全文获取类型
收费全文 | 401691篇 |
免费 | 43788篇 |
国内免费 | 169篇 |
专业分类
445648篇 |
出版年
2018年 | 3838篇 |
2016年 | 5117篇 |
2015年 | 6554篇 |
2014年 | 7765篇 |
2013年 | 11171篇 |
2012年 | 12321篇 |
2011年 | 12618篇 |
2010年 | 8726篇 |
2009年 | 7707篇 |
2008年 | 11150篇 |
2007年 | 11535篇 |
2006年 | 10762篇 |
2005年 | 10332篇 |
2004年 | 10434篇 |
2003年 | 10069篇 |
2002年 | 9818篇 |
2001年 | 17886篇 |
2000年 | 17677篇 |
1999年 | 14077篇 |
1998年 | 4913篇 |
1997年 | 5016篇 |
1996年 | 4689篇 |
1995年 | 4504篇 |
1994年 | 4495篇 |
1993年 | 4384篇 |
1992年 | 11806篇 |
1991年 | 11423篇 |
1990年 | 11256篇 |
1989年 | 10795篇 |
1988年 | 10108篇 |
1987年 | 9668篇 |
1986年 | 8754篇 |
1985年 | 8882篇 |
1984年 | 7309篇 |
1983年 | 6415篇 |
1982年 | 4992篇 |
1981年 | 4456篇 |
1980年 | 4212篇 |
1979年 | 7364篇 |
1978年 | 5535篇 |
1977年 | 5156篇 |
1976年 | 4816篇 |
1975年 | 5316篇 |
1974年 | 5745篇 |
1973年 | 5879篇 |
1972年 | 5395篇 |
1971年 | 4994篇 |
1970年 | 4260篇 |
1969年 | 4188篇 |
1968年 | 3700篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
22.
23.
A hitherto unknown defect in the immune responsiveness of B lymphocytes from SJL mice has enabled us to distinguish two qualitatively distinct classes of signal delivered to B cells by C8-substituted guanine ribonucleosides. This defect renders B cells from SJL mice unresponsive to the inductive (early acting) signal of 8-mercaptoguanosine (8MGuo) that culminates in mitogenesis and nonspecific secretion of immunoglobulin. Unresponsiveness is not attributable to a shift in either the dose-response or kinetic profiles, nor can the presence of suppressor cells be demonstrated. In striking contrast, however, SJL B cells exhibit normal responsiveness to the differentiative (T cell-like, or late acting) signal provided by the substituted nucleoside. This signal enables SJL B cells, depleted of T cells, to respond to T cell-dependent antigens, and synergizes with T cell-derived lymphokines. These data suggest 1) that nonspecific secretion of immunoglobulin is dependent on both inductive and differentiative signals, 2) that antigen alone can supply an effective inductive signal for antigen-specific responses, and 3) that the SJL mouse will provide a useful model for selective study of inductive vs differentiative events. 相似文献
24.
A A Sukhopiatkina O S Bat A V Levitskaia V G Potapova 《Zhurnal mikrobiologii, epidemiologii, i immunobiologii》1985,(4):80-81
The study of the specific epidemiological and clinical features of Q fever revealed the existence of an active focus of infection among humans due to their contacts with agricultural animals in one of the districts of the region. The focus was manifested by group morbidity among the cattle-tending personnel of a dairy farm. The source of this infection was cattle. The infection was transferred mainly through the air. The disease took a moderately severe course. The study of the rickettsial contamination of humans, animals and ticks suggested the presence of the active epidemic process and made it possible to work out concrete antiepidemic measures. 相似文献
25.
A specific decrease in collagen synthesis in acutely fasted, vitamin C-supplemented, guinea pigs 总被引:6,自引:0,他引:6
Weight loss often results from various experimental conditions including scurvy in guinea pigs, where we showed that decreased collagen synthesis was directly related to weight loss, rather than to defective proline hydroxylation (Chojkier, M., Spanheimer, R., and Peterkofsky, B. (1983) J. Clin. Invest. 72, 826-835). In the study described here, this effect was reproduced by acutely fasting normal guinea pigs receiving vitamin C, as determined by measuring collagen and non-collagen protein production after labeling tissues in vitro with [3H]proline. Collagen production (dpm/microgram of DNA) decreased soon after initiating fasting and by 96 h it had reached levels 8-12% of control values. Effects on non-collagen protein were much less severe, so that the percentage of collagen synthesis relative to total protein synthesis was 20-25% of control values after a 96-h fast. These effects were not due to changes in the specific radioactivity of free proline. Refeeding reversed the effects on non-collagen protein production within 24 h, but collagen production did not return to normal until 96 h. The effect of fasting on collagen production was independent of age, sex, ascorbate status, species of animal, and type of connective tissue and also was seen with in vivo labeling. Pulse-chase experiments and analysis of labeled and pre-existing proteins by gel electrophoresis showed no evidence of increased collagen degradation as a result of fasting. Procollagen mRNA was decreased in tissues of fasted animals as determined by cell-free translation and dot-blot hybridization with cDNA probes. In contrast, there was no decrease in translatable mRNAs for non-collagen proteins. These results suggest that loss of nutritional factors other than vitamin C lead to a rapid, specific decrease in collagen synthesis mainly through modulation of mRNA levels. 相似文献
26.
F F Dias M K Shaikh Y B Bhatt D C Modi V R Subramanyam 《The Journal of applied bacteriology》1986,60(4):271-275
Mutants of Bacillus amyloliquefaciens resistant to at least 10 micrograms/ml of tunicamycin were isolated and shown to be pleiotropic. The mutants were more resistant to streptomycin, chloramphenicol, kanamycin and neomycin than was the parent strain but less resistant to penicillin G and tetracycline. They were more autolytic, presumably due to an altered cell wall. The mutants produced reduced levels of amylase, penicillinase and both metal and serine protease besides having an enhanced sporulation frequency and being more motile. 相似文献
27.
28.
29.
30.
T. B. Lapirova V. R. Mikryakova A. S. Mavrin G. A. Yinogradova 《Journal of Evolutionary Biochemistry and Physiology》2000,36(1):47-51
The paper presents data on a change in lysozyme content in tissues of spleen, liver and heart in fry of the Lena River sturgeon
exposed to the presence of sublethal concentrations of Hg2+, Cd2+, and Cu2+ under conditions of chronic experiment. It has been shown that the lysozyme content in fish tissues varies and has a phasic
character. The amplitude of fluctuations of this parameter depends on the moment of sampling, nature of the toxicant, and
structural-functional organization of the studied organs. 相似文献