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   710987篇
  免费   77625篇
  国内免费   216篇
  788828篇
  2018年   6727篇
  2016年   8579篇
  2015年   10423篇
  2014年   12996篇
  2013年   18497篇
  2012年   20966篇
  2011年   21540篇
  2010年   14655篇
  2009年   13391篇
  2008年   19222篇
  2007年   20229篇
  2006年   19039篇
  2005年   18269篇
  2004年   18319篇
  2003年   17644篇
  2002年   17221篇
  2001年   29703篇
  2000年   30006篇
  1999年   23868篇
  1998年   8300篇
  1997年   8526篇
  1996年   8210篇
  1995年   7826篇
  1994年   7764篇
  1993年   7624篇
  1992年   20497篇
  1991年   20654篇
  1990年   20306篇
  1989年   19883篇
  1988年   18625篇
  1987年   17749篇
  1986年   16510篇
  1985年   16894篇
  1984年   13994篇
  1983年   12132篇
  1982年   9113篇
  1981年   8407篇
  1980年   7966篇
  1979年   13696篇
  1978年   10632篇
  1977年   9819篇
  1976年   9410篇
  1975年   10652篇
  1974年   11243篇
  1973年   11038篇
  1972年   10246篇
  1971年   9104篇
  1970年   7988篇
  1969年   7575篇
  1968年   6963篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
In a longitudinal prospective study of 1,529 women pregnant in 1974-1975, aspirin and acetaminophen were the two medications most frequently taken during the first half of pregnancy (46 and 41%, respectively). In a selected cohort of 421 offspring of these women, examined at 4 years of age, maternal aspirin use during the first half of pregnancy was significantly related to IQ and attention decrements in the exposed children. Multiple regression analyses were used to statistically adjust for a variety of potentially confounding factors including demographic characteristics, child characteristics, other exposures, and lifestyle/environmental variables. Continuous dose-response and step-function parameterizations of aspirin exposure were both statistically significant and not clearly distinguishable from each other. The estimated aspirin effect is significantly greater for girls than boys. Aspirin effects on offspring function were found in the absence of effects on physical size both at birth and at 4 years. Maternal acetaminophen use was not significantly related to child IQ or attention. As this exploratory research originated from observations of a data set gathered for other purposes, it would be desirable to have these findings replicated in other studies. Further follow-up of the children at a later age is planned.  相似文献   
102.
The chemical synthesis and physical data of several new esters of testosterone (17 beta-hydroxyandrost-4-en-3-one), which contain either a halogeno or an alkoxy substituent in the acid chain, are reported.  相似文献   
103.
Summary The exploitation of wild perennial species of subgenus Glycine has been formidable in soybean breeding programs because of extremely poor crossability and an early pod abortion. The combination of gibberellic acid application to hybridized gynoecia and improved seed culture media formulations resulted in a new intersubgeneric hybrid between Glycine max (L.) Merr. (2n=40) and G. clandestina Wendt. (2n=40). Of the 31 immature seeds cultured, 1 regenerated 21 plants through organogenesis while the remaining 30 failed to germinate. All the regenerated plants were similar morphologically, carried expected 2n=40, possessed hybrid isozyme patterns and were completely sterile. Complete absence of chromosome pairing was observed in 40.9% sporocytes. The occurrence of 1 to 6 loosely paired rod bivalents suggests some possibilities of allosyndetic pairing. Hybrid plants set aborted pods after backcrossing to G. max.  相似文献   
104.
Summary Culture-derived plants and cell cultures of potato (Solanum tuberosum L.) respond to the application of the herbicides SYS 67 ME (MCPA) and OMNIDEL (Na-2,2-dichloropropionate) in a comparable fashion. By gradually increasing the herbicide concentration, cell lines were developed which tolerated 50 mg/l of ME or 300 mg/l of OMNIDEL. Any further increase in concentration resulted in the death of all cell cultures. From cell cultures that had been able to grow on media supplemented with 30 mg/l of ME, regenerate plants were obtained that were also tolerant to this concentration. This new trait was retained even after repeated vegetative propagation of the plants.  相似文献   
105.
106.
107.
Studies have been made on the role of the thyroid in the development of rats. In the first group of experiments, newborn rat received within a month mercazolyl which inhibits the activity of the thyroid; in animals of the second group, mercazolyl injections were combined with cold exposures which stimulated motor activity in animals. It was found that hypothyroid rats in both groups exhibit retardation of growth as compared to normal animals. However, retardation is less significant in animals of the second group, as it is indicated by smaller changes in the protein content and total mass of skeletal muscles.  相似文献   
108.
109.
A simple electrophoretic method is introduced allowing to isolate five fractions of skeletal muscle ST-system vesicles. In a previous study differences in lipid content, 3H-ouabain binding and in presence of triads in individual fractions (Lehotsky et. al. 1986) were analysed. In the present study biochemical characterization was extended, and (in accordance with previous results) major differences were observed to exist between fraction 1 and fractions 3 and 4. SDS-PAGE showed that fractions 3 and 4 were enriched in a protein with m.w. 100 kD, these fractions showing the highest specific activities of (Mg2+ + Ca2+)-ATPase and oxalate-supported Ca2+-uptake; activities of Mg2+-ATPase and surface membrane marker enzymes were the lowest in these fractions. On the other hand, in fraction 1 the highest activities of Mg2+-ATPase and marker enzymes of the surface membrane were observed together with a decreased content of the 100 kD protein and activities of Ca2+ transport. It could be concluded that the method is suitable to differentiate between relatively pure SR (fractions 3 and 4) and fractions rich in sarcolemma or T-tubules components (fractions 1 and 5).  相似文献   
110.
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