全文获取类型
收费全文 | 535532篇 |
免费 | 66808篇 |
国内免费 | 178篇 |
专业分类
602518篇 |
出版年
2016年 | 5769篇 |
2015年 | 8010篇 |
2014年 | 9636篇 |
2013年 | 13610篇 |
2012年 | 15070篇 |
2011年 | 15179篇 |
2010年 | 9980篇 |
2009年 | 9419篇 |
2008年 | 13451篇 |
2007年 | 14252篇 |
2006年 | 13245篇 |
2005年 | 12870篇 |
2004年 | 12730篇 |
2003年 | 12373篇 |
2002年 | 11881篇 |
2001年 | 22915篇 |
2000年 | 23383篇 |
1999年 | 18731篇 |
1998年 | 6717篇 |
1997年 | 7062篇 |
1996年 | 6855篇 |
1995年 | 6598篇 |
1994年 | 6567篇 |
1993年 | 6501篇 |
1992年 | 16430篇 |
1991年 | 16314篇 |
1990年 | 15956篇 |
1989年 | 15549篇 |
1988年 | 14553篇 |
1987年 | 14011篇 |
1986年 | 13100篇 |
1985年 | 13259篇 |
1984年 | 11144篇 |
1983年 | 9828篇 |
1982年 | 7537篇 |
1981年 | 7067篇 |
1980年 | 6656篇 |
1979年 | 11175篇 |
1978年 | 8651篇 |
1977年 | 7970篇 |
1976年 | 7582篇 |
1975年 | 8411篇 |
1974年 | 8800篇 |
1973年 | 8650篇 |
1972年 | 8187篇 |
1971年 | 7144篇 |
1970年 | 6293篇 |
1969年 | 5939篇 |
1968年 | 5388篇 |
1967年 | 4697篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
241.
Results from a large-scale study, comprising 75 different breeding herds, are reported on predicting the halothane ( Hal ) genotypes of individual pigs by making use of the known close linkage between Hal and three C blood marker loci ( Phi, Po2, Pgd ). The parents haplotypes (involving Hal and marker loci) were determined from the HAL phenotypes (halothane test results) and marker loci phenotypes of their offspring in the first one or two litters studied. In subsequent litters of the Hal -marker loci haplotyped parents, the offspring's expected Hal genotypes could be predicted on the basis of the marker loci haplotypes inherited by them. By comparing the expected and observed HAL phenotypes of offspring in subsequent litters, the predicted Hal genotype was found to be correct in 90–95 % of the 4000 offspring (from Nn × Nn and Nn × nn matings) of Swedish Landrace and Yorkshire breeds studied.
The order of the three marker loci was confirmed as Phi-Po2-Pgd but the position of Hal with regards to Phi could not be resolved. The recombination frequencies between the most distant loci in this region, viz. Hal-Pgd and Phi-Pgd , were estimated to be 3–4.5 % and 4–6 % , respectively. The easy and rapid electrophoretic techniques described in the study to phenotype PHI, PO2, PGD, also allowed phe-notyping of six other polymorphic protein systems on the same gels. Thus Hal genotyping and effective parentage control can be conducted simultaneously. 相似文献
The order of the three marker loci was confirmed as Phi-Po2-Pgd but the position of Hal with regards to Phi could not be resolved. The recombination frequencies between the most distant loci in this region, viz. Hal-Pgd and Phi-Pgd , were estimated to be 3–4.5 % and 4–6 % , respectively. The easy and rapid electrophoretic techniques described in the study to phenotype PHI, PO2, PGD, also allowed phe-notyping of six other polymorphic protein systems on the same gels. Thus Hal genotyping and effective parentage control can be conducted simultaneously. 相似文献
242.
Carotenoids present in lipids extracted from the cyanobacterium Synechococcus 6716 indicate trans-membrane potential in proteoliposomes reconstituted from these lipids and the ATPase complex isolated from the same organism. A carotenoid absorbance band shift to a longer wavelength is obtained with valinomycin-induced potassium ion diffusion potentials, irrespective of the polarity of the potassium gradient. In contrast to this, the (externally added) probe oxonol VI only shows an absorbance band shift when the external potassium ion concentration is higher than the internal one. In liposomes without ATPase complex, no carotenoid absorbance band shifts were observed. 相似文献
243.
