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   76777篇
  免费   6532篇
  国内免费   39篇
  2022年   386篇
  2021年   944篇
  2020年   567篇
  2019年   721篇
  2018年   926篇
  2017年   871篇
  2016年   1449篇
  2015年   2403篇
  2014年   2739篇
  2013年   3755篇
  2012年   4607篇
  2011年   4760篇
  2010年   3138篇
  2009年   2833篇
  2008年   4193篇
  2007年   4313篇
  2006年   4121篇
  2005年   4089篇
  2004年   4188篇
  2003年   3774篇
  2002年   3786篇
  2001年   869篇
  2000年   610篇
  1999年   888篇
  1998年   1189篇
  1997年   854篇
  1996年   786篇
  1995年   761篇
  1994年   738篇
  1993年   686篇
  1992年   625篇
  1991年   603篇
  1990年   579篇
  1989年   615篇
  1988年   528篇
  1987年   509篇
  1986年   459篇
  1985年   593篇
  1984年   743篇
  1983年   638篇
  1982年   760篇
  1981年   795篇
  1980年   720篇
  1979年   499篇
  1978年   557篇
  1977年   527篇
  1976年   525篇
  1975年   402篇
  1974年   498篇
  1973年   452篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
The presence of definite cytoplasmic granulation in at least some of the malignant cells was used as the sole criterion to separate 156 patients with acute leukemia into two groups: 110 with myeloblastic (AML), and 46 with lymphoblastic or stem cell leukemia (ALL). The median survival from the onset of symptoms in patients with AML was 20 weeks, and those with ALL 37 weeks. The difference in survival in these two groups is much greater for patients under the age of 25 than for those over the age of 25.  相似文献   
922.
923.
924.
The parathyroid glands have received relatively little attentionwith regard to their impact on the internal milieu in amphibians.Present information, however, clearly indicates that distributionof mineral in frog tissues and body fluids is under hormonalregulation; further modifications are imposed by season andtemperature. Under conditions of cold acclimation or duringwinter normal amphibian bone shows minimal rates of carbohydrateutilization: simultaneously, phosphate is conserved by reducedexcretion and increased incorporation in tissue. Exogenous parathyroidhormone, injected under these conditions when endogenous titersmay be low due to glandular degeneration, reverses these effects,and phosphate is lost in a hyperphosphaturic urine. During summeror periods of warm acclimation, hormone promotes incorporationof phosphate in tissues, and increases rates of carbohydratemetabolism in bone; citric acid, which may act in solubilizingbone mineral, is elevated in the bone. Concurrently, pre-existingsulfated mucopolysaccharides of "old" bone may be degraded,while epiphyseal zones enlarge and may exhibit accelerated orabnormal elaboration of sulfated components of cartilage and/orosteoid matrix.  相似文献   
925.
Preparation, properties and metabolism of 5,6-monoepoxyretinoic acid   总被引:4,自引:3,他引:1  
1. Methyl retinoate has been converted into methyl 5,6-monoepoxyretinoate by reaction with monoperphthalic acid. The epoxy acid ester on alkaline hydrolysis gave 5,6-monoepoxyretinoic acid. 2. Treatment of the 5,6-monoepoxy compounds with ethanolic hydrochloric acid gave the corresponding 5,8-epoxy (furanoid) compounds. 3. With lithium aluminium hydride, the acid and the ester groups were selectively reduced to primary alcohols. 4. Administration of methyl 5,6-monoepoxyretinoate intraperitoneally and subcutaneously had good growth response in vitamin A-deficient rats. 5. 5,6-Monoepoxyretinoic acid, when given intraperitoneally as the sodium salt, was 157% as active as all-trans-retinyl acetate. 6. Methyl 5,6-monoepoxyretinoate was hydrolysed to the epoxy acid by rat-liver homogenate. It had 35% of the biological activity of all-trans-retinyl acetate in the rat when given orally.  相似文献   
926.
Electron microscopy was used to study pollen wall ontogeny in Zea mays. The initial stage of development consisted of compartmentalization of microspores within callose special walls. Microspore plasma membranes retracted and tubular elements of the endoplasmic reticulum became perpendicularly oriented to the plasma membranes. Evaginations of the endoplasmic reticulum into the microspore plasma membrane resulted in the establishment of a template or blueprint of the mature pollen wall. Sporopollenin deposition upon the template began immediately after dissolution of the callose special walls and release of the microspores into the anther locule. The columellae were the first pollen wall units to be formed; the tectum and foot layer became established shortly thereafter. The granular endexine was the last-formed unit. The relationships of membrane systems to the ontogeny of the pollen wall units and the mode of pollen wall growth are discussed.  相似文献   
927.
Indian bones from Mississippian and Archaic periods were examined by the techniques of electron microscopy and electron diffraction. Specimens of all ages containing intact mineral constituents showed numerous areas in which the apatite crystals exhibited typical periodic banding as seen in hard tissues of recent age. Diffraction patterns from the same areas verified the morphological appearance. Phosphotungstic acid stained, decalcified sections exhibited collagen fibrils with typical periodic banding regardless of age or soil conditions. In specimens of Archiac age there were a greater number of “poorly preserved” areas as compared with bones from Mississipian excavations. In these areas the collagen appeared to be degraded and the apatite crystals were randomly oriented. No attempt to quantitate the age changes was made in view of the inherent limitations of the method of investigation.  相似文献   
928.
Salivary Excretion of Coxsackie B-1 Virus in Rabbits   总被引:1,自引:1,他引:0       下载免费PDF全文
Coxsackie B-1 virus was injected into the ear vein of albino doe rabbits. Saliva and blood samples were taken before the injection of virus and at specific times thereafter. Virus was recovered in the whole saliva when the blood titer was approximately 10(4) TCID(50) per 0.1 ml or greater. The virus could be detected in the saliva as early as 2 min after the initiation of the viremia. The recovered virus was shown to be the same as the injected virus by serological identification of the recovered virus with neutralizing antibody for Coxsackie B-1 virus. These results suggest that virus may be transmitted to other animals in the saliva of animals who are in the viremic phase of infection without infection of the oropharyngeal tissues.  相似文献   
929.
Simultaneous sampling for microorganisms was accomplished at altitudes of 690, 1,600, and 3,127 meters. The location of temperature inversions in relation to the collection altitude determined, to a great extent, the micropopulation. High micropopulations were found when an inversion was above the sampling altitude, and low populations when the inversion was below the sampling altitude. Diurnal periodicity which could be generally correlated with periods of minimal and maximal convective activity was observed. Evidence is presented showing that the micropopulation is more stable at higher altitudes than at lower altitudes.  相似文献   
930.
Addition of lard or sodium oleate to the medium used for lipase production by Pseudomonas fragi resulted in a decreased accumulation of lipase in the culture supernatant fluid without affecting cell growth. The production and activity of lipase was inhibited by lard, sodium oleate, and the salts of other unsaturated fatty acids. Some divalent cations, Tweens, lecithin, and bovine serum prevented oleate inhibition, but did not reverse it. Similar inhibitory actions were observed with Geotrichum candidum lipase, but not with a staphylococcal lipase or pancreatic lipase. A protective effect by protein in crude enzyme preparations is indicated. The ability of oleate to lower surface tension does not appear to be related to its ability to inhibit lipase.  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