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   47162篇
  免费   4100篇
  国内免费   2472篇
  2024年   80篇
  2023年   437篇
  2022年   986篇
  2021年   1677篇
  2020年   1147篇
  2019年   1404篇
  2018年   1456篇
  2017年   1026篇
  2016年   1590篇
  2015年   2619篇
  2014年   2916篇
  2013年   3291篇
  2012年   4132篇
  2011年   3934篇
  2010年   2384篇
  2009年   2141篇
  2008年   2718篇
  2007年   2509篇
  2006年   2363篇
  2005年   1987篇
  2004年   1864篇
  2003年   1657篇
  2002年   1492篇
  2001年   864篇
  2000年   744篇
  1999年   742篇
  1998年   487篇
  1997年   422篇
  1996年   402篇
  1995年   313篇
  1994年   337篇
  1993年   254篇
  1992年   360篇
  1991年   343篇
  1990年   281篇
  1989年   274篇
  1988年   220篇
  1987年   178篇
  1986年   163篇
  1985年   196篇
  1984年   184篇
  1983年   129篇
  1982年   112篇
  1981年   85篇
  1980年   84篇
  1979年   87篇
  1978年   79篇
  1977年   70篇
  1976年   76篇
  1974年   79篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
993.
994.
Kevin Padian 《Ichnos》2013,20(2-4):115-126
The tracks ascribed to pterosaurs from the Late Jurassic limestones at Crayssac, France, must be pterosaurian because the manus prints are so far outside those of the pes, the pes print is four times longer than wide, and the manus prints appear to preserve distinct traces of a posteromedially directed wing-finger. These tracks are different in important ways from previously described Pteraichnus trackways, which have been variably considered pterosaurian, crocodilian, or indeterminate. No Pteraichnus (sensu stricto: those not from Crayssac) tracks have diagnostic features of pterosaurs and in none can a complete phalangeal or digital formula be reconstructed; however, all published Pteraichnidae tracks fulfill the criteria of poor preservation, and some have some diagnostic features of crocodile tracks. Reconstructions of pterosaurs walking in pteraichnid tracks do not fit those tracks well, but crocodiles do. In contrast, the Crayssac tracks demonstrate the erect stance and parasagittal gait previously reconstructed for pterosaurs. They also demonstrate that the footfall pattern was not as in typical reptiles (LH-RF-RH-LF), but that the manus must have been raised before the next forward step of the ipselateral foot (LH-LF-RH-RF), suggesting that the quadrupedal pattern was secondary. The metatarsus in pterosaurs was set low at the beginning of a stride, as it is in crocodilians and basal dinosaurs. The diagnosis of the Ichnofamily Pteraichnidae comprises features of possible crocodilian trackmakers, but not of possible pterosaurian trackmakers. Trackways considered for attribution to pterosaurs should show (1) manus prints up to three interpedal widths from midline of body, and always lateral to pes prints, (2) pes prints four times longer than wide at the metatarso-phalangeal joint, and (3) penultimate phalanges longest among those of the pes.  相似文献   
995.
996.
Recent studies indicate that there is a high diversity of pleurostomatid ciliates in the coastal waters of China. Here, three new congeners of Loxophyllum, L. caudatum sp. n., L. rugosum sp. n., and L. chinense sp. n., are described following observations of live cells and protargol‐impregnated specimens. All three species usually have two macronuclear nodules and prominent warts along the dorsal margin formed by clustered extrusomes. In addition, L. caudatum sp. n. is characterized by its long conspicuous tail, dot‐like cortical granules, 4 or 5 left and 9 or 10 right kineties, and a single subterminal contractile vacuole. Loxophyllum rugosum sp. n. is distinguished by possessing three prominent ridges on the left side, 7–11 right and 5–7 left kineties. Loxophyllum chinense sp. n. is characterized by having several contractile vacuoles distributed along the ventral margin, 13–18 right and 6–8 left kineties. The small subunit ribosomal DNA (SSU rDNA) sequence similarities among six congeners range from 96.46% to 99.94%. Phylogenetic trees based on the SSU rDNA sequences indicate that all Loxophyllum spp. form a well‐supported monophyletic group. A brief review of the marine and brackish Loxophyllum species is supplied and one new combination, Litonotus multiplicatus (Kahl 1931) comb. n. (basionym Loxophyllum multiplicatum Kahl 1931), and one new name, Litonotus dragescoi nom. n. (basionym L. fasciolatus Dragesco 1966), are suggested.  相似文献   
997.
