全文获取类型
收费全文 | 5042篇 |
免费 | 249篇 |
国内免费 | 292篇 |
专业分类
5583篇 |
出版年
2023年 | 68篇 |
2022年 | 81篇 |
2021年 | 109篇 |
2020年 | 131篇 |
2019年 | 141篇 |
2018年 | 95篇 |
2017年 | 113篇 |
2016年 | 118篇 |
2015年 | 135篇 |
2014年 | 223篇 |
2013年 | 322篇 |
2012年 | 190篇 |
2011年 | 241篇 |
2010年 | 168篇 |
2009年 | 186篇 |
2008年 | 244篇 |
2007年 | 238篇 |
2006年 | 219篇 |
2005年 | 194篇 |
2004年 | 190篇 |
2003年 | 177篇 |
2002年 | 149篇 |
2001年 | 158篇 |
2000年 | 146篇 |
1999年 | 145篇 |
1998年 | 110篇 |
1997年 | 101篇 |
1996年 | 91篇 |
1995年 | 97篇 |
1994年 | 95篇 |
1993年 | 84篇 |
1992年 | 62篇 |
1991年 | 62篇 |
1990年 | 46篇 |
1989年 | 57篇 |
1988年 | 54篇 |
1987年 | 53篇 |
1986年 | 56篇 |
1985年 | 57篇 |
1984年 | 35篇 |
1983年 | 37篇 |
1982年 | 44篇 |
1981年 | 29篇 |
1980年 | 46篇 |
1979年 | 31篇 |
1978年 | 34篇 |
1977年 | 21篇 |
1976年 | 18篇 |
1975年 | 24篇 |
1972年 | 17篇 |
排序方式: 共有5583条查询结果,搜索用时 11 毫秒
81.
Peter R. Bieck Karl-Heinz Antonin Gisbert Farger Erik B. Nilsson Eckhart K. Schmidt Philippe Dostert Margherita Strolin Benedetti Peter C. Waldmeier 《Neurochemical research》1993,18(11):1163-1167
CGP 28 014 is a specific inhibitor of catechol-O-methyltransferase (COMT) in vivo. In humans, the inhibition was assessed by measuring urinary excretion of isoquinolines and with the levodopa test. Following administration of CGP 28 014, urinary excretion of isoquinolines was significantly increased. In rats, CGP 28 014 reduced plasma and striatal concentrations of 3-O-methyldopa (30MD) in a dose-dependent manner. Acute and subchronic administration of CGP 28 014 alone or in combination with the peripherally acting decarboxylase inhibitor benserazide decreased plasma 30MD as an index of COMT inhibition by about 50%. There seems to be a close relationship between the time-course of plasma concentrations of CGP 28 014 and the extent of COMT inhibition assessed by the 30MD/DOPA ratio in plasma. 相似文献
82.
M. Anastassova-Kristeva H. Nicoloff S. Georgiev 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1978,53(5):229-231
Summary Six varieties of Triticum monococcum were analysed by means of the nucleolar test; i.e., estimation of the maximum number of primary nucleoli per nucleus. All of the varieties exhibited 4 primary nucleoli in telophase and early interphase. Following detailed karyological analysis four SAT chromosomes in all six karyotypes were found in accordance with the maximum nucleolar number. Secondary constrictions and microsatellites were localised on the short arms of chromosome pairs 3 and 5. A new order of the chromosomes in the idiogram of Tr. monococcum is proposed. 相似文献
83.
