全文获取类型
收费全文 | 6985篇 |
免费 | 494篇 |
出版年
2024年 | 4篇 |
2023年 | 36篇 |
2022年 | 103篇 |
2021年 | 192篇 |
2020年 | 104篇 |
2019年 | 132篇 |
2018年 | 195篇 |
2017年 | 178篇 |
2016年 | 266篇 |
2015年 | 384篇 |
2014年 | 433篇 |
2013年 | 540篇 |
2012年 | 710篇 |
2011年 | 649篇 |
2010年 | 384篇 |
2009年 | 348篇 |
2008年 | 455篇 |
2007年 | 422篇 |
2006年 | 374篇 |
2005年 | 354篇 |
2004年 | 293篇 |
2003年 | 264篇 |
2002年 | 231篇 |
2001年 | 32篇 |
2000年 | 33篇 |
1999年 | 46篇 |
1998年 | 49篇 |
1997年 | 29篇 |
1996年 | 30篇 |
1995年 | 28篇 |
1994年 | 22篇 |
1993年 | 19篇 |
1992年 | 20篇 |
1991年 | 12篇 |
1990年 | 11篇 |
1989年 | 13篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 4篇 |
1984年 | 13篇 |
1983年 | 7篇 |
1982年 | 5篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1973年 | 3篇 |
1972年 | 1篇 |
排序方式: 共有7479条查询结果,搜索用时 15 毫秒
11.
In this study we examine the effect on the centrosomes of cold treatment of early Drosophila embryos. Prolonged cold treatment during the mitotic divisions which lead to the formation of the blastoderm causes arrest at metaphase of the nuclear divisions. When examined with immunofluorescence microscopy the mitotic spindles show marked pole splitting with the formation of supernumerary and irregularly sized centers, all able to nucleate microtubules. In embryos recovered for longer periods the additional organizing centers become ring-shaped and lose their nucleating properties. Cold treatment of embryos during the cellularization of the blastoderm results in marked fragmentation of the centrosomes, but nucleating capacity is preserved. Sometimes the centrioles come away from the pericentriolar material and their structure is seen to be modified. 相似文献
12.
Daniela Daga Gordini Ines Castellani Dino Volpin Giorgio M. Bressan 《Cell and tissue research》1990,260(1):137-146
Summary Immuno-gold labeling at the electron-microscopy level was used to investigate the distribution of tropoelastin in the chick eye. Intense staining was found in the amorphous part of mature elastic fibers in different regions of the organ. In elaunin fibers, both the amorphous core and the surrounding microfibrils were clearly labeled. In addition, reactive sites were detected in the oxitalan fibers of the stroma of the cornea and in Descemet's membrane, which showed a gradient of reactive sites increasing from the center toward the periphery. Oxitalan fibers of the stroma often fused with Descemet's membrane; the pattern of immunological staining suggested a continuity between the two structures. In the ciliary zonule, labeling for tropoelastin was observed in discrete areas on the bundles of microfibrils. The results show a complex structural organization of elastic tissue; this may be important in endowing the various parts of the eye with different mechanical properties. 相似文献
13.
14.
Daniela Larizza Miryam Martinetti Cinzia Pizzochero Mariaclara Cuccia Francesca Severi 《Human genetics》1992,88(4):383-387
Summary Growth failure starting before birth is a common characteristic in Turner syndrome, and its pathogenesis is still not completely explained. Experiments performed in mice and rats to test whether a genetic disparity between mothers and offspring and maternal immunological status have any influence on litter size have demonstrated that allogenic litters are significantly larger in size than genetically compatible ones. Studies in humans have given contrasting results, but some authors have found that heterozygosity at enzyme loci and in blood groups is positively correlated with intrauterine growth. HLA class I and II polymorphisms were defined in 53 patients with Turner syndrome and in their parents, and lymphocytotoxic antibody detection was performed in 36 mothers. These data were related to the patients' birth weight. The frequency of the HLA-B16 allele in patients with a birth weight > 10th centile was significantly higher in comparison with those < 10th centile. HLA antigen sharing was present in 43 couples (81.1%). Mean birth weight was 2934 ± 472 g in patients without HLA antigen parental sharing and 2721 ± 529 g in those whose parents shared HLA antigens. The mean birth weight of the 10 patients whose parents do not share HLA antigens was significantly higher than that of the patients with parental HLA–B+DR sharing (P < 0.05) and not significantly higher than in those patients with parental HLA sharing at other HLA loci. Patients whose parents shared B+DR antigens also had significantly smaller birth weights than those with B and A+B+DR sharing (P < 0.025 and P < 0.025). No significant difference in mean birth weight was found in relation to other parameters, such as mother-child histocompatibility, HLA homozygosity and lymphocytotoxic production in the mothers. 相似文献
15.
