全文获取类型
收费全文 | 24578篇 |
免费 | 2297篇 |
国内免费 | 2346篇 |
出版年
2024年 | 60篇 |
2023年 | 327篇 |
2022年 | 728篇 |
2021年 | 1308篇 |
2020年 | 951篇 |
2019年 | 1120篇 |
2018年 | 1032篇 |
2017年 | 782篇 |
2016年 | 1133篇 |
2015年 | 1549篇 |
2014年 | 1977篇 |
2013年 | 1903篇 |
2012年 | 2242篇 |
2011年 | 1989篇 |
2010年 | 1239篇 |
2009年 | 1159篇 |
2008年 | 1369篇 |
2007年 | 1194篇 |
2006年 | 976篇 |
2005年 | 801篇 |
2004年 | 745篇 |
2003年 | 651篇 |
2002年 | 581篇 |
2001年 | 426篇 |
2000年 | 383篇 |
1999年 | 381篇 |
1998年 | 264篇 |
1997年 | 242篇 |
1996年 | 213篇 |
1995年 | 154篇 |
1994年 | 163篇 |
1993年 | 113篇 |
1992年 | 162篇 |
1991年 | 142篇 |
1990年 | 93篇 |
1989年 | 80篇 |
1988年 | 75篇 |
1987年 | 71篇 |
1986年 | 49篇 |
1985年 | 63篇 |
1984年 | 44篇 |
1983年 | 35篇 |
1982年 | 22篇 |
1981年 | 16篇 |
1980年 | 22篇 |
1979年 | 23篇 |
1978年 | 16篇 |
1977年 | 16篇 |
1974年 | 16篇 |
1970年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
53.
鼻蝇亚科(Rhiniinae)是双翅目丽蝇科(Calliphoridae)中比较特殊的一大类群,因此也有人把它从丽蝇科分出,独立成科的。其特征有口上片突出如鼻状,后头上部大半是裸出的,约占整个头的宽度的范围内连粉被也缺如;r脉的上方沿后侧有一列明晰的小刚毛,翅下大结节上无立纤毛,下腋瓣裸。主要分布在东洋区、非洲区、大洋洲区以及古北区南缘。目前全世界已知有22个属,近300种,曾有报道其幼虫寄生于白蚁巢内及蝗 相似文献
54.
High-fidelity gene targeting in embryonic stem cells by using sequence replacement vectors. 总被引:22,自引:8,他引:14 下载免费PDF全文
Mutations were targeted to the Hprt locus in murine embryonic stem cells by using sequence replacement vectors. When the vector was designed such that the mutated sequences were flanked on both sides by several kilobases of DNA homologous to the target locus, replacement of chromosomal sequences with the exogenous DNA occurred with precision. If, on the other hand, the target-homologous DNA on one arm of the vector was reduced to below 1 kb in length, the fidelity of recombination was diminished. 相似文献
55.
Culture and characterization of dental follicle cells from rat molars 总被引:12,自引:0,他引:12
Summary Because the dental follicle is necessary for the eruption of teeth of limited eruption, it was the objective of this study to determine if the cells of the follicle could be cultured in vitro. To achieve this, dental follicles and associated enamel organs were dissected from the first and second mandibular molars of 6–7-day-old rats (secretory stage of amelogenesis), and then cultured in a medium that promotes fibroblast growth — the predominant cell type of the dental follicle. The cultured cells grew to confluency and were kept through 3 passages before experimentation. The cultured cells were fibroblastic in shape, elongate with processes, and transmission electron microscopy revealed that they contained an abundant rough endoplasmic reticulum, but did not form desmosomes. Immunofluorescent staining for anti-vimentin showed that all the cells stained and electron-microscopic immunogold labeling indicated that the antibody was associated with intermediate filaments. As revealed by SDS-polyacrylamide gel electrophoresis and Western blotting, the cultured cells synthesized and secreted the extracellular matrix molecules fibronectin and procollagens. Subsequent immunofluorescence staining of permeabilized and non-permeabilized cells confirmed the presence of fibronectin and type I collagen both intra- and extracellularly. Thus, based on all the above characteristics, the cultured cells appeared to be fibroblasts derived from the dental follicle, although a few of the fibroblasts may be derived from undifferentiated mesenchymal cells interposed between the alveolar bone and follicle. Experiments now can be conducted to determine how these cultured cells respond directly to growth factors that alter the rates of tooth eruption. 相似文献
56.
