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   27617篇
  免费   121篇
  国内免费   267篇
系统科学   417篇
丛书文集   454篇
教育与普及   95篇
理论与方法论   101篇
现状及发展   10587篇
研究方法   1130篇
综合类   14570篇
自然研究   651篇
  2014年   172篇
  2013年   362篇
  2012年   614篇
  2011年   1266篇
  2010年   429篇
  2009年   388篇
  2008年   669篇
  2007年   779篇
  2006年   752篇
  2005年   697篇
  2004年   562篇
  2003年   468篇
  2002年   470篇
  2001年   775篇
  2000年   808篇
  1999年   588篇
  1992年   454篇
  1991年   393篇
  1990年   396篇
  1989年   382篇
  1988年   360篇
  1987年   390篇
  1986年   320篇
  1985年   444篇
  1984年   363篇
  1983年   278篇
  1982年   308篇
  1981年   270篇
  1980年   343篇
  1979年   762篇
  1978年   583篇
  1977年   607篇
  1976年   478篇
  1975年   556篇
  1974年   711篇
  1973年   627篇
  1972年   655篇
  1971年   727篇
  1970年   848篇
  1969年   737篇
  1968年   726篇
  1967年   734篇
  1966年   666篇
  1965年   478篇
  1959年   249篇
  1958年   414篇
  1957年   300篇
  1956年   249篇
  1955年   241篇
  1954年   203篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
R L Urry  L C Ellis 《Experientia》1975,31(8):891-892
Rat pituitary MAO activity was reduced by constant darkness and by additions of melatonin in vitro and was increased by constant light and by pinealectomy. Hypothalamic MAO activity followed the same pattern but was less dramatically affected. The data suggest that MAO may be a target enzyme for melatonin.  相似文献   
122.
Summary The extra-DNA body was found, for the first time, in oocytes of an insect with telotrophic ovary. Dispersion of this body is accompanied by an enormous increase of nuclear volume and production of multiple nucleoli. It is suggested that the extra-DNA contains a huge mass of nucleolus organizers.This research was supported in part by funds from the Cytobiology Committee of the Polish Academy of Science.  相似文献   
123.
Summary Acute toxicity induced by DMN was partially prevented by previously administering methyl mercuric chloride (MMC), a chemical inhibitor of the drug metabolizing enzyme system (DMES). We have studied the early changes occurring during the course of DMN-intoxication, namely disaggregation of polysomal profiles and necrosis, evaluated morphologically and by the release of S-GPT.This work was supported by Consiglio Nazionale delle Ricerche, Roma, Italy.  相似文献   
124.
Examination of ATCC stocks for HeLa marker chromosomes in human cell lines.   总被引:1,自引:0,他引:1  
K S Lavappa  M L Macy  J E Shannon 《Nature》1976,259(5540):211-213
  相似文献   
125.
126.
The crude petroleum ether, chloroform and alcoholic extracts from the roots of Aristolochia indica (Linn.) showed 100% interceptive activity in mature female mice at the single dose of 100 mg/kg body wt. The follow-up studies with the chloroform extract showed the most significant effect in the basic part and two acidic fractions at the single dose levels of 50 mg/kg body wt. No toxic effect was observed at the dose levels used.  相似文献   
127.
128.
Summary The C-11 proton of an aprophine possessing a C-1,2 methylenedioxy group falls relatively upfield between 7.47 and 7.86. Additionally, the i.c.s. for the two protons of the methylenedioxy group is large (4–12 Hz). The presence of a methylenedioxy at C-3,4 in a phenanthrene alkaloid also results in an upfield shift ( 8.95–9.00) of the C-5 proton.This project was supported by NIH research grant No. HL-12971, awarded by the National Heart and Lung Institute, PHS/DHEW.  相似文献   
129.
The hyperthermic effect of a bacterial pyrogen has been studied in rabbits pretreated or not with p-chlorophenylalanine or 5, 6-dihydroxytryptamine. The results obtained indicate that a selective reduction of cerebral 5-hydroxytryptamine levels by these drugs do not significantly affect pyrogen hypertermia.  相似文献   
130.
Summary The central and the peripheral areas of the head-fold stage rat embryo ectoderm develop into both the neural tissue and the epidermis when grafted under the kidney capsule of adult rats.This investigation was supported by the grant No. IV/3 from the Research Foundation of S.R. Croatia, and in part by NIH PL 480 Agreement No. 02-038-1.  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