全文获取类型
收费全文 | 11107篇 |
免费 | 1177篇 |
国内免费 | 1篇 |
出版年
2021年 | 140篇 |
2020年 | 82篇 |
2019年 | 109篇 |
2018年 | 162篇 |
2017年 | 136篇 |
2016年 | 205篇 |
2015年 | 400篇 |
2014年 | 450篇 |
2013年 | 545篇 |
2012年 | 657篇 |
2011年 | 643篇 |
2010年 | 389篇 |
2009年 | 338篇 |
2008年 | 542篇 |
2007年 | 582篇 |
2006年 | 503篇 |
2005年 | 521篇 |
2004年 | 496篇 |
2003年 | 459篇 |
2002年 | 437篇 |
2001年 | 204篇 |
2000年 | 196篇 |
1999年 | 178篇 |
1998年 | 168篇 |
1997年 | 159篇 |
1996年 | 120篇 |
1995年 | 123篇 |
1994年 | 136篇 |
1993年 | 127篇 |
1992年 | 179篇 |
1991年 | 143篇 |
1990年 | 134篇 |
1989年 | 135篇 |
1988年 | 122篇 |
1987年 | 139篇 |
1986年 | 104篇 |
1985年 | 152篇 |
1984年 | 152篇 |
1983年 | 118篇 |
1982年 | 137篇 |
1981年 | 121篇 |
1980年 | 114篇 |
1979年 | 97篇 |
1978年 | 72篇 |
1977年 | 73篇 |
1976年 | 82篇 |
1975年 | 65篇 |
1974年 | 81篇 |
1973年 | 68篇 |
1972年 | 60篇 |
排序方式: 共有10000条查询结果,搜索用时 141 毫秒
41.
William Russell 《BMJ (Clinical research ed.)》1889,1(1463):70-71
42.
43.
Often we must balance being prepared to act quickly with being prepared to suddenly stop. The stop signal task (SST) is widely used to study inhibitory control, and provides a measure of the speed of the stop process that is robust to changes in subjects’ response strategy. Previous studies have shown that preparation affects inhibition. We used fMRI to separate activity that occurs after a brief (500 ms) warning stimulus (warning-phase) from activity that occurs during responses that follow (response-phase). Both of these phases could contribute to the preparedness to stop because they both precede stop signals. Warning stimuli activated posterior networks that signal the need for top-down control, whereas response phases engaged prefrontal and subcortical networks that implement top-down control. Regression analyses revealed that both of these phases affect inhibitory control in different ways. Warning-phase activity in the cerebellum and posterior cingulate predicted stop latency and accuracy, respectively. By contrast, response-phase activity in fronto-temporal areas and left striatum predicted go speed and stop accuracy, in pre-supplementary motor area affected stop accuracy, and in right striatum predicted stop latency and accuracy. The ability to separate hidden contributions to inhibitory control during warning-phases from those during response-phases can aid in the study of models of preparation and inhibitory control, and of disorders marked by poor top-down control. 相似文献
44.
45.
Myosin associated with the male germ cells of angiosperms interacts with actin, promoting transport of the non-motile generative
and later sperm cells in the pollen tube. Myosin localizing on the sperm cell plasma membrane seems negligible in Plumbago, as reflected by the absence of: (i) anti-myosin labeling using immunoelectron microscopy, (ii) sperm motility on actin matrices,
and (iii) electrophoretic movement changes after addition of antibody. Sperm cells injected directly into actively streaming
Nitella internodal cells, however, follow actin bundles and their movement is sensitive to ATP and Mg2+. This may be based on simple charge binding since negatively charged latex beads also migrate on actin, whereas neutral or
positively-charged latex beads do not. Sperm cells are negatively charged according to capillary microelectrophoresis, whereas
killed sperm cells, which are positively charged do not migrate. The sperm cell that normally fertilizes the egg has a higher
calculated charge (8.277 × 103 esu/cm2) compared with the sperm cell that fuses with the central cell (6.120 × 103 esu/cm2).
Received: 15 December 1998 / Accepted: 21 January 1999 相似文献
46.
47.
48.
Mouse specific-locus test for the induction of heritable gene mutations by dibromochloropropane (DBCP) 总被引:1,自引:0,他引:1
The nematocide DBCP (1,2-dibromo-3-chloropropane) produced negative results in a specific-locus test for gene-mutation induction in the germline of male (101 X C3H)F1 mice, most of which were treated with 5 daily intraperitoneal injections of 80 mg/kg (total exposure, 400 mg/kg); a few received lower exposures. For treated spermatogonial stem cells, the finding of 2 mutations among 39519 offspring--a rate almost identical to the control rate--rules out (at the 5% significance level) an induced mutation frequency greater than 2.0 times the historical control rate. From treated poststem-cell stages, no mutants were found among 6240 offspring, ruling out (at the 5% significance level) a multiple of 8.0 times the control for these cell types. A multiple rearrangement (7 chromosomes involved in 3 translocations) found in one of the mutants probably arose as a postmeiotic event not associated with the DBCP treatment. The fertility of DBCP-treated males was not disturbed, in keeping with the absence of germ-cell toxicity and dominant lethals found by other investigators in these mice, and in contrast to results in certain other species. While the treated (101 X C3H)F1 mice are Ah-responsive, other findings make it questionable whether biotransformation of DBCP to reactive intermediates is accomplished via the Ah-receptor system. 相似文献
49.
50.