全文获取类型
收费全文 | 35755篇 |
免费 | 126篇 |
国内免费 | 171篇 |
专业分类
系统科学 | 173篇 |
丛书文集 | 478篇 |
教育与普及 | 55篇 |
理论与方法论 | 116篇 |
现状及发展 | 16333篇 |
研究方法 | 1496篇 |
综合类 | 16836篇 |
自然研究 | 565篇 |
出版年
2013年 | 239篇 |
2012年 | 535篇 |
2011年 | 1078篇 |
2010年 | 226篇 |
2008年 | 607篇 |
2007年 | 750篇 |
2006年 | 692篇 |
2005年 | 710篇 |
2004年 | 750篇 |
2003年 | 632篇 |
2002年 | 710篇 |
2001年 | 1083篇 |
2000年 | 1034篇 |
1999年 | 727篇 |
1992年 | 701篇 |
1991年 | 531篇 |
1990年 | 554篇 |
1989年 | 524篇 |
1988年 | 524篇 |
1987年 | 553篇 |
1986年 | 584篇 |
1985年 | 736篇 |
1984年 | 556篇 |
1983年 | 449篇 |
1982年 | 432篇 |
1981年 | 449篇 |
1980年 | 487篇 |
1979年 | 1159篇 |
1978年 | 953篇 |
1977年 | 837篇 |
1976年 | 740篇 |
1975年 | 733篇 |
1974年 | 983篇 |
1973年 | 820篇 |
1972年 | 908篇 |
1971年 | 1008篇 |
1970年 | 1344篇 |
1969年 | 1036篇 |
1968年 | 961篇 |
1967年 | 985篇 |
1966年 | 907篇 |
1965年 | 677篇 |
1964年 | 255篇 |
1959年 | 337篇 |
1958年 | 667篇 |
1957年 | 437篇 |
1956年 | 348篇 |
1955年 | 323篇 |
1954年 | 355篇 |
1948年 | 242篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
The genetic engineering of production traits in domestic animals 总被引:1,自引:0,他引:1
52.
Development and function of T cells in mice rendered interleukin-2 deficient by gene targeting. 总被引:69,自引:0,他引:69
Interleukin-2 (IL-2) is a lymphocytotropic hormone which is thought to have a key role in the immune response of mammalian cells. It is produced by a subpopulation of activated T-lymphocytes and acts in vitro as the principal auto- and paracrine T-cell growth factor (for reviews see refs 1-3). IL-2 is, however, not the sole T-cell growth factor, nor does it act exclusively on T cells, also promoting growth of NK cells and differentiation of B cells. A role for IL-2 in T-cell development has been postulated but remains controversial. Here we test the requirement for IL-2 in vivo using IL-2-deficient mice generated by targeted recombination. We find that mice homozygous for the IL-2 gene mutation are normal with regard to thymocyte and peripheral T-cell subset composition, but that a dysregulation of the immune system is manifested by reduced polyclonal in vitro T-cell responses and by dramatic changes in the isotype levels of serum immunoglobulins. 相似文献
53.
We develop in this paper an efficient way to select the best subset threshold autoregressive model. The proposed method uses a stochastic search idea. Differing from most conventional approaches, our method does not require us to fix the delay or the threshold parameters in advance. By adopting the Markov chain Monte Carlo techniques, we can identify the best subset model from a very large of number of possible models, and at the same time estimate the unknown parameters. A simulation experiment shows that the method is very effective. In its application to the US unemployment rate, the stochastic search method successfully selects lag one as the time delay and five best models from more than 4000 choices. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
54.
M E MacDonald A Novelletto C Lin D Tagle G Barnes G Bates S Taylor B Allitto M Altherr R Myers 《Nature genetics》1992,1(2):99-103
Analysis of 78 Huntington's disease (HD) chromosomes with multi-allele markers revealed 26 different haplotypes, suggesting a variety of independent HD mutations. The most frequent haplotype, accounting for about one third of disease chromosomes, suggests that the disease gene is between D4S182 and D4S180. However, the paucity of an expected class of chromosomes that can be related to this major haplotype by assuming single crossovers may reflect the operation of other mechanisms in creating haplotype diversity. Some of these mechanisms sustain alternative scenarios that do not require a multiple mutational origin for HD and/or its positioning between D4S182 and D4S180. 相似文献
55.
56.
57.
58.
59.
A. Valle 《Cellular and molecular life sciences : CMLS》1948,4(10):388-389
Riassunto Nella specieLycides idas L. si osservano variazioni numeriche dei cromosomi anche nelle spermatogonie di uno stesso individuo. Questo fatto è posto in relazione con la presenza nelle femmine dei caratteri secondari dei due sessi, come risultato dell'unione tra gameti con assetti sbilanciati. 相似文献
60.
1 Results In addition to single wall and multiwall carbon nanotubes[1], several structures,which are more or less related to fullerenes,including carbon nanohorns[2a], carbon nanospheres[2b] and onion like carbon structures[2c] have been reported.A new simple straight forward method to access some of these structures is the solid-state pyrolysis of different organometallic complexes in a sealed vessel,which led so far to carbon nanotubes[3a,b], carbon nanocables[3c] and onions[3d]. 相似文献