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   431865篇
  免费   47383篇
  国内免费   213篇
  479461篇
  2018年   4342篇
  2017年   4279篇
  2016年   5683篇
  2015年   7438篇
  2014年   8731篇
  2013年   12804篇
  2012年   14257篇
  2011年   14198篇
  2010年   9330篇
  2009年   8677篇
  2008年   12264篇
  2007年   12392篇
  2006年   11796篇
  2005年   11095篇
  2004年   10910篇
  2003年   10436篇
  2002年   10139篇
  2001年   19291篇
  2000年   19466篇
  1999年   15829篇
  1998年   5580篇
  1997年   5813篇
  1996年   5625篇
  1995年   5122篇
  1994年   5218篇
  1993年   5091篇
  1992年   13158篇
  1991年   12562篇
  1990年   12494篇
  1989年   12398篇
  1988年   11319篇
  1987年   10759篇
  1986年   9966篇
  1985年   10035篇
  1984年   8169篇
  1983年   7071篇
  1982年   5405篇
  1981年   4788篇
  1980年   4657篇
  1979年   7797篇
  1978年   6029篇
  1977年   5387篇
  1976年   5185篇
  1975年   5597篇
  1974年   5877篇
  1973年   5785篇
  1972年   5259篇
  1971年   4631篇
  1970年   4091篇
  1969年   3779篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
993.
994.
As compared to cutaneous leishmaniasis, vaccination against visceral leishmaniasis (VL) has received limited attention. In this study, we demonstrate for the first time that an UDP-Galactose: N-acetylglucosamine β 1–4 galactosyltransferase (GenBank Accession No. EF159943) expressing attenuated LD clonal population (A-LD) is able to confer protection against the experimental challenge with the virulent LD AG83 parasite. A-LD was also effective in established leishmania infection. The vaccinated animals showed both cell mediated (in vitro T-cell proliferation, and DTH response) and humoral responses (Th1 type). These results demonstrate the potential of the attenuated clones as an immunotherapeutic and immunoprophylactic agent against visceral leishmaniasis.  相似文献   
995.
996.
PUT cells were selected from the XD line of cultured tobacco cells (Nicotiana tabacum L. cv. Xanthi-nc) for the ability to utilize putrescine as sole nitrogen source. Previous work had indicated that hydroxycinnamoylputrescines (principally caffeoylputrescine) and 4-amino-n-butyric acid (GABA) are obligatory intermediates in the assimilation of putrescine by PUT cells. The apparent absence in these cells of diamine or polyamine oxidase and pyrroline dehydrogenase, enzymes which catalyze putrescine oxidation in some plant species, led us to propose the following pathway for putrescine oxidation in PUT cells: putrescine----hydroxycinnamoylputrescine----hydroxycinnamoyl - 4-aminobutyraldehyde----hydroxycinnamoyl-GABA----GABA. We tested the hypothesis by looking for the predicted compound, caffeoyl-GABA. A chemical synthesis was developed, and chromatographic and mass spectroscopic procedures were devised for identifying the compound in extracts of cells and plant tissues. Caffeoyl-GABA was found in extracts of PUT cells in micromolar concentrations but was not present in XD cells. Thus, its occurrence in PUT cells appears to be a direct result of selection for the ability to catabolize putrescine. Caffeoyl-GABA has the same distribution in tobacco plants as caffeoylputrescine, i.e. flower buds greater than open flowers greater than floral leaves, green fruit; absent in vegetative tissues.  相似文献   
997.
Energy expenditure was measured during pregnancy in seven primigravid women at 12-15, 25-28, and 34-36 weeks and after the cessation of lactation. On each occasion the resting metabolic rate and the increase in metabolic rate after ingestion of a liquid test meal were measured by indirect calorimetry. In absolute terms the resting metabolic rate increased steadily during pregnancy but when expressed per unit of body weight no change was found. The energetic response to a mixed constituent meal was significantly reduced by 28% in the middle trimester of pregnancy. These findings suggest a possible maternal adaptation to increase energetic efficiency at a time when the energy demands of the fetus are high.  相似文献   
998.
P J McLeod 《CMAJ》1987,136(7):709-712
Twelve Canadian medical schools that had an organized faculty development program were surveyed to evaluate the extent to which such programs were used and to estimate their effectiveness. Common practices included sabbaticals and programs designed to improve instructional skills. The main problems included underfunding, poor participation and inadequate instructor evaluation.  相似文献   
999.
The myeloarchitectonics of the specific human fields 40, 39, 37c, 37b of neocortex and also fields 17 and 19 which are the cortex end of visual analyzer has been studied with the help of neurohistological and quantitative methods. According to the character of the fibrous bundles, their diameter, the quantity of the fibrous in them and the diameter of the fibrous gradual changes from field 17 to field 19 and further to field 39 have been shown. It is an evidence of the succession between phylogenetically older and younger fields of the cortex. The peculiarities of the field quantitative myeloarchitectonics differing in the function and genesis are marked by heterochronous growth of these fields square in the process of evolution.  相似文献   
1000.
A simplified method of manual calculation for inbreeding coefficient (and/or relationship) from complicated pedigree tables presented in a nongraphic mode is offered. The method is based on the use of positional dual codes of ancestors in which all genealogical links of the proband are reflected.  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