全文获取类型
收费全文 | 435篇 |
免费 | 0篇 |
国内免费 | 1篇 |
专业分类
系统科学 | 8篇 |
理论与方法论 | 1篇 |
现状及发展 | 85篇 |
研究方法 | 36篇 |
综合类 | 286篇 |
自然研究 | 20篇 |
出版年
2017年 | 3篇 |
2014年 | 3篇 |
2013年 | 3篇 |
2012年 | 17篇 |
2011年 | 38篇 |
2010年 | 8篇 |
2009年 | 2篇 |
2008年 | 20篇 |
2007年 | 15篇 |
2006年 | 15篇 |
2005年 | 13篇 |
2004年 | 10篇 |
2003年 | 15篇 |
2002年 | 12篇 |
2001年 | 25篇 |
2000年 | 21篇 |
1999年 | 9篇 |
1995年 | 3篇 |
1993年 | 1篇 |
1992年 | 15篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 10篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1980年 | 7篇 |
1979年 | 1篇 |
1978年 | 11篇 |
1977年 | 5篇 |
1976年 | 8篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1973年 | 6篇 |
1972年 | 4篇 |
1971年 | 13篇 |
1970年 | 20篇 |
1969年 | 5篇 |
1968年 | 10篇 |
1967年 | 10篇 |
1966年 | 6篇 |
1965年 | 4篇 |
1964年 | 3篇 |
1963年 | 2篇 |
1960年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有436条查询结果,搜索用时 328 毫秒
411.
The status of Wnt signalling regulates neural and epidermal fates in the chick embryo 总被引:6,自引:0,他引:6
Wilson SI Rydström A Trimborn T Willert K Nusse R Jessell TM Edlund T 《Nature》2001,411(6835):325-330
The acquisition of neural fate by embryonic ectodermal cells is a fundamental step in the formation of the vertebrate nervous system. Neural induction seems to involve signalling by fibroblast growth factors (FGFs) and attenuation of the activity of bone morphogenetic protein (BMP). But FGFs, either alone or in combination with BMP antagonists, are not sufficient to induce neural fate in prospective epidermal ectoderm of amniote embryos. These findings suggest that additional signals are involved in the specification of neural fate. Here we show that the state of Wnt signalling is a critical determinant of neural and epidermal fates in the chick embryo. Continual Wnt signalling blocks the response of epiblast cells to FGF signals, permitting the expression and signalling of BMP to direct an epidermal fate. Conversely, a lack of exposure of epiblast cells to Wnt signals permits FGFs to induce a neural fate. 相似文献
412.
Naish TR Woolfe KJ Barrett PJ Wilson GS Atkins C Bohaty SM Bücker CJ Claps M Davey FJ Dunbar GB Dunn AG Fielding CR Florindo F Hannah MJ Harwood DM Henrys SA Krissek LA Lavelle M van Der Meer J McIntosh WC Niessen F Passchier S Powell RD Roberts AP Sagnotti L Scherer RP Strong CP Talarico F Verosub KL Villa G Watkins DK Webb PN Wonik T 《Nature》2001,413(6857):719-723
Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3-4 degrees C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1-23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event). 相似文献
413.
Dynamics of travelling waves in visual perception 总被引:7,自引:0,他引:7
Nonlinear wave propagation is ubiquitous in nature, appearing in chemical reaction kinetics, cardiac tissue dynamics, cortical spreading depression and slow wave sleep. The application of dynamical modelling has provided valuable insights into the mechanisms underlying such nonlinear wave phenomena in several domains. Wave propagation can also be perceived as sweeping waves of visibility that occur when the two eyes view radically different stimuli. Termed binocular rivalry, these fluctuating states of perceptual dominance and suppression are thought to provide a window into the neural dynamics that underlie conscious visual awareness. Here we introduce a technique to measure the speed of rivalry dominance waves propagating around a large, essentially one-dimensional annulus. When mapped onto visual cortex, propagation speed is independent of eccentricity. Propagation speed doubles when waves travel along continuous contours, thus demonstrating effects of collinear facilitation. A neural model with reciprocal inhibition between two layers of units provides a quantitative explanation of dominance wave propagation in terms of disinhibition. Dominance waves provide a new tool for investigating fundamental cortical dynamics. 相似文献
414.
Surprisingly few birds have penises, but among those that do, the Argentine lake duck (Oxyura vittata) tops the bill - the penis of this small stifftail duck from South America is shaped like a corkscrew and, at almost half a metre long, is the largest of any bird measured so far. Factors responsible for the evolution of this remarkable organ could include runaway selection, whereby drakes with longer penises gain dominance and copulate with more females, or preference by females for drakes with longer and more decorated penises. 相似文献
415.
416.
417.
418.
419.
420.
DNA sequences from the quagga, an extinct member of the horse family 总被引:18,自引:0,他引:18
To determine whether DNA survives and can be recovered from the remains of extinct creatures, we have examined dried muscle from a museum specimen of the quagga, a zebra-like species (Equus quagga) that became extinct in 1883 (ref. 1). We report that DNA was extracted from this tissue in amounts approaching 1% of that expected from fresh muscle, and that the DNA was of relatively low molecular weight. Among the many clones obtained from the quagga DNA, two containing pieces of mitochondrial DNA (mtDNA) were sequenced. These sequences, comprising 229 nucleotide pairs, differ by 12 base substitutions from the corresponding sequences of mtDNA from a mountain zebra, an extant member of the genus Equus. The number, nature and locations of the substitutions imply that there has been little or no postmortem modification of the quagga DNA sequences, and that the two species had a common ancestor 3-4 Myr ago, consistent with fossil evidence concerning the age of the genus Equus. 相似文献