Wolff A. H.; Coleshaw S. R.; Newstead C. G.; Keatinge W. R. 《Journal of applied physiology》1985,58(3):770-777
Flow of water under foam neoprene wet suits could halve insulation that the suits provided, even at rest in cold water. On the trunk conductance of this flow was approximately 6.6 at rest and 11.4 W . m-2 . C-1 exercising; on the limbs, it was only 3.4 at rest and 5.8 W . m-2 . degrees C-1 exercising; but during vasoconstriction in the cold, skin temperatures on distal parts of limbs were lower than were those of the trunk, allowing adequate metabolic responses. In warm water, minor postural changes and movement made flow under suits much higher, approximately 60 on trunk and 30 W . m-2 . degrees C-1 on limbs, both at rest and at work. These changes in flow allowed for a wide range of water temperatures at which people could stabilize body temperature in any given suit, neither overheating when exercising nor cooling below 35 degrees C when still. Even thin people with 4- or 7- mm suits covering the whole body could stabilize their body temperatures in water near 10 degrees C in spite of cold vasodilatation. Equations to predict limits of water temperature for stability with various suits and fat thicknesses are given. 相似文献
244.
245.
The RAINFOR database: monitoring forest biomass and dynamics 总被引:1,自引:0,他引:1
Problem: Data from over 100 permanent sample plots which have been studied for 10–20 years need a suitable system for storage which allows simple data manipulation and retrieval for analysis. Methods: A relational database linking tree records, taxonomic nomenclature and corresponding environmental data has been built in MS Access as part of the RAINFOR project. Conclusion: The database allows flexible and long‐term use of a large amount of data: more than 100 tree plots across Amazonia, incorporating over 80 000 records of individual trees and over 300 000 total records of tree diameter measurements from successive censuses. The database is designed to enable linkages to existing soil, floristic or plant‐trait databases. This database will be a useful tool for exploring the impact of environmental factors on forest structure and dynamics at local to continental scales, and long term changes in forest ecology. As an early example of its potential, we explore the impact of different methodological assumptions on estimates of tropical forest biomass and carbon storage. 相似文献
246.
Levels of 7.8, 18.5 and 26 mole % deuterium oxide were administered sequentially to (Japanese quail) via the drinking water. The primary effect observed was on egg frequency, which decreased from a normal level of 0.89 for 7.8 mole % D2O to a low of 0.38 during the administration of 26 mole % D2O. Adverse symptoms, such as hyperexcitability, convulsions, skin ulcerations, comatosity, weight loss, or death, which have been associated with deuterium toxicity in other animals, were not observed in these experiments. The amount of deuterium deposited in the water of the egg was 6.9, 13.98, and 19.83 mole % when 7.8, 18.5 and 26 mole % deuterium respectively was administered. For each period, the deuterium content of egg water rapidly reached a maximum concentration after which the concentration decreased slightly. This dilution effect has not been noted previously in body fluids from other animals. 相似文献
247.
R L Hallberg 《Developmental biology》1974,38(2):346-355
Displacement loops are found in mitochondrial DNA isolated from the ovaries of mature female Xenopus laevis. These displacement loops subtend some 7% of the contour length of a mitochondrial circular DNA. When mature oocytes are shed as unfertilized eggs at least 76% of the mitochondrial DNA in these eggs contains displacement loops. The implications of these findings are discussed with respect to displacement loop occurrence in other mitochondrial DNAs and especially with respect to mitochondrial DNA replication. 相似文献
248.
A contribution to the functional meaning of the substitution of primary by secondary bone tissue 总被引:1,自引:0,他引:1
AMPRINO R 《Acta anatomica》1948,5(3):291-300
249.
A 3-wk-old lamb died because of neurological disease. The predominant microscopic lesions were in the brain and spinal cord and consisted of nonsuppurative encephalomyelitis with severe gliosis throughout the gray and white matter. Immature and mature schizonts, 15.7 x 10.6 microns (8-30 x 6-18 microns), occurred in capillaries and were structurally similar to those of Sarcocystis tenella. 相似文献
250.
Monitoring signal transduction in cancer: cDNA microarray for semiquantitative analysis. 总被引:1,自引:0,他引:1
H B Hsieh R A Lersch D E Callahan S Hayward M Wong O H Clark H U Weier 《The journal of histochemistry and cytochemistry》2001,49(8):1057-1058
This study targeted the development of a novel microarray tool to allow rapid determination of the expression levels of 58 different tyrosine kinase (tk) genes in small tumor samples. The goals were to define a reference probe for multi-sample comparison and to investigate the variability and reproducibility of the image acquisition and RT-PCR procedures. The small number of tk genes on our arrays enabled us to define a reference probe by artificially mixing all genes on the arrays. Such a probe provided contrast reference for comparative hybridization of control and sample DNA and enabled cross-comparison of more than two samples against one another. Comparison of signals generated from multiple scanning eliminated the concern of photo bleaching and scanner intrinsic noise. Tests performed with breast, thyroid, and prostate cancer samples yielded distinctive patterns and suggest the feasibility of our approach. Repeated experiments indicated reproducibility of such arrays. Up- or downregulated genes identified by this rapid screening are now being investigated with techniques such as in situ hybridization. 相似文献