全文获取类型
收费全文 | 3609篇 |
免费 | 451篇 |
专业分类
4060篇 |
出版年
2021年 | 37篇 |
2019年 | 34篇 |
2017年 | 37篇 |
2016年 | 58篇 |
2015年 | 86篇 |
2014年 | 99篇 |
2013年 | 136篇 |
2012年 | 168篇 |
2011年 | 175篇 |
2010年 | 80篇 |
2009年 | 83篇 |
2008年 | 129篇 |
2007年 | 148篇 |
2006年 | 144篇 |
2005年 | 138篇 |
2004年 | 116篇 |
2003年 | 127篇 |
2002年 | 112篇 |
2001年 | 98篇 |
2000年 | 127篇 |
1999年 | 108篇 |
1998年 | 33篇 |
1997年 | 55篇 |
1996年 | 42篇 |
1995年 | 39篇 |
1994年 | 42篇 |
1993年 | 40篇 |
1992年 | 70篇 |
1991年 | 58篇 |
1990年 | 65篇 |
1989年 | 76篇 |
1988年 | 75篇 |
1987年 | 80篇 |
1986年 | 71篇 |
1985年 | 73篇 |
1984年 | 63篇 |
1983年 | 50篇 |
1982年 | 39篇 |
1981年 | 41篇 |
1980年 | 42篇 |
1979年 | 57篇 |
1978年 | 41篇 |
1977年 | 44篇 |
1976年 | 47篇 |
1975年 | 53篇 |
1974年 | 57篇 |
1973年 | 49篇 |
1971年 | 42篇 |
1970年 | 40篇 |
1967年 | 31篇 |
排序方式: 共有4060条查询结果,搜索用时 0 毫秒
61.
Procedure for Evaluating the Effects of 2,450- Megahertz Microwaves upon Streptococcus faecalis and Saccharomyces cerevisiae 总被引:5,自引:4,他引:1 下载免费PDF全文
Modifications of a commercial 2,450-megahertz microwave oven were made so that 6 ml of microbial suspension could be exposed to the microwave field for various periods of time. The microorganisms were contained in the central tube of a modified Liebig condenser positioned in the approximate geometric center of the oven cavity. Kerosene at -25 C was circulated through the jacket of the condenser during microwave exposure permitting microwaves to reach the microbial suspension. Flow rates of the kerosene were varied to permit the temperature of the suspension to range from 25 to 55 C during microwave exposure. Conductive heating experiments using similar temperatures were also conducted. A thermocouple-relay system was employed to measure the suspension temperature immediately after the magnetron shutoff. Continuous application of microwaves to suspensions of 10(8) to 10(9)Streptococcus faecalis or Saccharomyces cerevisiae per ml appeared to produce no lethal effects other than those produced by heat. Respiration rates of microwave-exposed Scerevisiae were directly related to decreases in viable count produced by increased microwave exposure times. 相似文献
62.
D. P. Fox 《Chromosoma》1967,20(4):386-412
Irradiation of Schistocerca gregaria embryo cells during the S and G2 stages of interphase produces the same general classes of chromatid aberration as have been seen in organisms, such as Vicia faba, which have been used widely in radiocytological work. In addition, however, a series of intrachange aberrations has been found which is novel. In order to account for these Revell's exchange hypothesis has been extended to include the involvement of isolocus primary lesions. The exchange-type hypothesis is preferred because it can more readily accomodate the whole spectrum of aberrations found. 相似文献
63.
64.
Expression and function of the UM4D4 antigen in human thymus 总被引:3,自引:0,他引:3
D A Fox L S Chan L Kan O Baadsgaard K D Cooper 《Journal of immunology (Baltimore, Md. : 1950)》1989,143(7):2166-2175
UM4D4 is a newly identified T cell surface molecule, distinct from the Ag receptor and CD2, which is expressed on 25% of peripheral blood T cells, resting or activated. Monoclonal anti-UM4D4 is mitogenic for T cells and T cell clones. Since alternative activation pathways independent of Ag/MHC recognition may be important in thymic differentiation, the expression and function of UM4D4 was examined in human thymus. UM4D4 was found on the surface of 6% of thymocytes. All thymocyte subsets contained UM4D4+ cells but expression was greatest on thymocytes that were CD1- (12%), CD3+ (11%) and especially CD4-CD8- (18%). CD3+CD4- CD8- cells, most of which bear the gamma delta-receptor, were greater than or equal to 50% + for UM4D4. Moreover, anti-UM4D4 was comitogenic for thymocytes together with PMA or IL-2. Anti-UM4D4 also reacted strongly with a subset of thymic epithelial cells in both cortex and medulla. Dual color fluorescence microscopy, with anti-UM4D4 and antibodies to other thymic epithelial Ag, showed UM4D4 expression on neuroendocrine thymic epithelium but not on thymic fibrous stroma. Thus, UM4D4 is expressed on, and represents an activation pathway for, a subset of thymic T cells. In addition, this determinant, initially identified as a novel T cell activating molecule, is broadly expressed by neuroendocrine thymic epithelium. Although the function of UM4D4 on the thymic epithelial cells is not yet clear, it is possible that UM4D4 represents a pathway for the functional activation of a subset of the thymic epithelium as well as a subset of thymocytes, thus playing a dual role in T cell differentiation. 相似文献
65.
