全文获取类型
收费全文 | 35036篇 |
免费 | 76篇 |
国内免费 | 118篇 |
专业分类
系统科学 | 338篇 |
丛书文集 | 720篇 |
教育与普及 | 87篇 |
理论与方法论 | 116篇 |
现状及发展 | 15314篇 |
研究方法 | 1400篇 |
综合类 | 16658篇 |
自然研究 | 597篇 |
出版年
2013年 | 200篇 |
2012年 | 487篇 |
2011年 | 1078篇 |
2010年 | 181篇 |
2008年 | 552篇 |
2007年 | 633篇 |
2006年 | 669篇 |
2005年 | 662篇 |
2004年 | 634篇 |
2003年 | 626篇 |
2002年 | 555篇 |
2001年 | 1067篇 |
2000年 | 1033篇 |
1999年 | 654篇 |
1992年 | 632篇 |
1991年 | 543篇 |
1990年 | 578篇 |
1989年 | 501篇 |
1988年 | 520篇 |
1987年 | 557篇 |
1986年 | 538篇 |
1985年 | 691篇 |
1984年 | 558篇 |
1983年 | 450篇 |
1982年 | 370篇 |
1981年 | 396篇 |
1980年 | 521篇 |
1979年 | 1119篇 |
1978年 | 916篇 |
1977年 | 882篇 |
1976年 | 701篇 |
1975年 | 782篇 |
1974年 | 1050篇 |
1973年 | 927篇 |
1972年 | 913篇 |
1971年 | 1113篇 |
1970年 | 1462篇 |
1969年 | 1098篇 |
1968年 | 914篇 |
1967年 | 1014篇 |
1966年 | 916篇 |
1965年 | 648篇 |
1964年 | 165篇 |
1959年 | 386篇 |
1958年 | 612篇 |
1957年 | 466篇 |
1956年 | 419篇 |
1955年 | 351篇 |
1954年 | 382篇 |
1948年 | 282篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
801.
802.
A selective deficit for writing vowels in acquired dysgraphia 总被引:2,自引:0,他引:2
Brain-damaged patients with acquired writing disorders provide important information about the normal processes of spelling and writing. Current models indicate that to produce a letter string, its 'abstract' representation is computed and stored in a temporary orthographic buffer, from which it is converted to a verbal code (if the word is to be spelled aloud) or to a physical letter code (if the word is to be written). The stored graphemic representations specify the identity and order of the component letters and their consonant/vowel status. Here I describe the spelling performance of two patients with a selective deficit in writing vowels. When writing words, the first patient omitted all vowels, leaving a blank space between consonants or consonant clusters, whereas the second produced errors that almost exclusively involved vowels. This pattern of performance supports the hypothesis that the consonant/vowel status of graphemes is differentially specified in the spelling process and may be selectively affected after brain damage. 相似文献
803.
A B cell-deficient mouse by targeted disruption of the membrane exon of the immunoglobulin mu chain gene 总被引:117,自引:0,他引:117
Of the various classes of antibodies that B lymphocytes can produce, class M (IgM) is the first to be expressed on the membrane of the developing cells. Pre-B cells, the precursors of B-lymphocytes, produce the heavy chain of IgM (mu chain), but not light chains. Recent data suggest that pre-B cells express mu chains on the membrane together with the 'surrogate' light chains lambda 5 and V pre B (refs 2-7). This complex could control pre-B-cell differentiation, in particular the rearrangement of the light-chain genes. We have now assessed the importance of the membrane form of the mu chain in B-cell development by generating mice lacking this chain. We disrupted one of the membrane exons of the gene encoding the mu-chain constant region by gene targeting in mouse embryonic stem cells. From these cells we derived mice heterozygous or homozygous for the mutation. B-cell development in the heterozygous mice seemed to be normal, but in homozygous animals B cells were absent, their development already being arrested at the stage of pre-B-cell maturation. 相似文献
804.
New use of BCG for recombinant vaccines 总被引:147,自引:0,他引:147
C K Stover V F de la Cruz T R Fuerst J E Burlein L A Benson L T Bennett G P Bansal J F Young M H Lee G F Hatfull 《Nature》1991,351(6326):456-460
BCG, a live attenuated tubercle bacillus, is the most widely used vaccine in the world and is also a useful vaccine vehicle for delivering protective antigens of multiple pathogens. Extrachromosomal and integrative expression vectors carrying the regulatory sequences for major BCG heat-shock proteins have been developed to allow expression of foreign antigens in BCG. These recombinant BCG strains can elicit long-lasting humoral and cellular immune responses to foreign antigens in mice. 相似文献
805.
806.
Complex patterns formed by motile cells of Escherichia coli 总被引:6,自引:0,他引:6
When chemotactic strains of the bacterium Escherichia coli are inoculated on semi-solid agar containing mixtures of amino acids or sugars, the cells swarm outwards in a series of concentric rings: they respond to spatial gradients of attractants generated by uptake and catabolism. Cells also drift up gradients generated artificially, for example by diffusion from the tip of a capillary tube or by mixing. Here we describe conditions under which cells aggregate in response to gradients of attractant which they excrete themselves. When cells are grown in semi-solid agar on intermediates of the tricarboxylic acid cycle, they form symmetrical arrays of spots or stripes that arise sequentially. When cells in a thin layer of liquid culture are exposed to these compounds, spots appear synchronously, more randomly arrayed. In either case, the patterns are stationary. The attractant is a chemical sensed by the aspartate receptor. Its excretion can be triggered by oxidative stress. As oxygen is limiting at high cell densities, aggregation might serve as a mechanism for collective defence. 相似文献
807.
Erythroid differentiation in chimaeric mice blocked by a targeted mutation in the gene for transcription factor GATA-1 总被引:135,自引:0,他引:135
L Pevny M C Simon E Robertson W H Klein S F Tsai V D'Agati S H Orkin F Costantini 《Nature》1991,349(6306):257-260
808.
Antigen presentation. The second class story 总被引:4,自引:0,他引:4
R N Germain 《Nature》1991,353(6345):605-607
809.
810.
The bcl-2 gene encodes a novel G protein 总被引:5,自引:0,他引:5
Little is known about the biochemical or functional nature of the proteins encoded by the bcl-2 gene, which undergoes chromosomal translocation in approximately 85% of follicular lymphoma, 20% of diffuse large cell lymphoma and 10% of chronic lymphocytic leukaemia of B cells. Translocation of bcl-2 sequences from chromosome 18 to the JH segment of the immunoglobulin gene at chromosome band 14q32 in B cells results in deregulated expression of this gene, causing high steady state levels of bcl-2 messenger RNA2. DNA sequence data indicate that bcl-2 encodes two proteins by virtue of alternative splicing, designated as Bcl-2 alpha and Bcl-2 beta, with relative molecular masses of 26,000 and 22,000 respectively. Cell fractionation experiments indicate that the bcl-2 alpha gene product is located at the inner surface of the cell membrane, suggesting a possible role in mitogenic signal transduction. We report here that Bcl-2 alpha has GTP-binding activity and a protein sequence that suggests it belongs to the small molecular weight GTP-binding protein (G protein) family. 相似文献