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   1123170篇
  免费   91236篇
  国内免费   2969篇
  2021年   18259篇
  2020年   13047篇
  2019年   16739篇
  2018年   17287篇
  2017年   15842篇
  2016年   27793篇
  2015年   42543篇
  2014年   50664篇
  2013年   76855篇
  2012年   30278篇
  2011年   16852篇
  2010年   41380篇
  2009年   43597篇
  2008年   16546篇
  2007年   13659篇
  2006年   20338篇
  2005年   21410篇
  2004年   20768篇
  2003年   18576篇
  2002年   16780篇
  2001年   21477篇
  2000年   18107篇
  1999年   21555篇
  1998年   24047篇
  1997年   23765篇
  1996年   23548篇
  1995年   21630篇
  1994年   21477篇
  1993年   20441篇
  1992年   18888篇
  1991年   17184篇
  1990年   15897篇
  1989年   17035篇
  1988年   15482篇
  1987年   14566篇
  1986年   14016篇
  1985年   16113篇
  1984年   17521篇
  1983年   15647篇
  1982年   17907篇
  1981年   17535篇
  1980年   16288篇
  1979年   13387篇
  1978年   13903篇
  1977年   13619篇
  1976年   13053篇
  1975年   11896篇
  1974年   11926篇
  1973年   12440篇
  1972年   10192篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Synthesis of prenylquinones in chloroplasts   总被引:3,自引:0,他引:3  
  相似文献   
142.
143.
144.
145.
An earlier report (1) of an adverse effect of high doses of oxytocin on human memory included results of studies on women receiving oxytocin as part of the treatment to induce 2nd trimester therapeutic abortion. These women served as their own controls. We have now been able to study a group of women who have been treated in all ways like the original group, with the exception that they did not receive oxytocin. The results from this external control corroborate the finding that oxytocin affected memory.  相似文献   
146.
The Thom gradient model of morphogenesis poses the followinga posteriori problem: “From the observed morphology of a given natural process (effect) determine the dynamics of the process (cause)”. In this paper we consider the classicala priori problem: “Given the cause (dynamics) determine the effect (resultant morphology)”. We find that in biochemical processes the mechanisms for energy activation, energy-matter interaction and energy dissipation determine the dynamics. Furthermore there exists basic energy mechanisms which drive the equilibrium states through the elementary catastrophes of Thom. A comparison with current theories shows that our models describe open ecological food chains and their dynamical systems generalize the equations of organisation posed by M. Eigen. Work supported by a Research Associateship of the International Centre for Theoretical Physics, P.O.B. 586, Miramare, 34100 Trieste, Italy.  相似文献   
147.
Several topics and concepts in developmental biology, such as induction of secondary axes (as in amphibians), the rules of distal transformation (valid, e.g., for insect and vertebrate limbs), and the rule one piece regenerates, the other one duplicates (valid also for imaginal discs) originally emerged from pioneering studies in hydroids. In addition, one finds phenomena of intercalation, i.e., the insertion of intervening parts when tissues of distant origin are confronted with each other. However, since not only short-range but also long-range interactions contribute to pattern regulation, one observes more regulative flexibility as compared to insect systems.  相似文献   
148.
O. Munk 《Zoomorphology》1984,104(3):180-183
Summary A conus-like structure, the hyaloid conus, located on the optic nerve head of the mesopelagic deep-sea teleost Radiicephalus elongatus is described. The hyaloid conus consists of a tapering sheath of unpigmented, vascularized connective tissue enveloping the proximal part of the hyaloid artery which proceeds from the optic nerve head through the vitreous body to the ventrally located falciform process and lens muscles. The hyaloid artery passes through the hyaloid conus without giving off any branches. The conus vessels encircling the hyaloid artery receive arterial blood from the choroid via small arteries and are drained to the choroid by a single vein. The hyaloid conus is compared with the lacertilian conus papillaris. The function of the hyaloid conus is unknown. Because of its small dimensions relative to those of the eyeball and its few capillaries, it is unlikely that the hyaloid conus is a supplemental nutritive device for the retina.  相似文献   
149.
Puerto Rican populations of two species of sea anemones (Bunodosoma cavernata and B. granulifera) which had previously been considered one were assayed electrophoretically for enzymes encoded by 12 loci. The two species shared no common allozymes at 6 of the 12 loci. Genetic distance and identity values based on these allozymes were computed for the Puerto Rican populations and for B. cavernata from Florida and B. granulifera from Panama. The Puerto Rican populations of both species had much higher genetic identities for their geographically distant conspecifics than for each other. These results indicate that the two species are reproductively isolated and should be considered as separate valid species. Average heterozygosities are presented which are the first published for coelenterate species.  相似文献   
150.
Several synthetic 7-hydroxyflavylium salts related to apigeninidin, a natural 3-deoxyanthocyanidin, have been studied in the Ames mutagenicity test using strain TA1537 of Salmonella typhimurium. Under the neutral pH conditions of the test, these flavylium salts are deprotonated through ionization of the C7-OH (pKa′ = 4.2–4.4) to form quinone methides. Only the quinone methides of 4-methyl-7-hydroxyflavylium chloride and 4′-methoxy-4-methyl-7-hydroxy-flavylium chloride showed mutagenicity. Responses of 4–8 times the background were observed at the higher doses (1000 μg/plate), both with and without metabolic activation. It was concluded that the induction of frameshift mutagenicity by this group of compounds is caused by those quinone methides that have non-ionic, stable polycyclic structures at neutral pH.  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