16.
Malgorzata Schmidt Desirée Du Sart Paul Kalitsis Margaret Leversha Sue Dale Leslie Sheffield Daniela Toniolo 《Human genetics》1991,86(5):519-521
Summary We have analysed two duplications of the X chromosome in male patients using chromosome replication and DNA methylation patterns as determinants of the functional status of the duplicated segments. In both cases, the large duplicated regions, Xq12-q22 and Xq26.3-qter, were not inactivated. A review of previously reported male cases revealed that these duplications were also not subject to inactivation. Taken together, the examined duplications cover almost the entire X chromosome except the pericentromeric region and Xq25–26. Thus, most regions of the X chromosome can be present in two functional copies without lethal consequences. 相似文献
17.
Daniela Zemlová 《Biologia Plantarum》1986,28(5):396-398
18.
Secondary components of visual evoked potentials (slow negative wave-SNW, and photically-evoked sensory afterdischarge-SAD) are known to be precursors of experimentally activated wave-spike discharges, similar to wave-spikes of petit mal epilepsy. Both SNW and SAD may be potently suppressed wither by amphetamine or GABAergic compounds such as diazepam and sodium valproate. A hypothesis was tested in the present study, that amphetamine-induced suppression of wave-spike discharges may require GABA-benzodiazepine activity for its expression.Electrocortical activity was recorded and averaged in unrestrained albino rats with chronically implanted epicortical electrodes. SNW and SAD obtained in habituated rats in the predrug state were potently suppressed by amphetamine (1 mg/kg, i.p.). Fifteen minutes after amphetamine injection, a challenging drug (metrazol, picrotoxin, convulsant benzodiazepine, Ro 5-3663, or imidazodiazepine, Ro 15-1788) was administered intraperitoneally. Subconvulsive doses of metrazol (10 mg/kg) reversed amphetamine suppression; imidazodiazepine (20 mg/kg) and picrotoxin (1.5 mg/kg) reliably opposed the SNW suppression; convulsant benzodiazepine, Ro 5-3663 (2 mg/kg), showed modest and nonsignificant effect in the same direction. It is proposed that the antiepileptic potency of amphetamine may be associated with its ability, apparently via modulatory effect of norepinephrine, to facilitate the activation of benzodiazepine-GABA receptors. 相似文献
19.
Sandro Cavicchi Daniela Guerra Vanna Natali Cristina Pezzoli Gianfranco Giorgi 《Journal of evolutionary biology》1989,2(4):235-251
From a laboratory stock of Drosophila melanogaster (Oregon), reared for more than 20 years at 18° C, a new population was derived and maintained at 28° C for 8 years. The chromosomal and cytoplasmic contribution to genetic divergence between the two populations was estimated. Six body traits and reproductive fitness were taken into account. The third chromosome is responsible for the adaptive difference for temperature between the two lines. Temperature-selected genes which control body size are located on the second and third chromosomes, although the contribution of each chromosome depends on the environment in which the flies develop. The correlation between the chromosomal and cytoplasmic contributions to different traits and fitness, changes with temperature. At 28° C the correlation between fitness and each body trait is proportional to the response to selection exhibited by each of them, but this is not true at 18° C. Body size has, therefore, an adaptive significance in relation to temperature, which is expressed only in the environment where selection occurs. Cytoplasmic genes affect almost all characters to an extent similar to that of chromosomal genes. Inter-chromosomal and nucleo-cytoplasmic interactions are present and also change with temperature. In general, genes selected in a given environment produce greater phenotypic changes in that environment than in another. The population that experienced both temperatures is fitter in both environments, suggesting that the capacity to adapt to warm temperatures depends on genes other than those which are involved in the adaptation to cold. 相似文献
20.
The effect of tryptophan on the biosynthesis of proline has been investigated. Cells of Daucus carota grown in B5 medium supplemented with 5×10–4M tryptophan acquired the ability to grow in the presence of inhibitory concentrations of azetidine-2-carboxylic acid, an analog of proline. When trp was added to carrot cell cultures at sub-growth inhibiting concentrations, overproduction of intracellular free proline was observed. An increase was also observed for lys, his, ala, leu and phe. Likewise, the addition of asparagine, glutamic acid and phenylalanine to the medium stimulated the intracellular increase of free proline and other amino acids.Abbreviations A2CA
azetidine-2-carboxylic acid
- 2,4-D
2,4-dichlorophenoxyacetic acid
- 5MT
5-methyltryptophan
- P5C
pyrroline-5-carboxylic acid
- f.wt.
fresh weight
- d.wt.
dry weight 相似文献