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   151741篇
  免费   5330篇
  国内免费   851篇
  2022年   612篇
  2021年   1214篇
  2020年   885篇
  2019年   968篇
  2018年   12790篇
  2017年   11464篇
  2016年   9083篇
  2015年   3237篇
  2014年   2916篇
  2013年   3604篇
  2012年   8081篇
  2011年   16110篇
  2010年   13907篇
  2009年   10117篇
  2008年   12133篇
  2007年   13660篇
  2006年   2617篇
  2005年   3142篇
  2004年   3568篇
  2003年   3200篇
  2002年   2572篇
  2001年   1790篇
  2000年   1635篇
  1999年   1356篇
  1998年   649篇
  1997年   622篇
  1996年   655篇
  1995年   502篇
  1994年   517篇
  1993年   506篇
  1992年   990篇
  1991年   912篇
  1990年   805篇
  1989年   799篇
  1988年   795篇
  1987年   720篇
  1986年   674篇
  1985年   655篇
  1984年   614篇
  1983年   481篇
  1982年   362篇
  1981年   357篇
  1980年   330篇
  1979年   458篇
  1978年   379篇
  1977年   315篇
  1975年   316篇
  1974年   323篇
  1972年   478篇
  1971年   482篇
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
911.
912.
913.
A new experimental model, the latissimus dorsi flap of the rabbit, was studied. This was found to be a relatively inexpensive research model. Its use is advocated for composite tissue transfer as transposition, island, or free myocutaneous flaps.  相似文献   
914.
After 36% of hepatic mass removal , rainbow trout recovered its initial liver weight in 20-30 days, i.e., with a regeneration rate clearly lower than in mammals. During early regeneration process hematocrit index and hemoglobin content were slightly decreased, but both parameters rapidly reached their normal values. The evolution of both glycaemia and hepatic glycogen content supported the idea of the existence of a late regeneration wave, which, in this case, could begin at about the 20th post-operative day.  相似文献   
915.
A new application of the Giemsa stain to demonstrate endocrine cells in deplasticized sections of Epon embedded material is described. Its application to the pancreas of Rana temporaria is illustrated. The technique does not require postfixation with OsO4 and is easily performed in 30 min. It allows the easy identification of three types of endocrine cells (A, B, and D). A cells, preferentially located at the islet periphery, stain purple-blue. B cells, which occupy the interior of the islet, display a lilac color. D cells give a strong purple color; they are located both in the periphery of the islets and scattered among acinar cells. Positive identification of the cell types was made by immunocytochemistry and electron microscopy.  相似文献   
916.
A simple and reliable method for G-banding chromosomes from human and mammalian cells is described. This rapid method combines hot saline and trypsin treatments and yields high quality G-bands in both bone marrow and cultured cells.  相似文献   
917.
The infection of murine macrophages and fibroblasts by recently isolated infective bloodstream trypomastigotes of Trypanosoma cruzi is inhibited by the addition of human plasma protease inhibitor alpha-2-macroglobulin (alpha 2M) or of soybean trypsin inhibitor. The ingestion of the non-infective epimastigotes by macrophages is not affected by the physiological protease inhibitor. Incubation of bloodstream trypomastigotes for 20 h in a serum-free axenic medium enhances their ability to infect macrophages in a process influenced by the temperature and sensitive to alpha 2M. After this period the infectivity of the parasites to cells was not sensitive to alpha 2M. These observations suggest that proteases located on the surface and/or secreted by the bloodstream trypomastigote form of T. cruzi may modulate its ability to infect host cells.  相似文献   
918.
To determine the influence of feeding, lighting and time of day on the copulating behavior of Panstrongylus megistus, 480 insect pairs were divided into four groups of 120 each and tested in the following respective situations: without food deprivation (F.D.), with five days of F.D., with ten days of F.D., and with 20 days of F.D. The tests were performed between 9:00 a.m. to 12:00 a.m. and 7:00 p.m. to 10:00 p.m., with light (700-1400 lux) and in the dark (1.4-2.8 lux) and behavior was recorded by the time sampling technique. Mating speed (MS) and duration of copulation (DC) were also calculated for each situation. The maximum frequency of copulation was observed after five days of F.D., at night, in the dark (n = 16), and the minimum was observed for recently-fed pairs, at night, with light (n = 4). Males approached females more often than females approached males. MS was lowest in pairs with twenty days of F.D., at night, with light (mean = 23.0 +/- 16.0 minutes), and highest in recently-fed pairs, during the day, with light (mean = 2.9 +/- 2.5 minutes). DC was shortest in recently-fed insects, during the day, in the dark (mean = 23.5 +/- 6.7 minutes), and longest in recently-fed animals, at night, in the dark (mean = 38.3 +/- 6.9 minutes).  相似文献   
919.
An aqueous solution of the latex of "coroa de cristo" (Euphorbia splendens var. hislopii) showed molluscicide action (LD90) at a concentration lower than 0.5 ppm on Biomphalaria glabrata and B. tenagophila reared in laboratory and at a concentration lower than 4.0 ppm for field B. tenagophila.  相似文献   
920.
Depending on the species, Cryptocoryne plants grow in different biotopes, viz. spring, river, dark forest, and tree-savanna. The species of Lagenandra are exclusively found in river biotopes. The chromosome numbers found in this study corroborate the numbers previously recorded. Some variation in respect to the morphology of the inflorescence of C. wendtii de Wit and L. ovata (L.) Thwaites is described. A map presenting the distribution of the of Cryptocoryne with their chromosome numbers is included.
The existing populations of Cryptocoryne on Sri Lanka should be protected from further exploitation by plant collectors.  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