B S Fox D Dordai R L Moore B E Lacy 《Journal of immunology (Baltimore, Md. : 1950)》1989,143(12):3887-3893
This paper describes an adjuvant-free immunization regimen that results in the priming of T cells but not B cells. B10.A mice were primed s.c. with syngeneic spleen cells that had been pulsed with the peptide 81-104 derived from pigeon cytochrome c. The T cell response was measured by using a sensitive limiting dilution assay that measures lymphokine production. The precursor frequency of Ag-specific cells found in these mice was indistinguishable from the frequency found in mice primed in the footpads with 81-104 in CFA. A striking difference in antibody induction was found, however, when these two immunization regimens were compared. Mice primed with 81-104 in CFA developed significant serum antibody responses against the peptide, whereas mice primed with Ag-pulsed spleen cells produced no detectable anti-peptide antibodies. This lack of antibody did not result from detectable differences in the T cells that were primed: no differences were seen in IL-2 and IL-4 production or in the ability to provide help to B cells in vitro. In vitro stimulation with LPS suggested that the B cells were not primed by the Ag-pulsed spleen cells. The B cells were not tolerized, however, because boosting the mice with Ag in CFA resulted in the induction of an antibody response. The failure to induce an antibody response by priming with Ag-pulsed spleen cells was not caused by the site of immunization or the total amount of Ag used for priming. The critical variable may be the introduction of the Ag on the surface of an APC; in this form, B cell Ag recognition was apparently inefficient, whereas T cell Ag recognition was optimal. 相似文献
66.
Purification and characterization of the adenosine A2-like binding site from human placental membrane 总被引:1,自引:0,他引:1
We have purified and characterized the adenosine A2-like binding site from human placental membranes. 5'-N-Ethylcarboxamido[2,8-3H]adenosine ([3H]NECA) binds to this site, with a Kd of 240 nM and a Bmax of 13.0 pmol/mg in human placental membranes. The adenosine A2-like binding site was purified after extraction from placental membranes with 0.1% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid. The purification included ammonium sulfate precipitation and concanavalin A, DEAE-Sephadex, and Sepharose 6B gel filtration chromatographies. The protein was purified 127-fold to homogeneity, with a final specific activity of 1.5-1.9 nmol/mg of protein and a 5.5-8.1% yield of binding activity from the membranes. The purified protein had similar binding properties and an identical potency order for displacement of [3H] NECA by adenosine analogs as the initial membranes. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of purified protein revealed a single band at 98 kDa which coeluted with [3H]NECA binding activity during Sepharose 6B gel filtration chromatography. In 0.1% Triton X-100, the binding complex has a Stokes radius of 70 A, a sedimentation coefficient of 6.9 S, and a partial specific volume of 0.698 ml/g. The detergent-protein complex has a calculated molecular mass of 230 kDa. The estimated frictional ratio is 1.5. The native binding complex appears to consist of a dimer of identical subunits. The function of this ubiquitous protein remains unclear. 相似文献
67.
Molecular characterization of a major autoantibody-associated cross-reactive idiotype in Sjogren's syndrome 总被引:8,自引:0,他引:8
T J Kipps E Tomhave P P Chen R I Fox 《Journal of immunology (Baltimore, Md. : 1950)》1989,142(12):4261-4268
Primary Sjogren's syndrome is an autoimmune disorder characterized by lymphocytic infiltration of the salivary and lacrimal glands, producing associated dry eyes (keratoconjunctivitis sicca), dry mouth, and intermittently swollen salivary glands. A high proportion of the infiltrating B lymphocytes express surface and cytoplasmic Ig bearing a kappa-L chain-associated CRI defined by reactivity with the murine mAb, 17.109. To determine the structural basis for CRI expression in this disease, we generated CRI+ lymphoblastoid cell lines and a cDNA library from lymphocytes extracted from Sjogren's syndrome patients' salivary gland biopsy specimens. Nucleic acid sequence analyses of the mRNA of one such 17.109-CRI+ lymphoblastoid cell line (NOV) reveals the expressed kappa light chain variable region gene (V kappa gene) to be homologous to Humkv325, a conserved V kappa gene used at relatively high frequency in certain B cell malignancies. In addition, synthetic oligonucleotides, corresponding to the first and third frameworks and the second complementarity determining region of the Humkv325 gene, were used to identify and isolate clones from a cDNA library generated from SS salivary gland lymphocytes. Clones annealing specifically with one or more of these oligonucleotide probes contained kappa light chain cDNA. The sequences corresponding to the variable region of two clones (Taykv320 and Taykv306) were homologous to Humkv325. The V kappa genes of four other cDNA clones (Taykv322, Taykv310, Taykv308, and Taykv312) most likely were generated somatically from the rearranged Humkv325 gene through a limited number of nucleic acid base substitutions. Our results suggest that the high frequency of 17.109-CRI expression in Sjogren's syndrome patients results from a multiclonal expansion of B cells using Humkv325, and that the expressed Humkv325 may undergo somatic diversification in an apparent Ag-driven response. 相似文献
68.
