首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   352223篇
  免费   158159篇
  国内免费   29887篇
  2016年   5034篇
  2015年   6616篇
  2014年   7371篇
  2013年   9327篇
  2012年   10614篇
  2011年   10998篇
  2010年   9802篇
  2009年   14307篇
  2008年   10886篇
  2007年   10997篇
  2006年   9501篇
  2005年   9526篇
  2004年   9277篇
  2003年   8934篇
  2002年   9340篇
  2001年   14953篇
  2000年   12954篇
  1999年   15339篇
  1998年   11645篇
  1997年   11701篇
  1996年   11029篇
  1995年   11122篇
  1994年   10561篇
  1993年   10203篇
  1992年   14881篇
  1991年   14709篇
  1990年   15202篇
  1989年   14470篇
  1988年   13426篇
  1987年   12526篇
  1986年   11616篇
  1985年   11291篇
  1984年   9196篇
  1983年   7922篇
  1982年   7134篇
  1981年   6705篇
  1980年   6258篇
  1979年   8753篇
  1978年   7201篇
  1977年   6927篇
  1976年   6586篇
  1975年   6562篇
  1974年   7183篇
  1973年   7088篇
  1972年   7008篇
  1971年   6349篇
  1970年   5639篇
  1969年   5680篇
  1968年   4991篇
  1967年   4270篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
112.
113.
114.
115.
Intraperitoneal stimulation of adoptively sensitized rats with bacterial antigen promotes the localization of lymphoblasts at the site of antigen deposition. Lymphoblast extravasation activity (LEA) is generated only when specifically immune donor lymphocytes and the recipients of these cells share at least on Ag-B haplotype. However, if the specificity criteria for its formation are satisfied, LEA promotes the local development of lymphoblasts of all available specificities and irrespective of their Ag-B genotype. Allogeneic lymphoblasts do not participate actively in the delayed inflammatory reaction even when they are passively recruited into exudates. The results suggest that LEA is a T cell-derived mediator that amplifies the delayed type hypersensitivity reaction by directing recently activated lymphocytes into lesions.  相似文献   
116.
The adaptive significance of inversion polymorphism of Drosophila subobscura is well established. However, little is known about gene combinations within inversions which are coadapted because of population-specific effects. We studied this aspect of Dobzhansky's coadaptation hypothesis, using the systematic inbreeding method. Differences in magnitude and quality of inversion polymorphism in two ecologically and topologically distinct habitats were compared after several generations of continuous full-sib inbreeding. Populations from the two habitats differ in frequency of homokaryotypes after third and fifth generation of inbreeding and in the levels of homozygosity of different gene arrangements. The effect of homozygosity appears population and chromosome specific.  相似文献   
117.
In many species of actinomycetes, carotenogenesis can be photoinduced. The capacity to respond to photoinduction is, however unstable and, in various strains of Streptomyces, is lost at a relatively high frequency. In Streptomyces setonii ISP5395, which normally produces no carotenoids, carotenoid-producing mutants can be obtained following protoplast regeneration. We report here the characterization of a gene, crtS, which was isolated from one such mutant and can confer on wild-type S. setonii ISP5395 cells the capacity to synthesize carotenoids. Sequence analysis of crtS reveals an open reading frame, which shows homology to genes that encode alternative sigma factors in Bacillus subtilis. We propose that crtS encodes a sigma factor which is necessary for the expression of a cryptic gene(s) for carotenoid biosynthesis in S. setonii ISP5395.  相似文献   
118.
119.
Rosette and single-element strain gauges were implanted on the tibia in 2 dogs and recordings were made during locomotion on a treadmill. At foot contact and during the swing phase of locomotion, bone strains were low and directions of the principal strains were variable. There was a large shift in the directions of the principal strains at the beginning of the stance phase and bone strains were considerably higher. Peak strain occurred midway through the stance phase. At that time, the maximum principal strain (tension) was directed upwards and anteriorly between 30 and 60 degrees with respect to the long axis of the tibia. These bone strain patterns in the dog are similar to those found in sheep while both differ markedly from those found in humans.  相似文献   
120.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号