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   436252篇
  免费   48792篇
  国内免费   388篇
  485432篇
  2018年   4176篇
  2016年   5606篇
  2015年   7442篇
  2014年   8679篇
  2013年   11898篇
  2012年   13559篇
  2011年   13964篇
  2010年   9526篇
  2009年   8875篇
  2008年   12704篇
  2007年   13141篇
  2006年   12280篇
  2005年   11732篇
  2004年   11814篇
  2003年   11040篇
  2002年   10832篇
  2001年   17725篇
  2000年   17708篇
  1999年   14160篇
  1998年   5147篇
  1997年   5356篇
  1996年   4996篇
  1995年   4728篇
  1994年   4571篇
  1993年   4615篇
  1992年   11748篇
  1991年   11705篇
  1990年   11410篇
  1989年   11045篇
  1988年   10615篇
  1987年   10192篇
  1986年   9462篇
  1985年   9344篇
  1984年   7843篇
  1983年   6778篇
  1982年   5214篇
  1981年   4673篇
  1980年   4518篇
  1979年   7482篇
  1978年   5928篇
  1977年   5455篇
  1976年   5237篇
  1975年   5645篇
  1974年   6349篇
  1973年   6213篇
  1972年   5792篇
  1971年   5261篇
  1970年   4671篇
  1969年   4602篇
  1968年   4438篇
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
971.
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   
972.
973.
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   
974.
Exercise thermoregulation after 14 days of bed rest   总被引:1,自引:0,他引:1  
  相似文献   
975.
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   
976.
977.
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   
978.
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   
979.
980.
A new species,Acanthocephaloides cyrusi, is described from the fishesSolea bleekeri andPomadasys commersoni from Lake St. Lucia, Natal, South Africa. It is distinguished from the other species in the genus by the more marked sexual dimorphism in length, the arrangement of hooks, the proboscis with the longest hooks at the anterior-most extremity and the greater size of the proboscis hooks and body spines. An acanthella, which may represent this species, was found in the tanaidApseudes digitalis.  相似文献   
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