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   833829篇
  免费   85048篇
  国内免费   1338篇
  920215篇
  2018年   17640篇
  2017年   16250篇
  2016年   15871篇
  2015年   12304篇
  2014年   14396篇
  2013年   20648篇
  2012年   26524篇
  2011年   34823篇
  2010年   26446篇
  2009年   21929篇
  2008年   29285篇
  2007年   31299篇
  2006年   20204篇
  2005年   19700篇
  2004年   19852篇
  2003年   19205篇
  2002年   18386篇
  2001年   39482篇
  2000年   39578篇
  1999年   30601篇
  1998年   9594篇
  1997年   10269篇
  1996年   9511篇
  1995年   8771篇
  1994年   8528篇
  1993年   8618篇
  1992年   24343篇
  1991年   23533篇
  1990年   22662篇
  1989年   22049篇
  1988年   20472篇
  1987年   19055篇
  1986年   17588篇
  1985年   17334篇
  1984年   14096篇
  1983年   11965篇
  1982年   8786篇
  1981年   7882篇
  1980年   7539篇
  1979年   13087篇
  1978年   10076篇
  1977年   9293篇
  1976年   8317篇
  1975年   9291篇
  1974年   10040篇
  1973年   9748篇
  1972年   9003篇
  1971年   8297篇
  1970年   7013篇
  1969年   6694篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
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   
43.
44.
45.
The content of neurospecific proteins S-100, GFA and D2 was measured in malignant cerebral tumors by electrophoresis with the use of monospecific antisera. Concomitant measurement of proteins S-100 and GFA is a more reliable diagnostic criterion as to the tumor histogenesis than study of each protein alone. D2 protein appeared to be the most stable specific marker.  相似文献   
46.
Mechanism and dynamics of conformational ordering in xanthan polysaccharide   总被引:4,自引:0,他引:4  
The thermally induced order-disorder transition of xanthan (extracellular bacterial polysaccharide from Xanthomonas campestris) has been investigated by optical rotation, differential scanning calorimetry, stopped-flow reaction kinetics and low-angle laser light scattering, and the results have been analysed in terms of Zimm -Bragg helix-coil transition theory. The reciprocal of the transition midpoint temperature (Tm) varies linearly with the logarithm of cation (K+) the salt dependence of Tm, is in agreement with Manning polyelectrolyte theory the ordered structure. The associated increase in cation binding, calculated from the salt dependence of tm, is in agreement with the Manning polyelectrolyte theory for one of the candidate structures from X-ray diffraction, a 5(1) single helix stabilized by packing of side-chains along the polymer backbone, but not for the alternative double-helix structure that has also been proposed. At each salt concentration, the two fundamental parameters of the Zimm -Bragg theory, s and sigma, were calculated. The equilibrium constant for growth of the ordered structure (s) is derived directly from calorimetric measurement of transition enthalpy (delta Hcal ), and sigma, which quantifies the relative instability of the helix nucleus, is derived from the ratio of delta Hcal to the apparent transition enthalpy (delta Happ ) obtained by van't Hoff analysis of the optical rotation data. The temperature course of conformational ordering calculated theoretically is in good quantitative agreement with experimental results from both optical rotation and scanning calorimetry. The calculated average length of stable, ordered chain-sequences increases with decreasing temperature, but equals or exceeds the total chain length from light scattering only at temperatures more than approximately equal to 70 K below Tm, suggesting that ordered and disordered regions may co-exist within the same xanthan molecule. Consistent with this interpretation, the observed rate of conformational ordering increases sharply under conditions where the starting solution for dynamic measurements is partially ordered, suggesting that ordered sequences within each chain may act as helix nuclei for adjacent disordered regions, so that helix growth, rather than the slower nucleation process, becomes rate limiting.  相似文献   
47.
Hostile intercommunity relations, including attacking and killing extra-community infants of both sexes have occurred at most wild chimpanzee sites. We describe three recent cases of intercommunity attacks on infants committed by members of the Ngogo chimpanzee community in Kibale National Park, Uganda. Two of the attacks resulted in confirmed infanticides while a third attack probably resulted in the infant's death. In common with previous accounts of chimpanzee infanticides, the attacks described here occurred during boundary patrols outside the Ngogo community's usual range, adult and adolescent males were the main participants, one infant was cannibalized after being killed, and the victims’ mothers did not accompany the attacking party back to the Ngogo range. However, the patrol parties during each infanticide were larger than before and included females from the Ngogo community. Our observations indirectly support both the range expansion and imbalance of power hypotheses, which address why and under which conditions chimpanzee intercommunity encounters lead to aggression. These cases of intercommunity infanticide add to the growing database of the phenomenon in wild chimpanzees.  相似文献   
48.
Formation of rings from Drosophila DNA fragments   总被引:1,自引:0,他引:1  
  相似文献   
49.
50.
Transcriptive expression during sea urchin embryogenesis   总被引:2,自引:0,他引:2  
  相似文献   
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