全文获取类型
收费全文 | 435280篇 |
免费 | 43948篇 |
国内免费 | 169篇 |
专业分类
479397篇 |
出版年
2021年 | 3687篇 |
2018年 | 5673篇 |
2017年 | 5332篇 |
2016年 | 7076篇 |
2015年 | 8990篇 |
2014年 | 10274篇 |
2013年 | 14197篇 |
2012年 | 17115篇 |
2011年 | 16983篇 |
2010年 | 10894篇 |
2009年 | 9485篇 |
2008年 | 14642篇 |
2007年 | 14616篇 |
2006年 | 14142篇 |
2005年 | 12942篇 |
2004年 | 12716篇 |
2003年 | 11986篇 |
2002年 | 11602篇 |
2001年 | 20826篇 |
2000年 | 20974篇 |
1999年 | 15904篇 |
1998年 | 5220篇 |
1997年 | 5177篇 |
1996年 | 4954篇 |
1995年 | 4500篇 |
1994年 | 4431篇 |
1993年 | 4442篇 |
1992年 | 11863篇 |
1991年 | 11672篇 |
1990年 | 11046篇 |
1989年 | 10958篇 |
1988年 | 10130篇 |
1987年 | 9533篇 |
1986年 | 8601篇 |
1985年 | 8583篇 |
1984年 | 6888篇 |
1983年 | 6038篇 |
1982年 | 4470篇 |
1981年 | 3963篇 |
1980年 | 3749篇 |
1979年 | 6355篇 |
1978年 | 4943篇 |
1977年 | 4554篇 |
1976年 | 4039篇 |
1975年 | 4731篇 |
1974年 | 4896篇 |
1973年 | 4792篇 |
1972年 | 4205篇 |
1971年 | 3884篇 |
1970年 | 3559篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Although many studies examine the form of sexual selection in males, studies characterizing this selection in females remain sparse. Sexual selection on females is predicted for sex‐role‐reversed Mormon crickets, Anabrus simplex, where males are choosy of mates and nutrient‐deprived females compete for matings and nutritious nuptial gifts. We used selection analyses to describe the strength and form of sexual selection on female morphology. There was no positive linear sexual selection on the female body size traits predicted to be associated with male preferences and female competition. Instead, we detected selection for decreasing head width and mandible length, with stabilizing selection as the dominant form of nonlinear selection. Additionally, we tested the validity of a commonly used instantaneous measure of mating success by comparing selection results with those determined using cumulative mating rate. The two fitness measures yielded similar patterns of selection, supporting the common sampling method comparing mated and unmated fractions. 相似文献
992.
Ildikó Vidra Kálmán Simon László Institóris Ingeborg Csöregh Mátyás Czugler 《Carbohydrate research》1982,111(1):41-57
After hydrolysis of 1,6-dibromo-1,6-dideoxygalactitol (1) and 1,2:5,6-dianhydrogalactitol (2), 11 compounds were isolated, three of them as tritylated derivatives. Their structures were established on the basis of chemical evidence and, for four compounds, by X-ray diffraction. The main product of the hydrolysis of 1 was 3,6-anhydro-1-bromo-1-deoxy-dl-galactitol; the end-products of the hydrolysis of 2 were 1,5-anhydro-dl-galactitol, 2,5-anhydro-dl-altritol, and galactitol. 相似文献
993.
994.
995.
T I Grand 《American journal of physical anthropology》1968,28(2):163-181
The howler monkey possesses unique anatomical adaptations associated with its arboreal habit. The behavioral elements are described by locomotor pattern, substrate, timing and rhythm of movement. The most significant motor adaptations are correlated clearly and directly with musculoskeletal features of the lower limb. The orientation of the joints within the limb, the shape of the joint surfaces, their bony environments, and the important planes of muscular control are the foundations for the observable locomotor behaviors. 相似文献
996.
The influence of diazepam (1; 5; 10 mg/kg, i. p.), chlorpromazine (1 mg/kg) and amphetamine (1; 5 mg/kg) on the Fourier's spectral EEG power of sensomotor cortex and dorsal hippocampus and conflict behavior freely moving albino and cotton (Sigmondon hispidus) rats was studied. Effects of diazepam (5 mg/kg) in cotton rats were similar, but influence on the theta-activity was more expressed. Correlation between slowing of theta-activity and extent of anxiolytic effect in conflict situation was showed. On the basis of the results obtained the authors discuss possible mutual relations between the influence of diazepam on EEG and anxiolytic effect of benzodiazepines. 相似文献
997.
998.
Purification and partial characterization of goose ceruloplasmin 总被引:2,自引:0,他引:2
M Hilewicz-Grabska A Zgirski T Krajewski A Plonka 《Archives of biochemistry and biophysics》1988,260(1):18-27
The preparation and properties of ceruloplasmin from goose blood plasma are described. Ammonium sulfate was used to precipitate the crude protein followed by adsorption and elution from DEAE-Sephadex A-50. Further treatment with an ethanol-chloroform mixture and Sephadex G-200 yielded an intensely blue protein possessing a high degree of chemical purity and biological activity. Goose ceruloplasmin, existing in two forms, appears to be a single polypeptide, apparent Mr121,300, with an A610/A280 ratio of 0.07. Copper represented 0.32%, which corresponded to six atoms of copper per protein molecule. Although the amount of EPR-detectable copper was the same as in mammalian ceruloplasmins there were some differences in EPR parameters, mainly in A parallel. Goose ceruloplasmin's amino acid composition, although similar in many residues to human ceruloplasmin, was lower in tyrosine, cystine/cysteine, and acidic amino acids. Valine was found as the N-terminal amino acid. Hexose, hexosamine, sialic acid, and fucose accounted for 6.65% of the weight. Goose protein contained only half the sialic acid of human ceruloplasmin. Two values for Km using either p-phenylenediamine (0.64 and 0.053 mM) or o-dianisidine (0.76 and 0.15 mM) were evaluated from Lineweaver-Burk plots. EPR studies on reactions with water radiolysis products at cryogenic temperatures allowed us to discover that goose ceruloplasmin, like human and bovine ceruloplasmins, possesses superoxide dismutase activity. 相似文献
999.
1000.
T F Kresina 《Experimental cell biology》1988,56(1-2):86-102
T cell hybridoma cell lines were generated by somatic cell fusion of BW 5147 myeloma cells and splenic cells from mice suppressed for collagen-induced arthritis (CIA). Two cell lines were characterized for their cell surface phenotype, antigen recognition and capacity to down-regulate the erythema and edema associated with CIA. Cell line T101N was determined to portray the cell surface phenotype Ly1+2- L3T4- Thy1+ by a direct binding assay. Cell line T104B1 was determined to express only the Thy1+ alloantigen. Panning studies, measurement of IL-2 production in vitro and the suppression of antibodies to type I and type II collagen in vivo indicate that the hybridoma cells are not isotype specific in their recognition of the polymorphic interstitial collagens. Down-regulation of the erythema and edema of CIA occurred on injection of 1 X 10(5) T101N cells but not T104B1 cells. Histology of the tarsus region of the hind paw of CIA mice 33 days after the administration of T101N cells showed contrasting histopathology compared to that of CIA mice. The joints of CIA mice given T101N cells showed aligned articular surfaces resembling normal joint structure and only residual pannus. The data indicate that collagen-specific cloned T cell lines can modulate the gross pathology and joint architecture of joints exhibiting CIA. 相似文献