全文获取类型
收费全文 | 33206篇 |
免费 | 54篇 |
国内免费 | 91篇 |
专业分类
系统科学 | 197篇 |
丛书文集 | 718篇 |
教育与普及 | 89篇 |
理论与方法论 | 234篇 |
现状及发展 | 13723篇 |
研究方法 | 1504篇 |
综合类 | 16440篇 |
自然研究 | 446篇 |
出版年
2013年 | 214篇 |
2012年 | 537篇 |
2011年 | 1060篇 |
2010年 | 232篇 |
2008年 | 700篇 |
2007年 | 702篇 |
2006年 | 743篇 |
2005年 | 775篇 |
2004年 | 712篇 |
2003年 | 718篇 |
2002年 | 749篇 |
2001年 | 982篇 |
2000年 | 909篇 |
1999年 | 606篇 |
1992年 | 574篇 |
1991年 | 465篇 |
1990年 | 487篇 |
1989年 | 494篇 |
1988年 | 490篇 |
1987年 | 499篇 |
1986年 | 492篇 |
1985年 | 600篇 |
1984年 | 498篇 |
1983年 | 411篇 |
1982年 | 355篇 |
1981年 | 349篇 |
1980年 | 452篇 |
1979年 | 970篇 |
1978年 | 862篇 |
1977年 | 852篇 |
1976年 | 580篇 |
1975年 | 631篇 |
1974年 | 933篇 |
1973年 | 790篇 |
1972年 | 806篇 |
1971年 | 1022篇 |
1970年 | 1345篇 |
1969年 | 1005篇 |
1968年 | 947篇 |
1967年 | 990篇 |
1966年 | 830篇 |
1965年 | 611篇 |
1964年 | 148篇 |
1959年 | 360篇 |
1958年 | 522篇 |
1957年 | 443篇 |
1956年 | 366篇 |
1955年 | 316篇 |
1954年 | 363篇 |
1948年 | 193篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
21.
Activation of a C. elegans Antennapedia homologue in migrating cells controls their direction of migration. 总被引:1,自引:0,他引:1
Anterior-posterior patterning in insects, vertebrates and nematodes involves members of conserved Antennapedia-class homeobox gene clusters (HOM-C) that are thought to give specific body regions their identities. The effects of these genes on region-specific body structures have been described extensively, particularly in Drosophila, but little is known about how HOM-C genes affect the behaviours of cells that migrate into their domains of function. In Caenorhabditis elegans, the Antennapedia-like HOM-C gene mab-5 not only specifies postembryonic fates of cells in a posterior body region, but also influences the migration of mesodermal and neural cells that move through this region. Here we show that as one neuroblast migrates into this posterior region, it switches on mab-5 gene expression; mab-5 then acts as a developmental switch to control the migratory behaviour of the neuroblast descendants. HOM-C genes can therefore not only direct region-specific patterns of cell division and differentiation, but can also act within migrating cells to programme region-specific migratory behaviour. 相似文献
22.
In this paper two alternative loss criteria for the least squares Procrustes problem are studied. These alternative criteria
are based on the Huber function and on the more radical biweight function, which are designed to be resistant to outliers.
Using iterative majorization it is shown how a convergent reweighted least squares algorithm can be developed. In asimulation
study it turns out that the proposed methods perform well over a specific range of contamination. When a uniform dilation
factor is included, mixed results are obtained. The methods also yield a set of weights that can be used for diagnostic purposes. 相似文献
23.
Studies of intracellular traffic in yeast and mammalian systems have implicated members of the Rab family of small GTP-binding proteins as regulators of membrane fusion. We have used the patch clamp technique to measure exocytotic fusion events directly and investigate the role of GTP-binding proteins in regulating exocytosis in mast cells. Intracellular perfusion of mast cells with GTP-gamma S is sufficient to trigger complete exocytotic degranulation in the absence of other intracellular messengers. Here we show that GTP is a potent inhibitor of GTP-gamma S-induced degranulation, indicating that sustained activation of a GTP-binding protein is sufficient for membrane fusion. We have found that synthetic oligopeptides, corresponding to part of the effector domain of Rab3a, stimulate complete exocytotic degranulation, similar to that induced by GTP-gamma S. The response is selective for Rab3a sequence and is strictly dependent on Mg2+ and ATP. This suggests that sustained activation of a Rab3 protein causes exocytotic fusion. The peptide response can be accelerated by GDP-beta S, suggesting that Rab3a peptides compete with endogenous Rab3 proteins for a binding site on a target effector protein, which causes fusion on activation. 相似文献
24.
25.
Shiga toxin and some other protein toxins that act on targets in the cytosol have previously been shown to enter the trans-Golgi network. Transport by this route may be necessary for translocation of the toxin to the cytosol and for intoxication, but it is not known whether the enzymatically active part of the toxins actually enters the cytosol from the trans-Golgi network. It has been suggested that such toxins are transported in a retrograde manner to the endoplasmic reticulum and that translocation occurs in this organelle, but retrograde transport of endocytosed material beyond the trans-Golgi network has never been demonstrated. Here we show that in butyric acid-treated A431 cells endocytosed Shiga toxin is not only transported to the trans-Golgi network, but also to all Golgi stacks, to the endoplasmic reticulum and to the nuclear envelope. Furthermore, butyric acid sensitizes the cells to Shiga toxin, which is consistent with the possibility that retrograde transport is required for translocation of the toxin to the cytosol. 相似文献
26.
27.
28.
29.
David H. Hubel 《自然科学进展(英文版)》2007,17(13):1-3
The visual field is topographically mapped onto the primary visual cortex (V1), forming a retinotopic map which is far more detailed for the foveal regions than for the periphery. We found that receptive field (RF) size in monkey V1 increases with eccentricity, and that a 1—2 m2 V1 region contains roughly a complete set of machinery necessary to analyze a visual-field area whose size is about that of the RFs of the cells. This allows V1 to be anatomically uniform, and is in sharp contrast with the retina. 相似文献
30.
Kang J.Y. Xu F.C. 《中国科学基金(英文版)》2004,(2):57-58,64
1 Introduction
Nanoscale science and technology have attracted greatattention since the novel properties become dominant for well-known materials as their sizes reduced to some critical dimension. These properties frequently originate in lattice distortions, structure transformations etc[1]. The variations of size and structure ofnanocrystals lead to the change of their electronic structures. Very recently, nanoscale materials with different electronic structures were utilized in fabricating nanoscale junctions[2].
…… 相似文献