S. S. Sandhu F. J. de Serres H. N. B. Gopalan W. F. Grant D. Svendsgaard J. Velemínský G. C. Becking 《Mutation research》1994,310(2):257-263
In the first phase of a collaborative study by the International Programme on Chemical Safety (PRCS), four coded chemicals, i.e. azidoglycerol (AG, 3-azido-1,2-propanediol), methyl nitrosurea (MNU), sodium azide (NaN3) and maleic hydrazide (MH), and ethyl methanesulfonate (EMS) as a positive control were tested in four plant bioassays, namely the Arabidopsis embryo and chlorophyll mutation assay, the Tradescantia stamen hair assay (Trad-SH assay), the Tradescantia micronucleus assay (Trade-MCN), and the Vicia faba root tip assay. Seventeen laboratories from diverse regions of the world participated with four to six laboratories each using one plant assay. For the Arabidopsis assay, laboratories were in agreement with MNU and AG giving positive responses and NaN3 giving a negative response. With the exception of one laboratory which reported MH as weakly mutagenic, no mutagenic response was reported for MH by the other laboratories. For the Vicia faba assay, all laboratories reported a positive response for MNU, AG, and MH, whereas two of the six laboratories reported a negative response for NaN3. For the Trad-SH assay, MH was reported as giving a positive response and a positive response was also observed for MNU with the exception of one laboratory. NaN3, which exhibited a relatively high degree of toxicity, elicited a positive response in three of the five laboratories. AG was found positive in only one of the two laboratories which tested this chemical. For the Trad-MCN assay, MNU and MH were reported as positive by all laboratories, while four out of five laboratories reported NaN3 to be positive. Only one of three laboratories reported AG to be positive. The major sources of variability were identified and considered to be in the same range as found in similar studies on other test systems. Recommendations were made for minor changes in methodology and for initiating the second phase of this study. 相似文献
84.
D. P. Sleeman J. Davenport S. J. More T. A. Clegg J. M. Griffin I. O’Boyle 《European Journal of Wildlife Research》2009,55(3):267-278
This study’s objective was to estimate the permeability of barriers to badger immigration during the Irish Four Area project.
These barriers were at the boundaries of removal areas, where there was proactive culling of badgers. Data from the last 3 years
of the study were used. Each length of barrier was allocated a space within the removal area. These were further sub-divided
into spaces of 0–2, 2–5 km and sometimes of more than 5 km from the edge of the removal area. It is assumed that all, or some,
of the badgers caught within these spaces came across the barriers. The barriers were one of the following: external buffers,
sea, rivers and political boundaries. The total lengths of the barriers in all areas were: external buffer 128.5 km; sea 70.9 km;
river 78.6 km; political 32.2 km. We assume three scenarios: (1) all badgers caught in the final 3 years were immigrants,
(2) 75% were immigrants or (3) 50% were immigrants. We test these scenarios using chi-square tests, applying internal buffers
of 1 km to counter movements of badgers across zones. Using this approach and multivariate analysis, we found that the permeability
of barrier types varied, with sea and external buffers being the most effective barriers. The combined capture data are further
examined by the sex ratio in each range, and then the sex ratio in total. Equal numbers of males and females were found, but
the source populations were probably predominantly female. If badger management options are to achieve maximum benefits, then
the field effectiveness of such barriers needs to be understood. 相似文献
85.
Alpha-glucosidase inhibition from a Chinese medical herb (Ramulus mori) in normal and diabetic rats and mice 总被引:9,自引:0,他引:9
Alpha-glucosidase inhibitors are oral antidiabetic drugs. A traditional Chinese medical herb, Sangzhi (Ramulus mori), appears to have properties similar to those of alpha-glucosidase inhibitors. The effects of an aqueous extract of Shangzhi (SZ) were studied in normal and alloxan diabetic rats and mice, and these results compared with those for acarbose, an alpha-glucosidase inhibitor. In our grade-dose studies, SZ was found to lower and prolong the zenith of blood glucose concentration (ZBG) after sucrose or starch loading and stabilize blood glucose levels in fasting normal and alloxan diabetic mice. After 2 weeks of SZ administration with high-calorie chow or a normal diet, the fasting and non-fasting blood glucose concentrations in alloxan diabetic mice and rats were decreased. In alloxan rats, the blood fructosamine concentration was lowered. Results for acarbose and SZ were similar. These indicate that SZ has alpha-glucosidase inhibitory effects. 相似文献
86.
