全文获取类型
收费全文 | 857793篇 |
免费 | 89866篇 |
国内免费 | 537篇 |
专业分类
948196篇 |
出版年
2018年 | 8719篇 |
2017年 | 8375篇 |
2016年 | 12733篇 |
2015年 | 18469篇 |
2014年 | 20583篇 |
2013年 | 27077篇 |
2012年 | 29345篇 |
2011年 | 27552篇 |
2010年 | 18944篇 |
2009年 | 17787篇 |
2008年 | 22948篇 |
2007年 | 23583篇 |
2006年 | 22305篇 |
2005年 | 26825篇 |
2004年 | 25301篇 |
2003年 | 22909篇 |
2002年 | 20025篇 |
2001年 | 40836篇 |
2000年 | 40571篇 |
1999年 | 32211篇 |
1998年 | 10271篇 |
1997年 | 10763篇 |
1996年 | 10032篇 |
1995年 | 9246篇 |
1994年 | 8998篇 |
1993年 | 9024篇 |
1992年 | 26129篇 |
1991年 | 25277篇 |
1990年 | 24484篇 |
1989年 | 23883篇 |
1988年 | 22206篇 |
1987年 | 20685篇 |
1986年 | 19112篇 |
1985年 | 18878篇 |
1984年 | 15256篇 |
1983年 | 12841篇 |
1982年 | 9363篇 |
1981年 | 8435篇 |
1980年 | 8048篇 |
1979年 | 14181篇 |
1978年 | 10897篇 |
1977年 | 9954篇 |
1976年 | 8983篇 |
1975年 | 10151篇 |
1974年 | 11024篇 |
1973年 | 10714篇 |
1972年 | 9631篇 |
1971年 | 8885篇 |
1970年 | 7737篇 |
1969年 | 7475篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
M. T. Vénec-Peyré 《Coral reefs (Online)》1987,5(4):205-212
The discovery of epipsammitic Foraminifera in sediments of Moorea and Scilly (French Polynesia) and the study of close relationships between embedded specimens and host-grains indicate that some species participate in the weakening and subsequent breakdown of skeletal grains and consequently contribute to the production of silt-sized particles. The study of 36 stations around the islands shows the factors that control the distribution and abundance of this incrusted microfauna. Specimens are more abundant on sand particles larger than 1,000 microns and in the high energy areas; this abundance decreases with depth. The physiological mechanism of penetrations is presumably chemical, but each species apparently has its own process: complete dissolution with removal of ions for the cytoplasm of calcareous species; partial dissolution with transport of silt-size aggregates on the test of agglutinated species. The purpose of such penetration may be to protect themselves against water turbulence and to provide material for test construction. 相似文献
992.
Evolution of the immunodominant domain of the circumsporozoite protein gene from Plasmodium vivax. Implications for vaccines 总被引:5,自引:0,他引:5
V F de la Cruz A A Lal J A Welsh T F McCutchan 《The Journal of biological chemistry》1987,262(14):6464-6467
Recent work directed toward the development of a malarial vaccine has focused on the identification and production of the immunodominant repeating peptide of the circumsporozoite protein of the human malaria parasites as an antigen. An important factor which relates to the usefulness of this antigen in a vaccine is the rate at which the molecule changes in sequence. We have determined the sequence and arrangement of the repeating epitope of the circumsporozoite protein gene from a Plasmodium vivax isolate from La Paz, El Salvador (Sal-I). This is compared with a portion of the previously published sequence of the circumsporozoite protein gene from a P. vivax isolate from Belém, Brazil. The genes appear to be very similar in the repeat region. There are 20 similar repeating units in the El Salvador strain and only 19 units are conserved in the Brazilian strain. Following this there are degenerate repeats in both strains. Even the pattern of silent mutations in the repeat area are similar; however, they are not necessarily in the identical location and appear to have shifted. The data suggest that the repeat region of these genes may be evolving by an accelerated mechanism(s). Such a phenomenon could severely decrease the long-term efficacy of a repeat-based anti-sporozoite vaccine. 相似文献
993.
During a 6-month period, 7252 faeces specimens were examined for Escherichia coli serotype O157:H7. Forty-nine specimens (0.7%) from 19 patients yielded this organism. Escherichia coli O157:H7 was the third most frequently isolated bacterial pathogen, following Campylobacter jejuni and (or) Salmonella sp. Although regional variation between laboratories determined whether Campylobacter jejuni or Salmonella was the primary bacterial pathogen isolated, E. coli O157:H7 was consistently isolated more frequently than either Shigella or Yersinia enterocolitica. 相似文献
994.
The authors demonstrate stereospecificity of the action of butaclamol enantiomers on substrate inhibition of hypothalamic tyrosine hydroxylase (TH) and regulation of the tyrosine hydroxylase response by the presynaptic membrane (presynaptic receptors) of rat hypothalamus synaptosomes under membrane activation with dopamine. The effect of (+)-butaclamol on the substrate inhibition of TH was noticeable at a concentration of 10(-8)M, reaching a maximum at 10(-5)M. (-)-Butaclamol administered at the same concentrations did not influence the substrate inhibition of the enzyme. (+)-Butaclamol added to the incubation medium containing hypothalamic synaptosomes concurrently with dopamine (10(-5)M) completely blocked the regulatory action of the latter on TH, with this action mediated via presynaptic receptors. (-)-Butaclamol (10(-5)M) antagonized the action of dopamine under the same conditions. The data obtained indicate high stereo-specificity of butaclamol enantiomers as regards their effect on presynaptic regulation of TH, suggesting that elimination of the substrate inhibition of hypothalamic TH is a stereoselective effect of neuroleptics and can be a prognostically important criterion in the appraisal of compounds with potential neuroleptic activity. 相似文献
995.
996.
997.
998.
999.
1000.