全文获取类型
收费全文 | 4260篇 |
免费 | 347篇 |
国内免费 | 3篇 |
出版年
2021年 | 50篇 |
2020年 | 29篇 |
2019年 | 35篇 |
2018年 | 56篇 |
2017年 | 44篇 |
2016年 | 87篇 |
2015年 | 121篇 |
2014年 | 139篇 |
2013年 | 220篇 |
2012年 | 286篇 |
2011年 | 270篇 |
2010年 | 199篇 |
2009年 | 214篇 |
2008年 | 264篇 |
2007年 | 265篇 |
2006年 | 221篇 |
2005年 | 242篇 |
2004年 | 264篇 |
2003年 | 218篇 |
2002年 | 195篇 |
2001年 | 61篇 |
2000年 | 42篇 |
1999年 | 51篇 |
1998年 | 60篇 |
1997年 | 56篇 |
1996年 | 34篇 |
1995年 | 35篇 |
1994年 | 33篇 |
1993年 | 45篇 |
1992年 | 43篇 |
1991年 | 48篇 |
1990年 | 35篇 |
1989年 | 25篇 |
1988年 | 33篇 |
1987年 | 30篇 |
1986年 | 34篇 |
1985年 | 32篇 |
1984年 | 41篇 |
1983年 | 31篇 |
1982年 | 33篇 |
1981年 | 40篇 |
1980年 | 26篇 |
1979年 | 25篇 |
1978年 | 20篇 |
1977年 | 38篇 |
1976年 | 18篇 |
1975年 | 21篇 |
1974年 | 27篇 |
1973年 | 18篇 |
1970年 | 16篇 |
排序方式: 共有4610条查询结果,搜索用时 15 毫秒
1.
2.
3.
Shaun P. Collin Wayne L. Davies Nathan S. Hart David M. Hunt 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2009,364(1531):2925-2940
Meeting the challenge of sampling an ancient aquatic landscape by the early vertebrates was crucial to their survival and would establish a retinal bauplan to be used by all subsequent vertebrate descendents. Image-forming eyes were under tremendous selection pressure and the ability to identify suitable prey and detect potential predators was thought to be one of the major drivers of speciation in the Early Cambrian. Based on the fossil record, we know that hagfishes, lampreys, holocephalans, elasmobranchs and lungfishes occupy critical stages in vertebrate evolution, having remained relatively unchanged over hundreds of millions of years. Now using extant representatives of these ‘living fossils’, we are able to piece together the evolution of vertebrate photoreception. While photoreception in hagfishes appears to be based on light detection and controlling circadian rhythms, rather than image formation, the photoreceptors of lampreys fall into five distinct classes and represent a critical stage in the dichotomy of rods and cones. At least four types of retinal cones sample the visual environment in lampreys mediating photopic (and potentially colour) vision, a sampling strategy retained by lungfishes, some modern teleosts, reptiles and birds. Trichromacy is retained in cartilaginous fishes (at least in batoids and holocephalans), where it is predicted that true scotopic (dim light) vision evolved in the common ancestor of all living gnathostomes. The capacity to discriminate colour and balance the tradeoff between resolution and sensitivity in the early vertebrates was an important driver of eye evolution, where many of the ocular features evolved were retained as vertebrates progressed on to land. 相似文献
4.
The endospore‐forming bacteria have persisted on earth perhaps 3Ga, leveraging the flexibility of their distinctive lifestyle to adapt to a remarkably wide range of environments. This process of adaptation can be investigated through the simple but powerful technique of laboratory evolution. Evolved strains can be analyzed by whole genome sequencing and an array of omics technologies. The intensively studied, genetically tractable endospore‐former, Bacillus subtilis, is an ideal subject for laboratory evolution experiments. Here, we describe the use of the B. subtilis model system to study the adaptation of these bacteria to reduced and stringent selection for endospore formation, as well as to novel environmental challenges of low atmospheric pressure, high ultraviolet radiation, and unfavourable growth temperatures. In combination with other approaches, including comparative genomics and environmental field work, laboratory evolution may help elucidate how these bacteria have so successfully adapted to life on earth, and perhaps beyond. 相似文献
5.
Harriet M. Jackson Kristen D. Onos Keating W. Pepper Leah C. Graham Ellen C. Akeson Candice Byers Laura G. Reinholdt Wayne N. Frankel Gareth R. Howell 《PloS one》2015,10(5)
Alzheimer’s disease (AD) is a leading cause of dementia in the elderly and is characterized by amyloid plaques, neurofibrillary tangles (NFTs) and neuronal dysfunction. Early onset AD (EOAD) is commonly caused by mutations in amyloid precursor protein (APP) or genes involved in the processing of APP including the presenilins (e.g. PSEN1 or PSEN2). In general, mouse models relevant to EOAD recapitulate amyloidosis, show only limited amounts of NFTs and neuronal cell dysfunction and low but significant levels of seizure susceptibility. To investigate the effect of genetic background on these phenotypes, we generated APPswe and PSEN1de9 transgenic mice on the seizure prone inbred strain background, DBA/2J. Previous studies show that the DBA/2J genetic background modifies plaque deposition in the presence of mutant APP but the impact of PSEN1de9 has not been tested. Our study shows that DBA/2J.APPswePSEN1de9 mice are significantly more prone to premature lethality, likely to due to lethal seizures, compared to B6.APPswePSEN1de9 mice—70% of DBA/2J.APPswePSEN1de9 mice die between 2-3 months of age. Of the DBA/2J.APPswePSEN1de9 mice that survived to 6 months of age, plaque deposition was greatly reduced compared to age-matched B6.APPswePSEN1de9 mice. The reduction in plaque deposition appears to be independent of microglia numbers, reactive astrocytosis and complement C5 activity. 相似文献
6.
7.
8.
9.
10.