全文获取类型
收费全文 | 95599篇 |
免费 | 20687篇 |
国内免费 | 37篇 |
出版年
2022年 | 348篇 |
2021年 | 1327篇 |
2020年 | 2713篇 |
2019年 | 4321篇 |
2018年 | 4659篇 |
2017年 | 4853篇 |
2016年 | 5469篇 |
2015年 | 6558篇 |
2014年 | 6320篇 |
2013年 | 7746篇 |
2012年 | 6460篇 |
2011年 | 6139篇 |
2010年 | 6053篇 |
2009年 | 4473篇 |
2008年 | 4630篇 |
2007年 | 4306篇 |
2006年 | 4089篇 |
2005年 | 4197篇 |
2004年 | 4070篇 |
2003年 | 3892篇 |
2002年 | 3833篇 |
2001年 | 1080篇 |
2000年 | 800篇 |
1999年 | 978篇 |
1998年 | 1030篇 |
1997年 | 740篇 |
1996年 | 742篇 |
1995年 | 740篇 |
1994年 | 699篇 |
1993年 | 749篇 |
1992年 | 653篇 |
1991年 | 592篇 |
1990年 | 506篇 |
1989年 | 512篇 |
1988年 | 526篇 |
1987年 | 444篇 |
1986年 | 429篇 |
1985年 | 507篇 |
1984年 | 608篇 |
1983年 | 510篇 |
1982年 | 588篇 |
1981年 | 552篇 |
1980年 | 484篇 |
1979年 | 347篇 |
1978年 | 413篇 |
1977年 | 382篇 |
1976年 | 317篇 |
1975年 | 311篇 |
1974年 | 318篇 |
1973年 | 292篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Lauren Petrullo Matthew J. Jorgensen Noah Snyder‐Mackler Amy Lu 《American journal of primatology》2019,81(10-11)
The human milk microbiome is vertically transmitted to offspring during the postnatal period and has emerged as a critical driver of infant immune and metabolic development. Despite this importance in humans, the milk microbiome of nonhuman primates remains largely unexplored. This dearth of comparative work precludes our ability to understand how species‐specific differences in the milk microbiome may differentially drive maternal effects and limits how translational models can be used to understand the role of vertically transmitted milk microbes in human development. Here, we present the first culture‐independent data on the milk microbiome of a nonhuman primate. We collected milk and matched fecal microbiome samples at early and late lactation from a cohort of captive lactating vervet monkeys (N = 15). We found that, similar to humans, the vervet monkey milk microbiome comprises a shared community of taxa that are universally present across individuals. However, unlike in humans, this shared community is dominated by the genera Lactobacillus, Bacteroides, and Prevotella. We also found that, in contrast to previous culture‐dependent studies in humans, the vervet milk microbiome exhibits greater alpha‐diversity than the gut microbiome across lactation. Finally, we did not find support for the translocation of microbes from the gut to the mammary gland within females (i.e., “entero‐mammary pathway”). Taken together, our results show that the vervet monkey milk microbiome is taxonomically diverse, distinct from the gut microbiome, and largely stable. These findings demonstrate that the milk microbiome is a unique substrate that may selectively favor the establishment and persistence of particular microbes across lactation and highlights the need for future experimental studies on the origin of microbes in milk. 相似文献
122.
Rats were trained to run spontaneously, without stress, in running wheels. The running activity increased gradually and could reach a plateau of 7 km/night after 3–4 weeks. During the first hour of running in the dark phase the squeak threshold increased significantly and remained high in the morning. The degree of increased threshold was correlated to the amount of running activity. The squeak threshold declined during the following 6 hours of inactivity. A rapid decrease in threshold occurred after naloxone (1–2 mg/kg i.p.). It is suggested that long-lasting muscle exercise (e.g. jogging), acupuncture, and low frequency electrical stimulation of afferent nerve fibres produce discharges in muscle afferents which influence central endorphin mechanics giving analgetic effects. 相似文献
123.
124.
125.
Alexander Wiedenmann Peter Dimroth Christoph von Ballmoos 《Molecular microbiology》2009,72(2):479-490
F1 F0 ATP synthases use the electrochemical potential of H+ or Na+ across biological membranes to synthesize ATP by a rotary mechanism. In bacteria, the enzymes can act in reverse as ATP-driven ion pumps creating the indispensable membrane potential. Here, we demonstrate that the F0 parts of a Na+ - and H+ -dependent enzyme display major asymmetries with respect to their mode of operation, reflected by the requirement of ∼100 times higher Na+ or H+ concentrations for the synthesis compared with the hydrolysis of ATP. A similar asymmetry is observed during ion transport through isolated F0 parts, indicating different affinities for the binding sites in the a/c interface. Together with further data, we propose a model that provides a rationale for a differential usage of membrane potential and ion gradient during ATP synthesis as observed experimentally. The functional asymmetry might also reflect an important property of the ATP synthesis mechanism in vivo . In Escherichia coli , we observed respiratory chain-driven ATP production at pH 7–8, while P -site pH values < 6.5 were required for ATP synthesis in vitro . This discrepancy is discussed with respect to the hypothesis that during respiration lateral proton diffusion could lead to significant acidification at the membrane surface. 相似文献
126.
Philippe Fernandez‐Fournier Samantha Straus Ruth Sharpe Leticia Avils 《Ecological Entomology》2019,44(2):157-162
1. Manipulation of host behaviour by parasitoids has long captured the imagination of ecologists. Parasitoid wasps in the Polysphincta group of genera develop as external parasitoids of spiders. 2. In the present study, the previously undescribed interaction between a Zatypota sp. wasp (Ichneumonidae) and a social spider Anelosimus eximius (Theridiidae) is described. The larva of this Zatypota wasp is found to induce its host to disperse from their communal web and build an entirely enclosed web consisting of densely spun silk. 3. The wasp is observed to target primarily immature A. eximius individuals, with 37.5–44% of nests in a given area being parasitised. Of those nests, approximately 1.3–2.0% of individuals are hosts to the parasitoid larvae. Larger spider colonies had a significantly higher probability of harbouring parasitoids. 4. This interaction results in unusual behaviours for A. eximius induced by the parasitoid: (i) leaving the protection of the social nest and (ii) building a unique, altered web that it would not otherwise build. It is suggested that the wasp may be tapping into ancestral dispersal behaviours in its host and that a social species provides this wasp an evolutionary advantage by allowing a stable host source. 相似文献
127.
128.
129.
David Gutirrez‐Larruscain Santiago Andrs‐Snchez Enrique Rico María Montserrat Martínez‐Ortega 《植物分类学报:英文版》2019,57(1):42-54
Forty-five populations of Pentanema corresponding to seven species included in the Pentanema conyzae clade have been studied using AFLP fingerprinting. The results show that allopolyploidization could have been involved in the diversification of this group, specifically in species P. langeanum and P. maletii. Molecular data confirm the presence of P. britannicum in the Iberian Peninsula and key steps are provided to identify the species that are morphologically the most challenging. 相似文献
130.
Abstract: Slices cut from five frozen human brains were dissected into 2-mm cubes and assayed for choline acetyltransferase (ChAT) activity and protein content. A pattern of enrichment of ChAT activity was found ventral to the anterior commissure; this finding is consistent with the location of the enzyme in the cells of the nucleus basalis of Meynert. The region beneath the anterior commissure was the only place a discrete enrichment of activity could be found, and the precise topography of the enrichment was somewhat variable from brain to brain. The results are discussed in the light of recent knowledge concerning the source of the cortical cholinergic innervation. 相似文献