 | 362篇 |
2013年 | 508篇 |
2012年 | 533篇 |
2011年 | 560篇 |
2010年 | 403篇 |
2009年 | 335篇 |
2008年 | 478篇 |
2007年 | 540篇 |
2006年 | 499篇 |
2005年 | 533篇 |
2004年 | 492篇 |
2003年 | 471篇 |
2002年 | 482篇 |
2001年 | 133篇 |
2000年 | 96篇 |
1999年 | 122篇 |
1998年 | 164篇 |
1997年 | 106篇 |
1996年 | 111篇 |
1995年 | 124篇 |
1994年 | 118篇 |
1993年 | 123篇 |
1992年 | 92篇 |
1991年 | 86篇 |
1990年 | 105篇 |
1989年 | 97篇 |
1988年 | 71篇 |
1987年 | 82篇 |
1986年 | 71篇 |
1985年 | 77篇 |
1984年 | 83篇 |
1983年 | 83篇 |
1982年 | 65篇 |
1981年 | 79篇 |
1980年 | 62篇 |
1979年 | 61篇 |
1978年 | 55篇 |
1977年 | 53篇 |
1976年 | 50篇 |
1975年 | 44篇 |
1974年 | 43篇 |
1973年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
42.
M Fischbach 《Journal of immunology (Baltimore, Md. : 1950)》1984,133(5):2365-2368
The effect of the single autosomal recessive gene lpr on antigen presentation was studied. MRL/Mp-lpr/lpr, C3H/HeJ-lpr/lpr, C57BL/6J-lpr/lpr, and their normal congenic partners were investigated. Mice bearing the lpr gene were unable to respond to TNP-KLH when presented by syngeneic antigen-presenting cells. The congenic normal partners gave a brisk response. Mixing experiments demonstrated that the defect resided with the lpr responding T cell and not with the lpr antigen-presenting cell. Antigen-presenting cells from lpr mice were capable of inducing T cell proliferation in normal congenic partners, whereas antigen-presenting cells from normal mice failed to stimulate lpr T cells. This defect was intrinsic to an Lyt-1+2- cell. Pharmacologic restoration was attempted by in vivo and in vitro administration of interleukin 2. However, cells from lpr mice remained unaffected. The relationship of these findings to autoimmunity is discussed. 相似文献
43.
Albrecht Guhlmann Wolfgang Hagmann Dietrich Keppler 《Prostaglandins & other lipid mediators》1987,34(1)
N-Acetyl-leukotriene E4, the end product of leukotriene C4 metabolism in the mercapturic acid pathway, was rapidly eliminated from the blood circulation into the bile of rats. Part of the N-acethyl-leukotriene E4 secreted from bile into the intestine undewent enterohepatic circulation. Leukotriene absorption occurred from the small intestine and from the colon. Biliary and urinary excretion within 5.5 h amounted to 15 and 2%, respectively, of the intraduodenally administered N-acetyl- H leukotriene E4 in animals anesthetized with ketamine. HPLC analyses indicated that 35% of the biliary radioactivity corresponded to unchanged N-acetyl- H leukotriene E4, while 65% in bile and 100% in urine were polar metabolites. Enterohepatic circulation extends the biological half-life of N-acetyl-leukotriene E4. 相似文献
44.
Prof. Dr. med. Wolfgang Schulze Martin Riemer Uwe Rehder Karl-Heinz Höhne 《Cell and tissue research》1986,244(1):1-8
Summary With the use of a digital image-processing method three-dimensional reconstructions of the arrangement of spermatocytes in human seminiferous tubules were performed. With this method it was possible to investigate the cellular distribution in the tubule in nearly any given perspective and projection. In addition, by means of simple mathematical procedures, such as by transformation of Cartesian coordinates into cylindrical coordinates, it was possible to vary the shape of a reconstruction, i.e., to convert the cylindrical image of a tubular portion into a right-angled r--z-representation.The present work not only confirms the existence of a complex helical plan of organization of the human seminiferous epithelium but also provides further aspects of the phenomenon of physiological germ-cell loss and its integration into the kinetics of spermatogenesis.Dedicated to Prof. E.C. Roosen-Runge, Seattle, on the occasion of his 75th birthday 相似文献
45.
