全文获取类型
收费全文 | 10984篇 |
免费 | 970篇 |
国内免费 | 4篇 |
专业分类
11958篇 |
出版年
2022年 | 51篇 |
2021年 | 114篇 |
2020年 | 72篇 |
2019年 | 116篇 |
2018年 | 149篇 |
2017年 | 126篇 |
2016年 | 178篇 |
2015年 | 320篇 |
2014年 | 377篇 |
2013年 | 491篇 |
2012年 | 678篇 |
2011年 | 735篇 |
2010年 | 466篇 |
2009年 | 420篇 |
2008年 | 577篇 |
2007年 | 597篇 |
2006年 | 600篇 |
2005年 | 602篇 |
2004年 | 589篇 |
2003年 | 545篇 |
2002年 | 558篇 |
2001年 | 143篇 |
2000年 | 108篇 |
1999年 | 162篇 |
1998年 | 165篇 |
1997年 | 124篇 |
1996年 | 112篇 |
1995年 | 111篇 |
1994年 | 122篇 |
1993年 | 138篇 |
1992年 | 115篇 |
1991年 | 107篇 |
1990年 | 93篇 |
1989年 | 113篇 |
1988年 | 90篇 |
1987年 | 94篇 |
1986年 | 81篇 |
1985年 | 113篇 |
1984年 | 114篇 |
1983年 | 125篇 |
1982年 | 131篇 |
1981年 | 113篇 |
1980年 | 124篇 |
1979年 | 87篇 |
1978年 | 62篇 |
1977年 | 67篇 |
1976年 | 71篇 |
1975年 | 68篇 |
1974年 | 73篇 |
1972年 | 49篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Bruce P. Squires 《CMAJ》1993,149(2):127-130
42.
The basal clade Burnetiamorpha is known from only two specimens representing two genera, Proburnetia from the Severodvinskian horizon of the Vyatka River Basin in the Kotelnich district of Russia, and Burnetia from the Dicynodon Assemblage Zone of the Beaufort Group of South Africa. Both genera are of Late Tatarian (Late Permian) age. This paper describes the cranial morphology of a new genus of burnetiamorph, Bullacephalus , from the Late Permian Tapinocephalus Assemblage Zone of the Beaufort Group of South Africa. It is known from a relatively complete skull and lower jaw and is the best preserved burnetiamorph yet discovered. Apart from being the oldest member of the clade, Bullacephalus is also morphologically the least derived and provides new evidence on the phylogeny of this poorly understood group of basal therapsids. 相似文献
43.
Schulze Peter C.; Williamson Craig E.; Hargreaves Bruce R. 《Journal of plankton research》1995,17(6):1233-1243
Conventional methods for measuring zooplankton distributionsare too laborious and time consuming to permit sufficient temporaland spatial resolution in many instances. An ability to makemore efficient and precise measurements would be useful. Weevaluated the potential for using the video system of a commerciallyavailable remotely operated vehicle (ROV) to measure the distributionand abundance of zooplankton by calibrating ROV counts withcounts based on a conventional sampling procedure (a Schindlertrap), and by using an ROV to measure the density of zooplanktonin a small lake. As configured here, this particular ROV wassuitable for measuring the density of the cladocerans Daphniaand Holopedium. It was also suitable for assessing the distribution,but not absolute densities, of Chaoborus and Leptodora. Imagequality was inadequate for quantitative estimates of copepod(Diaptomus minutus) abundance, and prevented us from studyingbehavioral responses of copepods to the vehicle. We concludethat the ROV has at least three useful features: it can be usedto locate patches of those species that are imaged effectively;a large number of samples (videotapes) can be collected almostsynoptically with high spatial resolution; the ROV enables insitu observation of zooplankton. The ROV also has three importantlimitations: the small image volume makes it difficult to studyrare organisms; inadequate image resolution precludes studiesof relatively small organisms (e.g. the calanoid copepod D.minutus);zooplankton respond to the presence of the ROV. 相似文献
44.
Bruce B. Jarvis C. S. Yatawara Sharon L. Greene Vivekanada M. Vrudhula 《Applied microbiology》1984,48(3):673-674
Verrucarol was obtained from a simple procedure that involved the hydrolysis of a crude extract of a culture of Myrothecium verrucaria ATCC 24571. 相似文献
45.
Glenn D. Prestwich Wai-Si Eng R.Michael Roe Bruce D. Hammock 《Archives of biochemistry and biophysics》1984,228(2):639-645
Four 3-alkylthio-1,1,1-trifluoro-2-propanones with juvenile hormone-like side chains were prepared from citronellol and homogeraniol. These substrates were designed as possible transition-state analogs for the juvenile hormone (JH)-specific esterases present in insects. These four isoprenoid trifluoromethyl ketones were assayed in vitro with JH esterase and general esterases from larvae of the cabbage looper, Trichoplusia ni (Lepidoptera, Noctuidae), and with eel acetylcholinesterase and bovine chymotrypsin. JH esterase inhibition I50 values were in the nanomolar range for all four compounds, while the other esterases had I50'S which were 103 to 105 higher. The high selectivity of these inhibitors is believed to be due to their similarity in size and functionality to natural JH III. Treatment of T. ni larvae in vivo with solutions of the most active analog, 3-[(E)-4,8-dimethyl-3,7-nonadienylthio]-1,1,1-trifluoro-2-propanone (DNTFP) causes a dose-dependent delay in pupation and a concurrent selective inhibition of JH esterase. These data support the hypothesis that the reduction in in vivo JH titer in larval T. ni is due, in part, to hydrolysis of the hormone by selective esterases. DNTFP appears to be competing with JH for the active site of JH esterase. 相似文献
46.
A new cell culture microcarrier that can be covalently bonded by cell attachment proteins and can be thin-sectioned for electron microscopy was synthesized. It was easily made by sulfonating cross-linked polystyrene beads for a negative surface charge followed by covalent attachment of polyethylenimine for a positive charge. Cell attachment proteins, e.g. collagen, was covalently bonded directly to the microcarrier using a carbodiimide or after activating the microcarrier surface with glutaraldehyde. HeLa-S3 cells attached, spread and grew to confluence more efficiently on the positive microcarriers and those coated with collagen than on the negative ones. Endothelial cells grew best on those with a negative surface charge. The nature of the microcarrier surface was not the only aspect involved in cell adhesion but also the type of serum proteins adsorbed. Qualitatively different proteins coated the microcarriers depending upon whether the carrier was negative, positive or coated with collagen. Comparison of various types of available microcarriers indicated that the modified cross-linked polystyrene beads used here were best for transmission and scanning electron microscopy. Endothelial cells grown on the microcarriers had the same ultrastructure as cells grown in monolayers in culture dishes. Of a variety of microcarriers tested the modified cross-linked polystyrene beads were the only ones that could be used for both ultrastructural and biochemical techniques. 相似文献
47.
48.
Significant polymorphism of serum complement components Bf, C2, C3, C6, and C8 in the chimpanzee has been demonstrated. The data are consistent with the hypothesis thatC2 andBf are closely linked toChLA and argue against close linkage ofChLA toC3 or toC8, as in man. In addition, a blank allele for C6 and C6 deficiency was detected in several chimps. 相似文献
49.
50.