全文获取类型
收费全文 | 429820篇 |
免费 | 48299篇 |
国内免费 | 165篇 |
专业分类
478284篇 |
出版年
2018年 | 4011篇 |
2016年 | 5443篇 |
2015年 | 7162篇 |
2014年 | 8416篇 |
2013年 | 11575篇 |
2012年 | 13207篇 |
2011年 | 13630篇 |
2010年 | 9316篇 |
2009年 | 8638篇 |
2008年 | 12419篇 |
2007年 | 12898篇 |
2006年 | 12086篇 |
2005年 | 11597篇 |
2004年 | 11605篇 |
2003年 | 10862篇 |
2002年 | 10652篇 |
2001年 | 17470篇 |
2000年 | 17472篇 |
1999年 | 13960篇 |
1998年 | 5071篇 |
1997年 | 5297篇 |
1996年 | 4944篇 |
1995年 | 4641篇 |
1994年 | 4523篇 |
1993年 | 4562篇 |
1992年 | 11634篇 |
1991年 | 11567篇 |
1990年 | 11315篇 |
1989年 | 10934篇 |
1988年 | 10508篇 |
1987年 | 10093篇 |
1986年 | 9387篇 |
1985年 | 9253篇 |
1984年 | 7773篇 |
1983年 | 6712篇 |
1982年 | 5175篇 |
1981年 | 4623篇 |
1980年 | 4475篇 |
1979年 | 7437篇 |
1978年 | 5874篇 |
1977年 | 5426篇 |
1976年 | 5217篇 |
1975年 | 5602篇 |
1974年 | 6308篇 |
1973年 | 6173篇 |
1972年 | 5778篇 |
1971年 | 5237篇 |
1970年 | 4635篇 |
1969年 | 4581篇 |
1968年 | 4427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.
Douglas G. Gilmour Walter M. Collins Tom L. Fredericksen Willard E. Urban Jr. Paul F. Ward Nancy L. DiFronzo 《Immunogenetics》1986,23(1):1-6
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
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.
Ante M. Krstulovi Laura Bertani-Dziedzic Sandra Bautista-Cerqueira Stanley E. Gitlow 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1982,227(2):379-389
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.