全文获取类型
收费全文 | 76192篇 |
免费 | 6010篇 |
国内免费 | 5450篇 |
专业分类
87652篇 |
出版年
2024年 | 183篇 |
2023年 | 979篇 |
2022年 | 2324篇 |
2021年 | 3803篇 |
2020年 | 2590篇 |
2019年 | 3127篇 |
2018年 | 3055篇 |
2017年 | 2302篇 |
2016年 | 3288篇 |
2015年 | 4666篇 |
2014年 | 5573篇 |
2013年 | 5800篇 |
2012年 | 6941篇 |
2011年 | 6259篇 |
2010年 | 3826篇 |
2009年 | 3433篇 |
2008年 | 4002篇 |
2007年 | 3585篇 |
2006年 | 3098篇 |
2005年 | 2606篇 |
2004年 | 2104篇 |
2003年 | 1976篇 |
2002年 | 1561篇 |
2001年 | 1263篇 |
2000年 | 1181篇 |
1999年 | 1092篇 |
1998年 | 680篇 |
1997年 | 593篇 |
1996年 | 578篇 |
1995年 | 533篇 |
1994年 | 489篇 |
1993年 | 396篇 |
1992年 | 540篇 |
1991年 | 420篇 |
1990年 | 368篇 |
1989年 | 326篇 |
1988年 | 263篇 |
1987年 | 250篇 |
1986年 | 220篇 |
1985年 | 198篇 |
1984年 | 138篇 |
1983年 | 145篇 |
1982年 | 104篇 |
1981年 | 56篇 |
1980年 | 59篇 |
1979年 | 84篇 |
1976年 | 62篇 |
1974年 | 64篇 |
1973年 | 53篇 |
1972年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Enantiomers of diastereomeric benzo[a]pyrene (BP) diol-epoxides, r-7,t-8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydro-BP (BP 7,8-diol-anti-9,10-epoxide), r-7,t-8-dihydroxy-c-9,10-epoxy-7,8,9,10-tetrahydro-BP (BP 7,8-diol-syn-9,10-epoxide), r-9,t-10-dihydroxy-t-7,8-epoxy-7,8,9,10-tetrahydro-BP (BP 9,10-diol-anti-7,8-epoxide), and several 7,8,9,10-tetrahydroxy-7,8,9,10-tetrahydrobenzo[a]pyrenes (BP tetrols) were resolved by high-performance liquid chromatography (HPLC) using columns packed with either (R)-N-(3,5-dinitrobenzoyl)phenylglycine[(R)-DNBPG] or (S)-N-(3,5-dinitrobenzoyl)leucine [(S)-DNBL], which is either ionically or covalently bonded to gamma-aminopropylsilanized silica. Resolution of enantiomers was confirmed by ultraviolet-visible absorption and circular dichroism spectral analyses. Resolved enantiomers of BP diol-epoxides were each hydrolyzed in acidic solution to a pair of diastereomeric tetrols which were separated by reversed-phase HPLC. Absolute stereochemistries of enantiomeric diol-epoxides were deduced by the absolute configuration of their hydrolysis products. 相似文献
82.
83.
The objectives of the present study were: 1) to develop a simple and more efficient technique for sperm microinjection than is currently available, using the rabbit as a model, and 2) to evaluate the development of rabbit oocytes fertilized by single or multiple sperm microinjection. Hyperosmotic sucrose in phosphate-buffered saline (SPBS) was employed to dehydrate oocytes to increase the perivitelline space for sperm microinjection and prevent possible injury to the vitellus. In the first experiment, 58% (n = 29) oocytes treated with 0.5 M SPBS developed to morulae following multiple sperm microinjection compared, respectively, to 47% (n = 34) and 60% (n = 15) for control IVF with or without sucrose exposure (P greater than 0.05). Blastocyst development from microinjected oocytes, however, was much lower (P less than 0.05) than that of controls (14% vs. 42% and 40%, respectively). Sham operation by puncturing the zona pellucida of the sucrose-treated oocytes with the microinjection pipette did not increase parthenogenesis (P greater than 0.05). In Experiment 2 a smaller-size injection pipette and shorter sucrose exposure time after sperm microinjection resulted in 41% (n = 42) of the oocytes developing into blastocysts for the microinjection group, whereas only 21% (n = 24) developed to blastocysts in the control IVF group (P less than 0.05). When relatively older oocytes (17 hr post ovulation injection) were used to test if microinjection could reduce the time to fertilization and cleavage (Expt. 3), an average of 27% (n = 63) blastocysts resulted from microinjection vs. 0% (n = 28) for the control IVF group. 相似文献
84.
Dissociated embryonic chick dorsal root ganglion (DRG) neurons maintained in culture exhibit a mixed Na+/Ca2+ action potential. The characteristic "shoulder" on the repolarizing phase is due to the relatively prolonged inward Ca2+ current. DRG neurons grown in an elevated K+ medium (25 versus. 5 mM) lack the plateau phase of the action potential. Voltage-clamp analysis showed that this plastic change in action potential duration is not due to the loss of the inward Ca2+ current but is partly due to the appearance of a Ca2(+)-dependent, 4-aminopyridine-(4-AP)-sensitive transient outward current. Faster activation of the purely voltage-dependent delayed rectifier outward current also contributes to the rapid repolarization observed in neurons cultured in elevated K+ medium. 相似文献
85.
