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   451349篇
  免费   50100篇
  国内免费   186篇
  501635篇
  2018年   4444篇
  2016年   6087篇
  2015年   8250篇
  2014年   9573篇
  2013年   12924篇
  2012年   14996篇
  2011年   15459篇
  2010年   10430篇
  2009年   9579篇
  2008年   13811篇
  2007年   14269篇
  2006年   13470篇
  2005年   12722篇
  2004年   12770篇
  2003年   11910篇
  2002年   11716篇
  2001年   17658篇
  2000年   17608篇
  1999年   14145篇
  1998年   5272篇
  1997年   5450篇
  1996年   5082篇
  1995年   4766篇
  1994年   4619篇
  1993年   4665篇
  1992年   11705篇
  1991年   11665篇
  1990年   11379篇
  1989年   10986篇
  1988年   10561篇
  1987年   10135篇
  1986年   9440篇
  1985年   9310篇
  1984年   7829篇
  1983年   6788篇
  1982年   5260篇
  1981年   4724篇
  1980年   4543篇
  1979年   7473篇
  1978年   5906篇
  1977年   5434篇
  1976年   5242篇
  1975年   5637篇
  1974年   6342篇
  1973年   6202篇
  1972年   5785篇
  1971年   5249篇
  1970年   4658篇
  1969年   4589篇
  1968年   4439篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
This paper reports a study of the heterosexual behavior of three male and four female captive adolescent chimpanzees living a semi-natural life style in a large field enclosure. Observations made with binoculars from an over-head deck were balanced over the daylight hours and the seven weekdays. We recorded 213 copulations in 741.25 hours of observation between February 14 and July 21, 1972. Analysis of the observations suggests that group-living chimpanzees in a large field enclosure behave more like free-living chimpanzees than like other captive chimpanzees paired in small cages. In this group, copulations were non-randomly distributed throughout the day, a finding in agreement with results on wild-living chimpanzees but not previously reported for captive chimpanzees. In this group, moreover, copulations were highly concentrated within each female's period of maximum tumescence, as are those of free-living chimpanzees; and individual differences in sexual attractiveness among females were apparently based on a maturational threshold of swelling size, a phenomenon that has also been reported for wild chimpanzees.  相似文献   
993.
994.
Chickens of Regional Poultry Research Laboratory (RPRL) inbred line 63 regress sarcomas induced by Bryan high-titer Rous sarcoma virus to a greater extent than chickens of line RPRL 100, although these lines are identical for the major histocompatibility B complex. They differ, however, at three independent autosomal loci: Ly-4 and Th-1 determine the surface alloantigens of partly overlapping subsets of T lymphocytes, and Bu-1 determines a surface alloantigen of B lymphocytes. The association of genotypes at these loci with quantitative variation in their ability to regress Rous sarcomas was tested in segregating F4 generation progeny derived from crosses of lines 100 and 63. The Ly-4 and Bu-1 genotypes showed association with Rous sarcoma regression, but the Th-1 genotype did not. Chickens of the Ly-4 a/Ly-4 a, Bu-1 b/Bu-1 b and Ly-4 b/Ly-4 b, Bu-1 a/Bu-1 a genotypes had a significantly higher regressor ability than the other two double homozygous genotypes. These results indicate that higher regression is associated with (1) interaction between the Ly-4 and Bu-1 loci, and (2) complementation between either the line 6 Ly-4 a allele and the line 100 Bu-1 b allele, or the line 100 Ly-4 b allele and the line 6 Bu-1 a allele.  相似文献   
995.
Exercise thermoregulation after 14 days of bed rest   总被引:1,自引:0,他引:1  
  相似文献   
996.
Antileukoprotease (ALP) was isolated from bronchial secretion. The amino acid composition as well as the N-terminal sequence were determined. No homology with other low molecular weight proteinase inhibitors was found.  相似文献   
997.
998.
In phenotype the mycoplasmas are very different from ordinary bacteria. However, genotypically (i.e., phylogenetically) they are not. On the basis of ribosomal RNA homologies the mycoplasmas belong with the clostridia, and indeed have specific clostridial relatives. Mycoplasmas are, however, unlike almost all other bacteria in the evolutionary characteristics of their ribosomal RNAs. These RNAs contain relatively few of the highly conserved oligonucleotide sequences characteristic of normal eubacterial ribosomal RNAs. This is interpreted to be a reflection of an elevated mutation rate in mycoplasma lines of descent. A general consequence of this would be that the variation associated with a mycoplasma population is augmented both in number and kind, which in turn would lead to an unusual evolutionary course, one unique in all respects. Mycoplasmas, then, are actually tachytelic bacteria. The unusual evolutionary characteristics of their ribosomal RNAs are the imprints of their rapid evolution.  相似文献   
999.
We describe a direct analysis for the simultaneous quantitative determination of 4-hydroxy-3-methoxyphenylacetic (homovanillic) acid and other monoamine metabolites in human lumbar cerebrospinal fluid, utilizing reversed-phase high-performance liquid chromatography with amperometric detection. In addition, a rapid isocratic separation was developed for homovanillic acid in the presence of other endogenous compounds.Twenty-five unselected diagnostic specimens of human lumbar cerebrospinal fluid were extracted with ethyl acetate and subsequently analyzed using the described method. Chromatographic peaks were identified on the basis of retention behavior and ratio of responses at several oxidation potentials.Although our quantitative results correlate well with the literature values, the data were not interpreted clinically since samples were obtained from routine, diagnostic testing of patients admitted to the medical or neurologic services at the Mount Sinai Hospital.  相似文献   
1000.
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