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   105137篇
  免费   6506篇
  国内免费   15篇
  2022年   723篇
  2021年   1529篇
  2020年   1265篇
  2019年   1412篇
  2018年   2643篇
  2017年   2375篇
  2016年   3224篇
  2015年   4233篇
  2014年   4519篇
  2013年   5873篇
  2012年   6819篇
  2011年   6147篇
  2010年   4128篇
  2009年   3408篇
  2008年   5018篇
  2007年   4795篇
  2006年   4632篇
  2005年   3924篇
  2004年   3983篇
  2003年   3512篇
  2002年   3288篇
  2001年   2713篇
  2000年   2508篇
  1999年   2056篇
  1998年   1010篇
  1997年   755篇
  1996年   771篇
  1995年   712篇
  1994年   655篇
  1992年   1236篇
  1991年   1159篇
  1990年   1106篇
  1989年   1171篇
  1988年   969篇
  1987年   991篇
  1986年   905篇
  1985年   956篇
  1984年   781篇
  1983年   701篇
  1982年   587篇
  1979年   798篇
  1978年   601篇
  1977年   586篇
  1975年   693篇
  1974年   755篇
  1973年   714篇
  1972年   655篇
  1970年   624篇
  1969年   686篇
  1968年   647篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
181.
Aims of this study were to analyze the long-term effects of postovulatory aging of mouse oocytes on 1) reproductive traits of parental (F(0)) and first (F(1))-generation females (pregnancy rate, gestation length, litter size, perinatal death, and sex ratio of offspring) and 2) developmental and behavioral variables of F(1) and second-generation (F(2)) offspring (birth weight and weight gain during preweaning development, postnatal day of attainment of immediate righting, spontaneous motor activity, and passive and active conditioned learning ability). Hybrid (C57BL/6JIco x CBA/JIco) females were artificially inseminated at 13 h (control group) or 22 h (oocyte-aged group) after GnRH injection. Experimental (oocyte-aged group) F(0) females exhibited lower pregnancy rate, shortened gestation length, decreased litter size, higher perinatal death of their pups, and increased percentage of male offspring compared to control F(0) females. Postovulatory aging of oocytes was also associated with increased number of growth-retarded pups, delayed development of the righting reflex, and higher spontaneous motor activity and emotionality of F(1) offspring. Postovulatory aging of F(0) oocytes did not affect birth weight, weight gain during preweaning development, passive and active conditioned learning ability of F(1) offspring, or reproductive traits of F(1) females or developmental and behavior variables of F(2) offspring.  相似文献   
182.
During batch cultivation ofEscherichia coli in a medium deficient in inorganic phosphate, the growth curve after exhaustion of phosphate is linear. Results obtained in batch cultivation were used for deriving expressions for bacterial growth at a constant rate in a single-and multi-stage continuous system. It was found experimentally that the theoretical relations derived are satisfactory.  相似文献   
183.
184.
From the roots of Ferula loscosii the coumarins umbelliprenin, coladin and coladonin and the new natural isovaleryl derivative of the latter have been isolated. A study which confirms the structure of coladonin and its difference from farnesiferol A is also given.  相似文献   
185.
186.
T-cell dysfunction and hyperimmunoglobulinemia E in paracoccidioidomycosis   总被引:1,自引:1,他引:0  
Various aspects of T and B cell mediated immunity were investigated in 20 well documented cases of active (10) or inactive (10) paracoccidioidomycosis (Pcm), as well as in 8 healthy individuals living in the endemic area of the disease. The results confirm previous reports that active Pcm produces diverse grades of depression of T cell mediated immunity. Such T cell dysfunction is not associated with a reduction in the number of peripheral E rosette-forming cells, and the immunodepression is reversed by chemotherapy. Sera from Pcm (active or inactive) patients have significantly increased levels of total IgE, but the actual proportion of IgE antibodies against P. brasiliensis was very low (0.4–0.6%). The highest levels of total IgE were found in active patients with disease-related immune depression, suggesting that T cell dysfunction might contribute to the excessive IgE production.  相似文献   
187.
188.
M Esmann  L I Horváth  D Marsh 《Biochemistry》1987,26(26):8675-8683
The sodium and potassium ion activated adenosinetriphosphatase [(Na+,K+)-ATPase] in membranous preparations from Squalus acanthias has been spin-labeled on sulfhydryl groups after prelabeling with N-ethylmaleimide. Saturation-transfer electron spin resonance spectroscopy has been used to study the rotational motions of the labeled protein on the microsecond time scale. Effective rotational correlation times deduced from the diagnostic line-height ratios in the second-harmonic, 90 degrees out-of-phase (V2') spectra are much larger than those deduced from the spectral integrals, indicating the presence of large-scale segmental motions, in addition to rotation of the protein as a whole. Experiments involving controlled cross-linking of the protein by glutaraldehyde, as well as measurements of the line broadening of the conventional electron spin resonance spectra, support this interpretation. Both the spectral integrals and diagnostic line-height ratios are found to increase irreversibly with time on incubation at temperatures greater than 20 degrees C, corresponding to a decrease in the segmental motion of the protein and probably also in the overall protein rotation. The native enzyme displays a marked nonlinearity in the Arrhenius temperature dependence of the activity at temperatures above 20 degrees C, and the activity decreases with a half-life of ca. 70 min on incubation at 37 degrees C (but not on incubation at low temperature), paralleling the time- and temperature-dependent changes in the saturation-transfer spectra of the labeled protein. Both of these observations suggest that the changes observed in the molecular dynamics could correspond to functional properties of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
189.
Summary This study is concerned with electron-microscopic observations on endocrine or paracrine cells in the fundic gastric mucosa of the bullfrog. Also, an attempt was made to identify the histamine-releasing cells involved in the secretagogue response. At least three distinct endocrine-like cell types were found. The classification is based on the appearance of secretory granules and other organelles, and the relationship of endocrine-like cells with other cells in the tissue. The amphibian endocrine-like cells resemble the ECL, D and EC cells of mammals. Type-I (ECL) cells showed degranulation after repeated stimulation with tetragastrin (TG), acetylcholine (ACh) and K+ depolarizing solution, all of which release histamine.  相似文献   
190.
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