SUMMARY: In a recent article Begg et al. (2007, Biometrics 63, 522-530) proposed a statistical test to determine whether or not a diagnosed second primary tumor is biologically independent of the original primary tumor, by comparing patterns of allelic losses at candidate genetic loci. The proposed concordant mutations test is a conditional test, an adaptation of Fisher's exact test, that requires no knowledge of the marginal mutation probabilities. The test was shown to have generally good properties, but is susceptible to anticonservative bias if there is wide variation in mutation probabilities between loci, or if the individual mutation probabilities of the parental alleles for individual patients differ substantially from each other. In this article, a likelihood ratio test is derived in an effort to address these validity issues. This test requires prespecification of the marginal mutation probabilities at each locus, parameters for which some information will typically be available in the literature. In simulations this test is shown to be valid, but to be considerably less efficient than the concordant mutations test for sample sizes (numbers of informative loci) typical of this problem. Much of the efficiency deficit can be recovered, however, by restricting the allelic imbalance parameter estimate to a prespecified range, assuming that this parameter is in the prespecified range. 相似文献
87.
符号检验法及其在动物生态学研究中的应用 总被引:1,自引:1,他引:1
符号检验法是非参数统计法的一种重要方法,在动物生态学的实验数据统计分析领域有一定的实用价值,本文简要介绍这一方法的使用。 相似文献
88.
流行性感冒裂解疫苗免疫原性试验及电镜观察 总被引:1,自引:0,他引:1
实验报告了以三硝基甲苯X-100(Triton X-100)为裂解剂对流行性感冒病毒裂解效果的电镜观察结果及裂解疫苗的免疫原性。通过对病毒纯化、裂解、再纯化制备的3批流行性感冒裂解疫苗样品的电镜观察,表明使用此裂解剂能使疫苗中的病毒裂解完全,无完整病毒颗粒,裂解效果好。安全试验和免疫试验结果表明疫苗的安全性好,免疫效果好。 相似文献
89.
The connectivity among marine populations is determined by the dispersal capabilities of adults as well as their eggs and larvae. Dispersal distances and directions have a profound effect on gene flow and genetic differentiation within species. Genetic homogeneity over large areas is a common feature of coral reef fishes and can reflect high dispersal capability resulting in high levels of gene flow. If fish larvae return to their parental reef, gene flow would be restricted and genetic differentiation could occur. Larabicus quadrilineatus (Labridae) is considered as an endemic fish species of the Red Sea and Gulf of Aden. The juveniles of this species are cleaner fish that feed on ectoparasites of other fishes. Here, we investigated the genetic population structure and gene flow in L. quadrilineatus among five locations in the Red Sea to infer connectivity among them. To estimate genetic diversity, we analysed 369 bp of 237 mitochondrial DNA control region sequences. Haplotype and nucleotide diversities were higher in the southern than in the northern Red Sea. Analysis of molecular variance (amova) detected the highest significant genetic variation between northern and central/southern populations (Phi(CT) = 0.01; P < 0.001). Migration analysis revealed a several fold higher northward than southward migration, which could be explained by oceanographic conditions and spawning season. Even though the Phi(ST) value of 0.01 is rather low and implies a long larval dispersal distance, estimates based on the isolation-by-distance model show a very low mean larval dispersal distance (0.44-5.1 km) compared to other studies. In order to enable a sustainable ornamental fishery on the fourline wrasse, the results of this study suggest that populations in the northern and southern Red Sea should be managed separately as two different stocks. The rather low larval dispersal distance of about 5 km needs to be considered in the design of marine protected areas to enable connectivity and self-seeding. 相似文献
90.
Ten urine chemistry parameters were measured on 74 voided urine samples from 34 wild chimpanzees (Pan troglodytes). Multi-reagent urine dipstick tests were performed and results determined using colorimetric scales. Urine pH measured between 8 and 9 units in 91% of the chimpanzees. Test pads detected protein, erythrocytes, leukocyte esterase activity, and nitrites, ketones and bilirubin in 47, 32, 29, and <10% of the chimpanzees, respectively. No apparent association between positive test results for blood in adult females and reproductive status was found. Overall, 17 of the 34 chimpanzees had positive urine test results for protein, hemoglobin, erythrocytes, leukocytes, nitrites, ketones, and/or bilirubin. Dipstick urinalysis alone is an unreliable method for assessing health and physiological status of wild chimpanzees. However, if combined with other diagnostics it could prove to be a valuable health-monitoring tool. Limitations associated with this methodology need to be considered when interpreting urinary dipstick test results. 相似文献