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   739490篇
  免费   177721篇
  国内免费   30448篇
  947659篇
  2018年   8317篇
  2017年   7817篇
  2016年   10283篇
  2015年   13692篇
  2014年   15791篇
  2013年   20547篇
  2012年   23852篇
  2011年   24864篇
  2010年   18611篇
  2009年   21925篇
  2008年   23013篇
  2007年   23447篇
  2006年   21387篇
  2005年   20638篇
  2004年   20595篇
  2003年   19191篇
  2002年   19494篇
  2001年   36175篇
  2000年   34434篇
  1999年   31165篇
  1998年   15609篇
  1997年   15055篇
  1996年   14145篇
  1995年   14020篇
  1994年   13324篇
  1993年   12977篇
  1992年   26322篇
  1991年   26069篇
  1990年   25973篇
  1989年   24916篇
  1988年   23091篇
  1987年   21411篇
  1986年   20053篇
  1985年   19332篇
  1984年   15659篇
  1983年   13291篇
  1982年   10786篇
  1981年   9665篇
  1980年   9188篇
  1979年   14239篇
  1978年   11578篇
  1977年   10619篇
  1976年   9777篇
  1975年   10650篇
  1974年   11571篇
  1973年   11366篇
  1972年   10605篇
  1971年   9818篇
  1970年   8588篇
  1969年   8483篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
The content of the diterpene Marrubiin was assessed by GC-FID in leaves of Marrubium vulgare plants along their ontogeny. Maximum accumulation occurred just before flowering time and in fully expanded leaves. After feeding the plants with radio labeled [3H]-geranyl geranyl diphosphate, up to 70% of the radioactivity was recovered in HPLC-Rt coincidental with authentic Marrubiin, which was also characterized by GC-EIMS, thus confirming that the biosynthesis of Marrubiin proceeds through the 1-deoxy-D-xylulose-5-phosphate pathway. The major accumulation of radioactivity occurred in glandular trichome cells, and the product remained stable throughout.  相似文献   
972.
Electron micrographs of skin from xanthic (gold) sailfin mollies revealed numerous xanthophores, as well as scattered melanophores. The melanophores were seen to contain premelanosomes in various stages of development. This is consistent with the fact that xanthic mollies have been shown to be tyrosinase positive. Melanosomes in xanthic mollies appear to develop by one of two pathways: 1) from an endoplasmic reticulum-derived vesicle which develops an internal lamellar framework, and 2) by fusion of multiple Golgi-derived vesicles which lack an internal lamellar framework. Analysis of the pigments in the skin of the xanthic mollies identified four colorless pteridine pigments (xanthopterin, isoxanthopterin, neopterin, and pterin) and a carotenoid with an absorbance spectrum similar to beta-carotene. It appears that, unlike some other poeciliid fishes, sailfin mollies do not use pteridine pigments for orange coloration. Rather, they appear to rely primarily on carotenoids.  相似文献   
973.
974.
975.
976.
977.
978.
979.
980.
S Meleth  L S Dahlgren  R Sankaran  K Sankaran 《CMAJ》1995,153(4):415-419
OBJECTIVE: To determine the vaccination rate among infants discharged from a neonatal intensive care unit (NICU) and factors affecting that rate. DESIGN: Cross-sectional survey conducted when the children were 12 to 18 months of age. SETTING: NICU at the Royal University Hospital, Saskatoon, Sask. PARTICIPANTS: All 395 infants discharged from the NICU between Jan. 1 and June 30, 1992. MAIN OUTCOME MEASURES: Vaccination rate, ethnic background (native or non-native), place of residence (urban or rural), health status (number of days spent in the NICU), reasons for delay in or incomplete vaccinations (those involving parents'' responsibility, infant illness or contraindications). RESULTS: Of the 395 infants, 20 (5.0%) had died and incomplete information was available for 30 (7.6%). Complete data were available for 345 (87.3%). Of the infants for whom data were available, 8 (2.3%) had never been vaccinated and 142 (41.2%) had a delayed vaccination schedule or had not completed their scheduled vaccinations. Only 195 (56.6%) of the infants had received a full vaccination series. Non-native ethnic background was a predictor of completed vaccinations (odds ratio [OR] 5.40, 95% confidence interval [CI] 3.05 to 9.52). In a univariate model, urban area of residence was not a significant predictor of vaccination status, but when ethnic background was controlled for in a multivariate logistic regression analysis, urban area of residence was found to be inversely associated with completed vaccinations (OR 0.34, 95% CI 0.15 to 0.79). The number of days the child had spent in the NICU was not a significant predictor of vaccination status. CONCLUSION: The vaccination rate of infants discharged from the NICU is not optimal. Urban native children appears to be at risk of not being vaccinated. Non-native infants are five times more likely than native infants to have completed all of their scheduled vaccinations. Methods to improve the rate of completed vaccinations, especially for native children, must be sought and tested.  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