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   519313篇
  免费   55798篇
  国内免费   451篇
  575562篇
  2016年   6110篇
  2015年   7658篇
  2014年   9226篇
  2013年   13167篇
  2012年   14695篇
  2011年   15334篇
  2010年   10478篇
  2009年   9584篇
  2008年   13655篇
  2007年   14065篇
  2006年   13334篇
  2005年   12979篇
  2004年   12755篇
  2003年   12648篇
  2002年   12219篇
  2001年   21902篇
  2000年   21934篇
  1999年   17403篇
  1998年   6014篇
  1997年   6216篇
  1996年   6136篇
  1995年   5679篇
  1994年   5636篇
  1993年   5538篇
  1992年   14684篇
  1991年   14320篇
  1990年   14200篇
  1989年   13932篇
  1988年   13076篇
  1987年   12662篇
  1986年   11565篇
  1985年   11999篇
  1984年   9807篇
  1983年   8658篇
  1982年   6532篇
  1981年   6208篇
  1980年   5681篇
  1979年   9655篇
  1978年   7596篇
  1977年   7091篇
  1976年   6828篇
  1975年   7616篇
  1974年   8238篇
  1973年   8205篇
  1972年   7385篇
  1971年   6791篇
  1970年   6024篇
  1969年   5812篇
  1968年   5363篇
  1967年   4612篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
The effects of β-endorphin under the conditions of naloxone hydrochloride blockade of opiate receptors, as well as the effects of the selective agonists of μ-and δ-receptors DAGO and DADLE and the effects of melanocyte-potentiating factor (MPF), on the in vitro proliferative response of lymphocytes were studied. The dose-effect dependence indicated stimulating effects of β-endorphin, DAGO, and DADLE on the proliferative response in the presence of phytohemagglutinin (PHA). The tetrapeptide MPF, which is the C-terminal sequence of β-endorphin, had almost no effect on the proliferative activity of lymphocytes. β-Endorphin, naloxone, and the μ-and δ-receptor selective agonists enhanced the proliferative response of lymphocytes in an unfractionated cell culture, whereas β-endorphin, naloxone, and DAGO suppressed the proliferative activity of lymphocytes in the mononuclear fraction purified of monocytes. In both cases, the naloxone blockade of opiate receptors enhanced rather than eliminated the β-endorphin effect.  相似文献   
993.
994.
Summary A child with cri-du-chat syndrome, 46,XY,5p-, was born to a mother who had two normal children and two abortuses. The mother was shown to carry a balanced translocation. Giemsa and fluorescent banding demonstrate the exact location of the translocated segment. The deleted short arm of chromosome number 5 was shown to be attached to the long arm of chromosome number 11.
Zusammenfassung Ein Kind mit Cri-du-chat Syndrom und Karyotyp 46,XY,5p- wurde von einer Mutter geboren, die zuvor zwei gesunde Kinder und zwei Aborte gehabt hatte. Bei der Mutter wurde eine balancierte Translokation nachgewiesen; Chiemsa- und Fluorescens-Bandenmuster zeigten die genaue Lokalisation des translozierten Segmentes: Der deletierte kurze Arm des Chromosoms Nr.5 war an den langen Arm vom Chromosom Nr. 11 angeheftet.


We are indeed indebted to Dr. G. R. Hennigar for facilities and Dr. C. D. Barnett for financial assistance.  相似文献   
995.
Observations are made on the development and the presence of a Saccus Vasculosus in Tilapia mossambica and other cichlids.
In those cichlids that do not possess a saccus vasculosus the development of this organ is apparently suppressed in a very early stage of life.
A saccus vasculosus is present in all those American and Indian cichlids that were investigated, but this organ was not found in cichlids indigenous to the African continent.  相似文献   
996.
997.
998.
A simple procedure for induction of nistatin-resistant mutants in yeasts under the action of 6-N-hydroxylaminopurine (HAP) is proposed. Some differences between the spectra of mutations induced by HAP and UV-light are observed: HAP induces dominant and double recessive mutants more frequently.  相似文献   
999.
We have studied the effects of immune complexes on the expression of macrophage surface proteins in vitro. Increased expression of the H-2 molecules I-A, I-E, and K on the macrophage membrane was induced by in vitro culture with crude lymphokine or interferon-gamma. Expression of all three of the molecules was additionally increased by stimulating the cultures with heat-killed Listeria monocytogenes. Addition of soluble immune complexes to the cultures did not have any effect on macrophage expression of these proteins. However, significant inhibition of lymphokine or interferon-gamma induction of I-A, I-E, and H-2K was observed when macrophages were cultured on plates to which immune complexes had been bound. This inhibition was dose dependent, required an immunoglobulin (Ig) molecule with an intact Fc portion, did not require the presence of T cells, and occurred in the presence of indomethacin. Complexes containing IgG1, IgG2a, IgG2b, and IgE, but not IgM or IgA, antibodies mediated the inhibitory effect.  相似文献   
1000.
Fast atom bombardment mass spectrometry (FAB-MS) has been used to measure positional isotope exchange rates in enzyme-catalyzed reactions. The technique has been applied to the reactions catalyzed by acetyl-CoA synthetase and argininosuccinate synthetase. The FAB technique is also able to quantitatively determine the oxygen-18 or oxygen-17 content of nucleotides on as little as 10 nmol of material with no prior derivatization. Acetyl-CoA synthetase has been shown by FAB-MS to catalyze the positional exchange of an oxygen-18 of ATP from the beta-nonbridge position to the alpha beta-bridge position in the presence of acetate. These results are consistent with acetyl adenylate as a reactive intermediate in this reaction. Argininosuccinate synthetase was shown not to catalyze a positional isotope exchange reaction designed to test for the formation of citrulline adenylate as a reactive intermediate. Argininosuccinate synthetase was also found not to catalyze the transfer of oxygen-18 from [ureido-18O]citrulline to the alpha-phosphorus of ATP in the absence of added aspartate. This experiment was designed to test for the transient formation of carbodiimide as a reactive intermediate. These results suggest that either argininosuccinate synthetase does not catalyze the formation of citrulline adenylate or the enzyme is able to completely suppress the rotation of the phosphoryl groups of PPi.  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