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   21413篇
  免费   2113篇
  国内免费   18篇
  23544篇
  2022年   205篇
  2021年   430篇
  2020年   232篇
  2019年   300篇
  2018年   389篇
  2017年   309篇
  2016年   456篇
  2015年   884篇
  2014年   861篇
  2013年   1194篇
  2012年   1519篇
  2011年   1462篇
  2010年   928篇
  2009年   833篇
  2008年   1184篇
  2007年   1204篇
  2006年   1110篇
  2005年   1154篇
  2004年   1100篇
  2003年   1017篇
  2002年   1042篇
  2001年   237篇
  2000年   181篇
  1999年   267篇
  1998年   277篇
  1997年   185篇
  1996年   168篇
  1995年   157篇
  1994年   150篇
  1993年   173篇
  1992年   161篇
  1991年   157篇
  1990年   143篇
  1989年   166篇
  1988年   162篇
  1987年   141篇
  1986年   127篇
  1985年   157篇
  1984年   148篇
  1983年   149篇
  1982年   194篇
  1981年   195篇
  1980年   144篇
  1979年   122篇
  1978年   137篇
  1977年   107篇
  1976年   106篇
  1975年   93篇
  1974年   101篇
  1973年   86篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Cytogenetic examination was made of 103 13-14-day-old blastocysts and 116 24-32-day-old fetuses from untreated and androstenedione-7-HSA-immunized Merino ewes. There were no differences in the chromosome composition of blastocysts or fetuses from treated or untreated ewes and so the data were combined. At Days 13-14 a 1N/2N mosaic and a 2N - 1/2N/4N mosaic embryo were observed. In addition, 52 of the blastocysts were 2N/4N mosaics, with 8 of these also containing 8N cells, and one blastocyst was a 2N/8N mosaic. No aneuploid fetuses were observed, but 80 of the 116 fetuses contained polyploid cells, including 4N, 6N and 8N cells. The polyploid cells observed in the blastocysts and fetuses should not be considered as abnormal cells as they appear to be a normal part of the developmental processes leading to trophoblast formation and fetal differentiation.  相似文献   
32.
33.
34.
35.
The misaligned undulating membrane (mum) mutant of Tetrahymena thermophila is a non-conditional, single gene recessive mutation. The major effect of the mum mutation is the production of multiple undulating membrane (UM) fragments in the oral apparatus (OA). The ultrastructure of the UM fragments of mum OAs is identical to that of the single UM of wild-type OAs. Analysis of OA development at midbody using a combination of light microscopy of protargol-stained cells and SEM of demembranated whole cells showed that the phenotypic effect of the mum mutation first becomes evident during mid to late stage 4 and is fully manifested in early stage 5. The effect of the mutation involves a proliferation of excess basal bodies in the UM field. Subsequent events in the development of the mum OA from mid to late stage 5 are identical to those in wild-type OAs. This study suggests that the mum mutation establishes conditions that allow the production of multiple UMs and thus reveals that the UM field is competent for the complete and coordinated development of several adjacent UMs. This level of regional control is not clearly evident when a single UM is present. The comparison of development of wild-type and mum OAs required an extensive reanalysis of stages 4 and 5 of normal oral development. On the basis of current and previous observations, we propose a new and more subdivided staging system for oral development in Tetrahymena.  相似文献   
36.
Dye coupling in the organ of Corti   总被引:3,自引:0,他引:3  
Summary Dye-coupling in an in vitro preparation of the supporting cells of the guinea-pig organ of Corti was evaluated by use of the fluorescent dyes, Lucifer Yellow, fluorescein and 6 carboxyfluorescein. Despite the presence of good electrical coupling in Hensen cells (coupling ratios >0.6) the spread of Lucifer yellow was inconsistent. Hensen cells are very susceptible to photoinactivation, i.e., cell injury upon illumination of intracellular dye; and this in conjunction with Lucifer Yellow's charge and K+-induced precipitability may account for its variability of spread. Fluorescein and 6 carboxyfluorescein, on the other hand, spread more readily and to a greater extent than Lucifer Yellow, often spreading to cell types other than those of Hensen. Dye spread is rapid, occurring within a few minutes. These results suggest that molecules of metabolic importance also may be shared by the supporting cells of the organ of Corti.  相似文献   
37.
38.
Interaction of the exr and lon Genes in Escherichia coli   总被引:6,自引:5,他引:1  
Strains of Escherichia coli carrying the gene lon typically produced excess capsular polysaccharide, and were sensitive to ultraviolet light (UV) irradiation, thymine starvation, and nalidixic acid, forming long filaments after these treatments. Sensitivity was reduced by a number of posttreatments. In the presence of a second UV sensitivity gene, exr, some of these properties were suppressed: long filaments were not formed, the effect of lon on UV and nalidixic acid sensitivity was greatly reduced, and irradiation posttreatments gave an enhancement of survival characteristic of exr rather than lon strains. Production of capsular polysaccharide was not affected by the exr gene.  相似文献   
39.
40.
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