血管内皮舒张因子在氧自由基所致慢性缺氧大鼠肺内动... 总被引:1,自引:1,他引:0
The role of endothelium-derived relaxing factor (EDRF) on the effect of oxygen-derived free radicals (generated by xanthine-xanthine oxidase system) on intrapulmonary arterial in chronic hypoxic rats was studied by a microbioassay method. Intrapulmonary artery rings with intact or denuded endothelium of hypoxic (5,000 m, 10 days) and normoxic rats were prepared for observation of oxygen-derived free radicals induced contraction. It was shown that oxygen-derived free radicals induced contractions of intrapulmonary arterial rings with intact endothelium were obviously augmented in hypoxic rats than in normoxic controls. The augmented responses could be further potentiated by the addition of EDRF inactivator reduced hemoglobin (RHb), but diminished or even abolished by applying superoxide dismutase (Cu-Zn SOD). However, no effect on denuded rings was observed when RHb or SOD was added. It is concluded that chronic hypoxia may attenuate the action of EDRF in the enhancement of the reactivity of intrapulmonary artery to oxygen-derived free radicals. 相似文献
57.
电损毁海马CA3区及连合前穹窿对大鼠血浆胰岛素水平... 总被引:1,自引:0,他引:1
Bilateral electrical lesioning of the hippocampal CA3 region (HCA3-EL) or anterior commissura hippocampi (ACHF-EL) caused marked elevations in plasma basal levels of insulin. 2 weeks later, fasting blood glucose levels were also augmented with decreased glucose tolerance. In contrast, the secretory response of pancreatic B cells to glucose stimulation was markedly enhanced. Following intravenous glucose tolerance test (IVGTT), the relative amounts of glucagon-like and insulin-like immunoreactants were reduced in the pancreatic islets of both HCA3-EL and ACHF-EL rats in comparison with the controls. In the HCA3-EL group, the relative amounts of somatostatin-like immunoreactants and gross numbers of such immunostained cells in islets were also decreased as compared with the control. No difference was seen in pancreatic-polypeptide-like immunoreactivities as assessed by immunohistochemistry plus microphotometry method. The above results suggest strongly that HCA3 and ACHF exert a tonic inhibitory action on the insulin secretion in the rat. 相似文献
58.
鹅掌楸种子和胚胎发育的研究 总被引:23,自引:0,他引:23
应用控制授粉、软 X-射线法、常规石蜡制片法和荧光检测等手段,研究了鹅掌楸(Lirio-dendron chinense(Hemsl.)Sarg.胚胎发育和控制授粉与结籽率的相关性。控制授粉后2小时花粉萌发,6小时萌发率最高,柱头可授期持续30小时左右。花粉管借助于柱头毛之间的分泌物进入柱头沟,经花柱沟、珠孔塞和珠心冠原进入胚囊,行珠孔受精。授粉后2周,胚乳为2至3细胞厚的狭组织;第6周,胚乳充满胚囊腔,珠心随之解体殆尽;第7到8周,球形胚、心形胚发生;第14到16周,子叶形成;第22周种子或熟,胚乳丰富。单株自然授粉结籽率不足1%。控制授粉后,单个聚合果的最高结籽率可达39%,9个聚合果的平均结籽率为17.7%。 相似文献
59.
In vitro vegetative propagation of Chinese cabbage 总被引:1,自引:0,他引:1
Explants from cotyledons, axillary buds, inflorescence stems and flower buds of Brassica campestris ssp. pekinensis (Lour.) Olsson (Chinese cabbage, cv. Wongbok) were cultured on MS medium with growth regulators. Multiple shoots were obtained from cotyledons, axillary buds and flower buds but not from inflorescence stems. Propagation of shoots from cotyledons was more successful than from axillary buds and flower buds. The vegetative propagation rates varied amongst clones derived from cotyledons of the same cultivar and seed lot. The propagation rates of the cotyledon-derived material followed a normal distribution with an average propagation rate of 2.6 shoots per two weeks subculture when cultured on MS media plus 44.4 m benzyladenine (BA) and 14.8 m -indolebutyric acid (IBA). Shoots from three clones were cultured on MS medium with nine different concentrations of BA. The concentration of BA which promoted the highest rate of shoot propagation varied for the three clones and was in the range 44.4 to 177.6 m. 相似文献
60.
应用毛细管气相色谱标准样品叠加法和色谱-质谱-计算机联用技术分析了巴山冷杉(Abies fargesii Franch. )针叶精油的化学成分。从气相色谱分离出的225个色谱峰中,初步鉴定出43个组分,占精油总量的94.15%,主要成分为α-蒎烯(13.25%)、柠檬烯(10.82%)、石竹烯(10.75%)、莰烯(10.40%)、乙酸龙脑酯(6.9%)、γ-杜松烯(6.28%)、α-蛇麻烯(3.97%)、檀烯(3.52%)、芳萜醇(3.02%)、α-依兰油烯(2.76%)、β-甜没药醇(2.56%)及α-橙花叔醇(2.54%)等。 相似文献