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   336782篇
  免费   22922篇
  国内免费   1065篇
  2018年   14170篇
  2017年   12997篇
  2016年   10715篇
  2015年   4308篇
  2014年   4938篇
  2013年   6955篇
  2012年   11453篇
  2011年   20522篇
  2010年   17133篇
  2009年   12774篇
  2008年   16444篇
  2007年   18170篇
  2006年   7018篇
  2005年   6931篇
  2004年   7281篇
  2003年   7168篇
  2002年   6787篇
  2001年   12275篇
  2000年   12129篇
  1999年   9180篇
  1998年   2687篇
  1997年   2785篇
  1996年   2726篇
  1995年   2490篇
  1994年   2441篇
  1993年   2358篇
  1992年   7210篇
  1991年   7021篇
  1990年   7082篇
  1989年   6854篇
  1988年   6377篇
  1987年   6027篇
  1986年   5356篇
  1985年   5667篇
  1984年   4472篇
  1983年   3884篇
  1982年   2676篇
  1981年   2500篇
  1980年   2313篇
  1979年   4119篇
  1978年   3144篇
  1977年   2886篇
  1976年   2835篇
  1975年   3279篇
  1974年   3500篇
  1973年   3529篇
  1972年   3302篇
  1971年   3111篇
  1970年   2538篇
  1969年   2302篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
F N Dodoo 《Social biology》1992,39(1-2):102-108
Data from the 1988 Ghana Demographic and Health Survey is employed to determine the existence of a relationship between the amount of education a woman has and her intent to cease childbearing. The findings are that with increasing levels of education, a female's desire to cease reproduction also increases. However, this relationship is obscured if parity is not controlled.  相似文献   
982.
983.
Baby hamster kidney cells were seeded onto Western blots of fetal serum proteins which had been extracted from several foreign surfaces. This revealed that the major cell adhesive proteins adsorbed onto these surfaces from fetal serum were (1) fibronectin of Mr 220,000 Da and (2) vitronectin of Mr 65,000 and 78,000 Da. Two minor bands of cell attachment were observed at Mr 153,000 and Mr 134,000 Da in the fetal serum proteins extracted from heparin-agarose and serotonin-agarose. However, by exposing the Western blots of separated proteins to a second round of serum proteins, prior to cell blotting, very strong cell adhesive bands were revealed at Mr 153,000, 134,000, and 120,000 Da. By (i) modifying the composition of the serum proteins used to treat the Western blots, (ii) using specific antibodies to fibronectin, and (iii) using radiolabeled fibronectin, it was conclusively demonstrated that the new cell adhesive bands owed their increased cell attachment activity to secondary binding of fibronectin. The new bands were shown (i) to be trypsin sensitive and collagenase sensitive and therefore to be collagen-like proteins and (ii) to react negatively in immunoblots using anti-fibronectin, anti-vitronectin, anti-fibrinogen, anti-fetuin or anti-thrombospondin. In SDS-PAGE (i) the Mr 120,000-Da protein comigrated with the alpha 2-chain of Type I collagen, (ii) the Mr 134,000-Da protein comigrated with the alpha 1-chain of Type I collagen, and (iii) the Mr 153,000-Da protein comigrated with the pN-alpha 1-chain of Type III collagen. Since the novel collagen-like proteins acted as strong sites of cell attachment on nitrocellulose blots by binding fibronectin, they might well promote cell attachment on the foreign surfaces from which they were extracted.  相似文献   
984.
985.
986.
987.
988.
Salmonella enterica serovar Typhimurium is a Gram-negative bacterium that has a significant impact on both human and animal health. It is one of the most common food-borne pathogens responsible for a self-limiting gastroenteritis in humans and a similar disease in pigs, cattle and chickens. In contrast, intravenous challenge with S. Typhimurium provides a valuable model for systemic infection, often causing a typhoid-like infection, with bacterial replication resulting in the destruction of the spleen and liver of infected animals. Resistance to systemic salmonellosis in chickens is partly genetically determined, with bacterial numbers at systemic sites in resistant lines being up to 1000-fold fewer than in susceptible lines. Identification of genes contributing to disease resistance will enable genetic selection of resistant lines that will reduce Salmonella levels in poultry flocks. We previously identified a novel resistance locus on Chromosome 5, designated SAL1 . Through the availability of high-density SNP panels in the chicken, combined with advanced back-crossing of the resistant and susceptible lines, we sought to refine the SAL1 locus and identify potential positional candidate genes. Using a 6th generation backcross mapping population, we have confirmed and refined the SAL1 locus as lying between 54.0 and 54.8 Mb on the long arm of Chromosome 5 ( F  = 8.72, P  = 0.00475). This region spans 14 genes, including two very striking functional candidates; CD27-binding protein ( Siva ) and the RAC -alpha serine/threonine protein kinase homolog , AKT1 ( protein kinase B , PKB ).  相似文献   
989.
990.
Delivery was induced by an intravenous infusion of prostaglandin F2alpha (PGF2alpha) in gradually increasing doses in 30 consecutive cases of fetal death in utero after the 28th week of gestation. Twenty patients delivered during the first day of prostaglandin administration, 9 on the second day, and 1 patient not until the third day of infusion. It is concluded, that intravenous PGF2alpha appears to be superior to oxytocin in termination of pregnancy under these conditions.  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