Altered expressions of receptor for advanced glycation end-products (RAGE) and its ligand (S100A9) are observed in many cancers and play a key role in inflammation-associated cancer. In our previous study, by two-dimensional gel electrophoresis followed by mass spectrometry, the expression of S100A9 protein was found to increase in squamous cervical cancer compared with adjacent normal cervical tissues. Therefore, in the present study we observed the expressions of S100A9 and RAGE in 30 chronic cervicitis, 50 cervical intraepithelial neoplasia (CIN), and 40 squamous cervical cancer (SCC) using immunohistochemical analysis and analyzed the differential expression and possible role of S100A9 and RAGE in cancer development. Immunohistochemical findings were as follows: the expressions of S100A9 and RAGE were demonstrated in chronic cervicitis, CIN, and SCC. Moreover, their expressions were gradually increasing as the tumor progressed. In SCC, the staining scores of S100A9 and RAGE were significantly higher in well-differentiated tumors compared to moderately and poorly differentiated tumors. The expression of S100A9 in epithelial cells exhibited a positive correlation to RAGE expression in chronic cervicitis, CIN, and SCC. There were no significant difference of S100A9 immunoreactivity in stromal cells among chronic cervicitis, CIN, and SCC. Moreover, there was no correlation between S100A9 immunoreactivity in stromal cells of SCC and clinicopathological parameters. Finally, double immunohistochemistry illustrated that RAGE and S100A9 co-express in SCC. In conclusion, RAGE binds its ligand (S100A9), which plays an important role in the development of SCC. In addition, the expressions of S100A9 and RAGE in SCC tumor cells were closely associated with histological differentiation.  相似文献   
998.
To develop a transurethral endoscopy technique of the transurethral seminal vesiculoscopy to examine and treat seminal vesicle disease. A total of 61 patients with seminal vesicle disease were diagnosed and treated with the transurethral seminal vesiculoscopy through the distal seminal tracts and vesicles. 58 cases were successfully treated using transurethral seminal vesiculoscopy via the seminal vesicles. The operation took 25 ~ 85 min, with an average of (35.6) mins. In this group, seven cases were diagnosed as ejaculatory orifice cyst, 14 cases had blood clots in the seminal vesicles, and nine patients had stones in the seminal vesicles. All patients were treated properly. Follow-up occurred at 3 months, with two cases showing post-operative discomfort in perineal region. One patient had recurrence with seminal vesiculitis, which improved with treatment. Four infertile patients had a significant increase in sperm count and ejaculation volume and two of these patients were able to naturally inseminate within seven to 18 months post-surgery. This approach enables a new endoscopic technique with the transurethral seminal vesiculoscopy to diagnose and treat seminal vesicle disease through the normal anatomic pathway which can be easily performed with few post-operative complications.  相似文献   
999.
Extended producer responsibility (EPR), which assigns significant responsibility to producers to take back their end‐of‐life products to create incentives for redesign of products with lower life cycle environmental impacts, has come to a crossroad facing a trade‐off between the original innovation‐oriented regime design and the cost‐efficiency challenges in practice. This is particularly true in its implementation in non‐Organization for Economic Co‐operation and Development (OECD) countries as they are trying to transplant the “best practices” from OECD countries, for there is increasing skepticism as to whether EPR is suitable for developing countries at all. As an important producer of electronic products and destination of electronic waste (e‐waste) flows in the world, China has been expected to play a vital role in the evolution of global governance based on the idea of EPR, either to create new ways for producers to perform their end‐of‐life strategies, or to reshape the mode of production and consumption with its fast‐growing market. However, the establishment of EPR in China has been long and full of difficulties. This article reviews the status and trends in the establishment of an EPR system for waste electrical and electronic equipment (WEEE) management in China. We use the framework of a multilevel perspective of transition theory in our analysis to characterize the complex interactions among various agents in the evolution of the Chinese system from initial innovation‐oriented design to the current efficiency‐oriented version. An ongoing research framework for evaluation of the EPR program in China is outlined as the research agenda in coming years.  相似文献   
1000.
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