James P. Chalcroft Harald Engelhardt Wolfgang Baumeister 《Archives of microbiology》1986,144(3):196-200
The regular surface layer of Pseudomonas acidovorans was investigated by computer processing of a series of tilted view electron micrographs, and a reconstruction of the three-dimensional structure was obtained. The pattern is tetragonal and consists of massive identical subunits, block-like in face-view, which interlock loosely in a simple cobblestone pattern. The square unit cell has a lattice constant of 11 nm. The surface layer pattern of P. acidovorans appears to be more dependent on the underlying membrane for maintaining its integrity than those so far studied in other bacteria. 相似文献
46.
Abstract: Membranes of the secretory vesicles from bovine adrenal medulla were investigated for the presence of the endogenous protein phosphorylation activity. Seven phosphoprotein bands in the molecular weight range of 250,000 to 30,000 were observed by means of the sodium dodecyl sulphate electrophoresis and autoradiography. On the basis of the criteria of molecular weight, selective stimulation of the phosphorylation by cyclic AMP (as compared with cyclic GMP) and immunoprecipitation by specific antibodies, band 5 (molecular weight 60,300) was found to represent the phosphorylated form of the secretory vesicle-bound tyrosine hydroxylase. The electrophoretic mobility, the stimulatory and inhibitory effects of cyclic AMP in presence of Mg2+ and Zn,2+ respectively, and immunoreactivity toward antibodies showed band 6 to contain two forms of the regulatory subunits of the type II cyclic AMP-dependent protein kinase, distinguishable by their molecular weights (56,000 and 52,000, respectively). Phosphorylation of band 7 (molecular weight 29,800) was stimulated about 2 to 3 times by Ca2+ and calmodulin in the concentration range of both agents believed to occur in the secretory tissues under physiological conditions. 相似文献
47.
In addition to known derivatives, four new sesquiterpene-coumarin ethers were isolated from the roots of Achillea ochroleuca and Artemisia tripartita and identified by 1H and 13C NMR spectroscopy, including lanthanide induced shifts. The new compounds are isofraxidin derived ethers which differ from the previously described derivatives by ring cleavage and methyl migration within the terpenoid unit. The chemosystematic importance of sesquiterpene-coumarin ether accumulation within the two genera is briefly discussed. 相似文献
48.
In various applications one faces the problem of estimating a signal from discontinuous observations. For example, in biomedical applications the signal may be the ‘state’ of a given organ and one observes through an external counter the amount of radioactivity sequestered by the organ after injection of a radioactive tracer. Here the problem is studied in the context of nonlinear filtering when the signal can be modelled as either a random variable or a diffusion process, and the observations have a continuous and a purely discontinuous component; both components may be affected by the signal. When the signal is a random variable an explicitly computable solution is obtained; for the diffusion case the solution is given as a sequence of approximating filters that can be computed recursively. 相似文献
49.
A phylogenetic analysis of Legionella 总被引:2,自引:0,他引:2
Four species of Legionella, L. pneumophila NCTC 11192, L. bozemanii NCTC 11368, L. micdadei NCTC 11371 and L. jordanis ATCC 33623 have been characterized by oligonucleotide cataloguing of their 16S ribosomal RNA. All four species are phylogenetically closely related, while no specific relationship could be detected with any other group of organisms investigated so far with respect to this method. At a low level of relationship legionellae are members of the broad group of purple photosynthetic bacteria and their non-phototrophic relatives, in which Legionella form an independent line of descent. 相似文献
50.
Summary Sieve-tube exudate which appears on cut surfaces of stems of Cucurbita maxima as distinct droplets has been depicted in electron micrographs of longitudinal sections of the phloem. The exudate, which was produced from mature sieve tubes only, contained filaments of P-protein, but no mitochondria or vesicles of endoplasmic reticulum. The water-soluble part of the exudate contained at least 12 proteins, as shown by disc-electrophoresis. Enzymic activity was found for peroxidases, acid phosphatases, and aldolases. Color tests and assays for other enzymes, including ATPase, fructokinase, several dehydrogenases, and UDP-glucose: D-fructose-2-glucosyl transferase, gave negative results. With repeated cutting of a stem, the protein content of the exudate increased, while the amount of exudate decreased.Supported by Deutsche Forschungsgemeinschaft and Stiftung Volkswagenwerk. During part of this investigation the senior author held a U.S. National Science Foundation Senior Foreign Scientist Fellowship at the University of Wisconsin. 相似文献