Long-chain unsaturated fatty acids (UFA) having up to six double bonds are derivatized to 2-substituted 4,4-dimethyloxazolines (DMOX) and then analyzed by combined in-beam electron impact (IBEI)-B/E-linked scan mass spectrometry. This technique provides highly characteristic mass spectra and may serve as an auxiliary means for direct structure determination of individual UFA in mixtures. 相似文献
86.
We developed an efficient neural network algorithm for solving the Multiple Traveling Salesmen Problem (MTSP). A new transformation of the N-city M-salesmen MTSP to the standard Traveling Salesmen Problem (TSP) is introduced. The transformed problem is represented by an expanded version of Hopfield-Tank's neuromorphic city-position map with (N + M-1)-cities and a single fictitious salesmen. The dynamic model associated with the problem is based on the Basic Differential Multiplier Method (BDMM) [26] which evaluates Lagrange multipliers simultaneously with the problem's state variables. The algorithm was successfully tested on many problems with up to 30 cities and five salesmen. In all test cases, the algorithm always converged to valid solutions. The great advantage of this kind of algorithm is that it can provide solutions to complex decision making problems directly by solving a system of ordinary differential equations. No learning steps, logical if statements or adjusting of parameters are required during the computation. The algorithm can therefore be implemented in hardware to solve complex constraint satisfaction problems such as the MTSP at the speed of analog silicon VLSI devices or possibly future optical neural computers. 相似文献
87.
向日葵离体孤雌生殖的超微结构研究 总被引:5,自引:1,他引:4
本文是研究未受精胚珠培养诱导的孤雌生殖过程超微结构变化的首次报道。向日葵(Heliaanthus annuus L.)的卵细胞在离体条件下被激活,发生细胞核移位、极性丧失、细胞器增多并转变成活动状态、液泡化程度增大、合点端形成细胞壁等一系列变化,预示即将启动孤雌生殖。孤雌生殖的原胚具有若干显著特征,如极性颠倒、有自体吞噬活动、壁的自由生长、游离核分裂等。对这些现象作了初步的讨论。 相似文献
88.
Cardiac and vascular actions of sarafotoxin S6b and endothelin-1 总被引:3,自引:0,他引:3
Snake venom-derived sarafotoxin S6B (SRT) and porcine endothelium-derived endothelin-1 (ET) have striking structural similarities. In conscious, freely-moving rats, ET (0.67 nmol/kg) produced a transient tachycardia and fall in arterial blood pressure which was followed by a long-lasting increase in arterial pressure, bradycardia, decrease in cardiac output (CO) and marked increase in total peripheral resistance. In contrast, SRT (0.67 nmol/kg) produced only the sustained cardiovascular responses. The sustained cardiovascular effects of SRT or ET were similarly attenuated by nifedipine. SRT and ET (30 nM) produced vasoconstriction in the isolated perfused mesenteric vascular bed without initial vasodilation. SRT and ET had potent positive inotropic and negative chronotropic effects on isolated perfused hearts and induced toxic reactions including coronary vasospasm, arrhythmias, A-V block and ventricular fibrillation. In addition to SRT lacking the initial depressor response in vivo, several differences in the activities of the peptides were also observed. ET produced greater and longer-lasting actions than SRT in producing pressor and vasoconstrictor responses in all 3 preparations, and in its ability to induce toxic effects on the heart. 相似文献
89.
Water-storage capacity was measured inThuja occidentalis L.,Tsuga canadensis (L.) Carr., andAcer saccharum Marsh. during the dehydration of stem segments 1.5–2.5 cm in diameter. Stem water potential was measured with a temperature-corrected
stem hygrometer and cavitations were detected acoustically. Water loss was measured by weight change. Dehydration isotherms
consistently displayed three phases. The first phase, from water potential (Ψ) 0 to about −0.2 MPa, had a high capacitance
(C>0.4kg water lost· (1 of tissue)−1· MPa−1) and we have attributed this high C to capillary water as defined by Zimmermann (1983, Xylem structure and the ascent of
sap, Springer-Verlag). The second phase from Ψ=−0.5 to about −2.0 had the lowest C values (<0.02 kg·l−1·MPa−1) and was accompanied by a few cavitation events. This phase may have been a transition zone between capillary storage and
water released by cavitation events as well as water drawn from living cells of the bark. The third phase also had a high
C (about 0.07–0.22kg·l−1·MPa−1) and was associated with many cavitation events while Ψ declined below about −2.5 MPa; we presume the high capacitance was
the consequence of water released by cavitation events. We discuss the ecological adaptive advantage of these three phases
of water-storage in trees. In moist environments, water withdrawn from capillary storage may be an important fraction of transpiration,
but may be of little adaptive advantage. For most of the growth season trees draw mainly on elastic storage, but stem elastic
storage is less than leaf elastic storage and therefore unlikely to be important. In very dry environments, water relased
by cavitation events might be important to the short-term survival of trees. 相似文献
90.
Functional antagonism between oncoprotein c-Jun and the glucocorticoid receptor 总被引:121,自引:0,他引:121
R Schüle P Rangarajan S Kliewer L J Ransone J Bolado N Yang I M Verma R M Evans 《Cell》1990,62(6):1217-1226