全文获取类型
收费全文 | 5876篇 |
免费 | 638篇 |
国内免费 | 397篇 |
出版年
2023年 | 134篇 |
2022年 | 111篇 |
2021年 | 188篇 |
2020年 | 250篇 |
2019年 | 333篇 |
2018年 | 279篇 |
2017年 | 251篇 |
2016年 | 241篇 |
2015年 | 249篇 |
2014年 | 390篇 |
2013年 | 458篇 |
2012年 | 230篇 |
2011年 | 293篇 |
2010年 | 257篇 |
2009年 | 257篇 |
2008年 | 299篇 |
2007年 | 297篇 |
2006年 | 290篇 |
2005年 | 253篇 |
2004年 | 230篇 |
2003年 | 212篇 |
2002年 | 169篇 |
2001年 | 128篇 |
2000年 | 102篇 |
1999年 | 106篇 |
1998年 | 106篇 |
1997年 | 85篇 |
1996年 | 77篇 |
1995年 | 67篇 |
1994年 | 69篇 |
1993年 | 44篇 |
1992年 | 58篇 |
1991年 | 37篇 |
1990年 | 25篇 |
1989年 | 30篇 |
1988年 | 26篇 |
1987年 | 36篇 |
1986年 | 29篇 |
1985年 | 23篇 |
1984年 | 23篇 |
1983年 | 15篇 |
1982年 | 18篇 |
1981年 | 15篇 |
1980年 | 24篇 |
1979年 | 11篇 |
1978年 | 15篇 |
1977年 | 13篇 |
1976年 | 8篇 |
1975年 | 16篇 |
1973年 | 9篇 |
排序方式: 共有6911条查询结果,搜索用时 31 毫秒
91.
John J. Shea 《Evolutionary anthropology》1992,1(4):143-150
Stone tools are the most durable and ubiquitous residue of prehistoric hominid activity. For this reason, archeologists attempt to learn as much as possible about hominid behavior from the analysis of lithic artifacts. Lithic microwear analysis reconstructs aspects of stone-tool use from patterned variation in the traces of microscopic wear on those tools. The analysis of lithic microwear traces has increased our understanding of how stone tools were used in contexts ranging from the early Pleistocene to the ethnographic present. 相似文献
92.
Enhanced glucan formation of filamentous fungi by effective mixing,oxygen limitation and fed-batch processing 总被引:2,自引:0,他引:2
U. Rau E. Gura E. Olszewski F. Wagner 《Journal of industrial microbiology & biotechnology》1992,9(1):19-25
Summary Glucan formation ofSchizophyllum commune andSclerotium glucanicum were investigated. Process data obtained during batch cultivation are presented. Glucan release can be improved by oxygen limitation. Thus, growth and glucan release are influenced by oxygen in opposite ways. Possible pathways of this oxygen-dependent regulation are discussed. A draft-tube/propeller system, rushtonturbine-, fan- and helicon-ribbon-impeller as well as a fundaspi and intermig agitator were tested. The 4-bladed fan impeller withd
*=0.64 yielded the best results, since effective bulk mixing is much more important than bubble break up (micromixing) with regard to this system. Fed-batch cultivation always resulted in higher rates of glucan formation than the batch process. 相似文献
93.
Marie-Renée de Roubin Michael D. Cailas Shi-Hsiang Shen Denis Groleau 《Journal of industrial microbiology & biotechnology》1992,9(1):69-72
Summary A central composite design (CCD) was used to evaluate, for the purpose of future process optimization, the influence of pH, yeast extract and ammonium chloride concentrations on the proportion of periplasmic hepatitisB pre-S2 antigen in the recombinant yeastHansenula polymorpha. Each factor was tested at five levels, and a second order polynomial model for the proportion of periplasmic antigen was fitted to the treatment combinations. pH showed the greatest effect: the proportion of periplasmic antigen was greatly increased at the higher pH levels. At the higher pH levels used, the proportion of periplasmic antigen was enhanced by a high concentration of ammonium chloride. Additional experiments have confirmed both the validity of the selected model and the optimal conditions found. A significant correlation was found between the proportion of periplasmic antigen and the total yield of antigen. These results indicated that is should be possible to modulate the distribution of the pre-S2 antigen between the periplasm and the cytoplasm of the yeast. 相似文献
94.
The possibility that the feeding rates of larvae of S. austeni and S. lineatum are influenced by the nature of the food material is considered in relation to natural river conditions and laboratory feeding experiments. Diatoms appeared to be the most satisfactory food source and bacteria (Pseudomonas) the poorest. Simuliid larvae attain high assimilation efficiencies when feeding on diatoms but the proportion of river suspended solids which they assimilate is probably very low. 相似文献
95.
Summary Each primary micromere and macromere of the D-quadrant ofDentalium was deleted, through the mesentoblast stage, to investigate the way in which the polar lobe cytoplasm exerts its influence on development.-D and -1D embryos form an apical tuft but no posttrochal structures.-2D embryos form an apical tuft and a reduced posttrochal region without a shell. -3D and -4D are externally similar to control embryos. -1d embryos and -1c embryos have an apical tuft with a reduced number of cilia. Embryos in which both 1c and 1d are deleted lack the apical tuft.-2d embryos lack shell and most other posstrochal structures. -3d and-4d embryos appear externally equivalent to controls.The polar lobe cytoplasm exerts its influence sequentially, and as inIlyanassa the maximal effect is at the third quartet stage. 相似文献
96.
E. Y. Lasfargues W. G. Coutinho A. S. Dion 《In vitro cellular & developmental biology. Plant》1979,15(9):723-729
Summary A human breast tumor cell line BT-474 derived from an invasive ductal carcinoma was experimentally infected in vitro with
a mouse mammary tumor virus from the RIII strain (RIII-MuMTV). The virus that replicated in the human cells was characterized
as a mouse virus by immunofluorescence, electron microscopy and the presence of a specific RNA-directed DNA polymerase. The
cells themselves were human as per the karyotype and isoenzyme migration patterns. It is concluded that human cells are susceptible
to the mouse mammary tumor virus and can, eventually, support its replication.
This work was supported by USPHS Grant CA-08515 from the National Cancer Institute and by NIH Contract N01-CP-81003. 相似文献
97.
98.
99.
100.