Rapid in situ hybridization technique using 16S rRNA segments for detecting and differentiating the closely related gram-positive organisms Bacillus polymyxa and Bacillus macerans. 总被引:8,自引:5,他引:3 下载免费PDF全文
A rapid, sensitive, inexpensive in situ hybridization technique, using 30-mer 16S rRNA probes, can specifically differentiate two closely related Bacillus spp., B. polymyxa and B. macerans. The 16S rRNA probes were labeled with a rhodamine derivative (Texas Red), and quantitative fluorescence measurements were made on individual bacterial cells. The microscopic fields analyzed were selected by phase-contrast microscopy, and the fluorescence imaging analyses were performed on 16 to 67 individual cells. The labeled 16S rRNA probe, POL, whose sequence was a 100% match with B. polymyxa 16S rRNA but only a 60% match with B. macerans 16S rRNA, gave quantitative fluorescence ratio measurements that were 34.8-fold higher for B. polymyxa cells than for B. macerans cells. Conversely, the labeled probe, MAC, which matched B. polymyxa 16S rRNA in 86.6% of its positions and B. macerans 16S rRNA in 100% of its positions, gave quantitative fluorescence measurements that were 59.3-fold higher in B. macerans cells than in B. polymyxa cells. Control probes, whose 16S rRNA sequence segment (P-M) was present in both B. polymyxa and B. macerans as well as a panprokaryotic probe (16S), having a 100% match with all known bacteria, hybridized equally well with both organisms. These latter hybridizations generated very high fluorescence signals, but their comparative fluorescence ratios (the differences between two organisms) were low. The control paneukaryotic probe (28S), which had less than 30% identity for both B. macerans and B. polymyxa, did not hybridize with either organism. 相似文献
69.
Egg size variation often has large effects on the fitness of progeny in insects. However, many studies have been unable to detect an advantage of developing from large eggs, suggesting that egg size variation has implications for offspring performance only under adverse conditions, such as during larval competition, periods of starvation, desiccation, or when larvae feed on low-quality resources. We test this hypothesis by examining the consequences of egg size variation for survivorship and development of a seed-feeding insect, Stator limbatus, on both a low-quality (Cercidium floridum) and a high-quality (Acacia greggii) host plant. Our results are consistent with the hypothesis. S. limbatus larval performance was affected by egg size only when developing on the poor-quality host (C. floridum); larvae from large eggs survived better on C. floridum than those from small eggs, while there was no evidence of an effect of egg size on progeny development time, body weight, or survivorship when larvae developed on A. greggii. These results indicate intense selection for large eggs within C. floridum-associated populations, but not in A. greggii-associated populations, so that egg size is predicted to vary among populations associated with different hosts. Our results also support this hypothesis; females from a C. floridum-associated population (Scottsdale) laid larger eggs than females from an A. greggii-associated population (Black Canyon City). 相似文献
70.
Acceleration of cheese ripening 总被引:14,自引:0,他引:14
P. F. Fox J. M. Wallace S. Morgan C. M. Lynch E. J. Niland J. Tobin 《Antonie van Leeuwenhoek》1996,70(2-4):271-297
The characteristic aroma, flavour and texture of cheese develop during ripening of the cheese curd through the action of numerous enzymes derived from the cheese milk, the coagulant, starter and non-starter bacteria. Ripening is a slow and consequently an expensive process that is not fully predictable or controllable. Consequently, there are economic and possibly technological incentives to accelerate ripening. The principal methods by which this may be achieved are: an elevated ripening temperature, modified starters, exogenous enzymes and cheese slurries. The advantages, limitations, technical feasibility and commercial potential of these methods are discussed and compared. 相似文献