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   208165篇
  免费   17768篇
  国内免费   76篇
  226009篇
  2018年   1974篇
  2016年   2472篇
  2015年   2429篇
  2014年   3303篇
  2013年   4812篇
  2012年   5381篇
  2011年   5942篇
  2010年   4157篇
  2009年   3729篇
  2008年   5388篇
  2007年   5706篇
  2006年   5387篇
  2005年   5114篇
  2004年   5268篇
  2003年   5176篇
  2002年   5154篇
  2001年   10052篇
  2000年   10213篇
  1999年   7612篇
  1998年   2429篇
  1997年   2468篇
  1996年   2232篇
  1995年   2062篇
  1994年   1978篇
  1993年   2069篇
  1992年   6203篇
  1991年   6370篇
  1990年   6126篇
  1989年   6070篇
  1988年   5613篇
  1987年   5319篇
  1986年   4795篇
  1985年   4928篇
  1984年   3959篇
  1983年   3380篇
  1982年   2270篇
  1981年   2048篇
  1980年   2031篇
  1979年   3740篇
  1978年   2963篇
  1977年   2666篇
  1976年   2396篇
  1975年   2972篇
  1974年   3336篇
  1973年   3200篇
  1972年   3031篇
  1971年   2709篇
  1970年   2365篇
  1969年   2253篇
  1968年   2123篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Clastogenic effects of acrylamide in mouse bone marrow cells   总被引:5,自引:0,他引:5  
Acrylamide, known to induce dominant-lethal mutations (Shelby et al., 1986; Smith et al., 1986) and heritable translocations (Shelby et al., 1987) in rodent germ cells, was hitherto a questionable clastogen in rodent bone marrow (Shiraishi, 1978). Therefore, it was tested for chromosomal aberrations in mouse bone marrow cells, spermatogonia and by the micronucleus test. The intraperitoneally injected doses ranged from 50 to 150 mg/kg. In the chromosomal bone marrow test and the micronucleus assay positive results were obtained with acrylamide, and in the latter test the effect increased linearly with dose. Chromosomal aberrations were not induced in differentiating spermatogonia by the acute acrylamide treatment. Cisplatin was used as a positive control and gave the expected positive response in all 3 tests. The present results demonstrate that acrylamide is no exception among clastogens. It breaks chromosomes not only in mammalian germ cells but also in somatic cells.  相似文献   
992.
993.
R. E. Falk  L. Makowka  U. Ambus  J. A. Falk  R. Bugala  S. Landi 《CMAJ》1983,128(12):1385-8,1422
The experience of the Toronto General Hospital in the use of nonspecific stimulation of the immune system with bacille Calmette-Guérin (BCG) for the treatment of cancer of the gastrointestinal tract, malignant melanoma and breast cancer is described. The results are presented in terms of survival curves. The use of BCG administered intraperitoneally in a randomized study of patients with gastric, pancreatic and colorectal cancer proved of no benefit. On the other hand, when BCG was given orally in a randomized study of patients with resectable cancer of the colon and in nonrandomized consecutive studies of patients with malignant melanoma and stage IV carcinoma of the breast survival was increased. In a group of patients with advanced gastrointestinal cancer selective stimulation of the immune system with NED 137 produced a significant increase in survival when compared with the survival of historical controls (the patients given BCG intraperitoneally along with 5-fluorouracil for gastrointestinal cancer). The results of these studies suggest the need for a more rational approach in manipulating the immune response that would combine chemotherapy with selective stimulation of the immune system.  相似文献   
994.
Administration of delta-sleep-inducing peptide (DSIP) in vivo in a dose of 30 microgram/kg bw brings about MAO-A (substrate-serotonin) activation in synaptosome subfractions and cellular mitochondria from the brain structures (motor cortex, nucleus caudatus, thalamus). Activity of MAO-B (substrate-p-nitrophenylethylamine) and acetylcholinesterase was inhibited negligibly and specifically in subcellular fractions of the test brain structures. The results suggest that DSIP effects the regulatory or modulation function in the synapse. As one of the elements of sleep mechanisms this peptide induces a number of processes, particularly in serotonin metabolism.  相似文献   
995.
996.
The characteristic feature of replicas obtained from the freeze-fractures of B. pertussis unfixed cultures developing on casein charcoal agar for 1-7 days is the associative growth of highly polymorphic cells, ensured by the ramified system of intercellular connections (IC) formed by the derivatives of the outer layers of the cell wall. This proves that the associative location of bacterial cells, linked by numerous IC, in the preparation is not the artefact appearing in the process of their chemical fixation. In replicas obtained from the freeze-fractures of B. pertussis cultures, previously fixed with glutaraldehyde, osmic acid and uranyl acetate, oval cells with the cytoplasm having a relatively homogeneous structure and with the smoothed-out three-layer cell wall prevail. As a rule, IC are limited to the sites of direct contacts between individual cells.  相似文献   
997.
The presence of 5 alpha-sitostanol (24-ethyl-5 alpha-cholestan-3 beta-ol) in serum of a patient with the rare genetic disease phytosterolemia was confirmed. This study aimed at clarifying the pathway(s) for the formation of 5 alpha-sitostanol, by use of rats with bile fistula. 5 alpha-Sitostanol was formed only slowly from sitosterol, but readily from 24-ethyl-4-cholesten-3-one. Some conversion was also obtained with 7 alpha-hydroxysitosterol as precursor. In view of the low rate of 7 alpha-hydroxylation of sitosterol, however, a pathway from sitosterol to 5 alpha-sitostanol involving 7 alpha-hydroxysitosterol as intermediate is probably of small physiological importance. Intestinal microorganisms are not essential for the above conversions, since the 5 alpha-sitostanol was found in bile from bile fistula rats. 5 alpha-Sitostanol was converted to water soluble metabolites (bile acids) much more slowly than was cholestanol (5 alpha-cholestan-3 beta-ol), and was accumulated serum to a much larger extent.  相似文献   
998.
Bovine-heart mitochondrial complex III was purified in the presence of Triton X-100, and the size and shape of the resulting protein-surfactant-phospholipid mixed micelles were investigated by laser light-scattering. The protein appears to be present in the form of a dimer, irrespective of temperature (between 25 and 40 degrees C) and protein concentration (between 0.5 and 5 mg/ml). The molecular weight of the micelle increases with temperature from 600 000 (25 degrees C) to 692 000 (40 degrees C). The variation of the solvent second virial coefficient in this temperature range suggests that, with increasing temperature, some of the free surfactant molecules become integrated in the mixed micelles. The average quadratic radius of gyration of these is of 42 +/- 5 nm, corresponding in our case to an ellipsoidal shape.  相似文献   
999.
Twenty-four thermophilic bacteriophages have been isolated from diverse sources such as compost, soil, silage and rotting straw. Although considerable individual host specificity was observed, the phages were able to infect most of the major taxonomic groups of Bacillus thermophiles. The phages varied considerably in morphology and size; the phage heads were either cylindrical or polyhedral with tails varying in length between 15 and 500 nm. Most of the phages were stable at 50 degrees C for 4-5 h but at 70 degrees C the plaque-forming units decreased by between 10(2)- and 10(7)-fold in 2 h. The DNA of morphologically similar phages was examined by restriction enzyme analysis, and some differences in the DNA fragment patterns were found. Efficiency of plating data indicated that 'B. caldotenax' has a restriction and modification system. These phages may be valuable for the study of the genetics of thermophilic bacilli: transduction of 'B. caldotenax' and 'B. caldovelox' by phage JS017 has been observed.  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