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   3630篇
  免费   455篇
  4085篇
  2021年   37篇
  2019年   34篇
  2017年   38篇
  2016年   58篇
  2015年   89篇
  2014年   100篇
  2013年   135篇
  2012年   170篇
  2011年   178篇
  2010年   80篇
  2009年   83篇
  2008年   129篇
  2007年   147篇
  2006年   144篇
  2005年   140篇
  2004年   116篇
  2003年   127篇
  2002年   113篇
  2001年   98篇
  2000年   129篇
  1999年   109篇
  1998年   35篇
  1997年   55篇
  1996年   43篇
  1995年   39篇
  1994年   42篇
  1993年   40篇
  1992年   70篇
  1991年   58篇
  1990年   65篇
  1989年   77篇
  1988年   78篇
  1987年   80篇
  1986年   73篇
  1985年   73篇
  1984年   63篇
  1983年   50篇
  1982年   39篇
  1981年   42篇
  1980年   42篇
  1979年   57篇
  1978年   41篇
  1977年   45篇
  1976年   47篇
  1975年   53篇
  1974年   57篇
  1973年   49篇
  1971年   42篇
  1970年   40篇
  1967年   31篇
排序方式: 共有4085条查询结果,搜索用时 8 毫秒
41.
Domain protolife     
We propose the Thermal Protein First Paradigm (protocell theory) that affirms that first life was cellular. The first cells emerged from molecular (chemical) evolution as protocells (heated amino acids self-order in copolymerization reactions to form thermal proteins which self-organize when in contact with water to form protocells). Metaprotocells are specialized protocells capable of synthesizing ATP (light energy conversion to chemical energy), polypeptides, and polynucleotides. Aggregations of protocells in thermal protein matrices form distinctive morphologies (protocellular networks). Prokaryotic cells emerged from metaprotocells. We classify protocells and metaprotocells as members of the Domain Protolife. We revised the cell theory to include protolife.  相似文献   
42.
The primal role of the origins of proteins in molecular evolution is discussed. On the basis of this premise, the significance of the experimentally established self-sequencing of amino acids under simulated geological conditions is explained as due to the fact that the products are highly nonrandom and accordingly contain many kinds of information. When such thermal proteins are aggregated into laboratory protocells, an action that occurs readily, the resultant protocells also contain many kinds of information. Residue-by-residue order, enzymic activities, and lipid quality accordingly occur within each preparation of proteinoid (thermal protein).In this paper are reviewed briefly the phenomenon of self-sequencing of amino acids, its relationship to evolutionary processes, other significance of such self-ordering, and the experimental evidence for original polyfunctional protocells.  相似文献   
43.
Techniques for using the triploid cell marker for studying cell lineage during the development and regeneration of the axolotl limb are described. Triploid animals possess cells with three nucleoli while diploid animals possess cells with two nucleoli. We have developed a technique for isolating the limb dermis as a sheet of cells for whole-mount analysis of cellular ploidy. Whole-mount tissue preparations as well as paraffin-embedded sectioned tissues were stained specifically for nucleoli with bismuth. Cell counts from a number of triploid and diploid dermal preparations show that (1) diploid dermal cells never possess three nucleoli, (2) the frequency of trinucleolate cells in whole-mount triploid dermal preparations is not 100% but varies between animals from 30 to 76%, (3) within a single triploid animal, the frequency of trinucleolate cells in different dermal preparations is constant. These data establish the usefulness of this technique and emphasize the need for appropriate control cell counts when using the triploid cell marker in the axolotl.  相似文献   
44.
J M Stoffman  M J Bass  A M Fox 《CMAJ》1984,131(6):573-575
To determine what proportion of head injuries in children under 24 months of age who presented to an emergency department were related to the use of baby walkers, we reviewed the charts of 52 such children. Walkers were involved in 42% of the head injuries in the children under 12 months of age and in none of those in the children aged 12 to 24 months. All walker-related injuries, including skull fractures in three children, involved stairs (p less than 0.001). Questionnaires were also sent to all families with children aged 3 to 18 months attending a private pediatric practice to determine the prevalence of falls involving baby walkers among these children and the factors associated with such falls. Of the 152 responding families 82% reported using or having used a walker. Thirty-six percent of the families reported that their child had a fall while in a walker, with 8.8% of the falls resulting in contact with a doctor. Walker-related falls were directly associated with time spent in the walker (p less than 0.001) and with a previous fall from the walker by an older sibling (p less than 0.03). Since there is no demonstrated benefit of walkers, their use should not be encouraged, and parents should be advised of their potential danger.  相似文献   
45.
46.
47.
48.
Data from the 1971 census population were used to evaluate the effects of age differences of married partners on mortality rates. Different age groups were isolated to highlight the association between mortality and age of spouse for specific ages of married men and women. Men married to much younger or to older women exhibited a higher mortality rate than men married to women who were only a few years younger than themselves. A similar trend was observed among women married to much younger or much older men compared with those whose spouses were a few years older or of similar ages as themselves. Trends for other age groups (women aged 60-69 years, men below 40, and women below 30) did not exhibit a clear pattern. Although statistical biases within age groups may in part account for the differences in the findings, the trend which emerged from the analysis suggests that lower morbidity is associated with the most common age combinations (husbands same age or slightly older than wives). Other factors could also account for the differences (e.g., selection of healthy partners in 1st marriages, differences in lifestyles between married and single).  相似文献   
49.
The size distribution and numbers of oocytes were examined in samples of ovaries from two sites and in fish in tanks whose past spawning activities were known. In an upland stream in northern England Cottus gobio would seem to be an annual spawner, maturing a single batch of eggs each year. In a more productive stream in southern England, where the reproductive life is short, fish mature and lay a succession of batches of eggs during a long spawning season. The physiological and ecological implications of these characteristics are discussed.  相似文献   
50.
An electrophysiological response of axons, referred to as the "propagation potential," was investigated. The propagation potential is a sustained voltage that lasts as long as an action potential propagates between two widely spaced electrodes. The sign of the potential depends on the direction of action potential propagation. The electrode towards which the action potential is propagating is positive with respect to the electrode from which it is receding. For normal frog sciatic nerves the magnitude of the propagation potential was 17% of the peak of the extracellular action potential; TEA increased it to 32%. For normal earthworm median or lateral giant fibers it was 30%. A ripple pattern on the propagation potential was attributed to variation in resistance along the length of the worm. Cooling increased the duration of the propagation potential and attenuated the higher frequency components of the ripple pattern. Differential records from two widely spaced intracellular microelectrodes in the same axon differed from the propagation potential. The amplitude of the plateau relative to the peak was smaller, it decreased as the action potential propagated from one electrode site to the other, and the potential did not return to zero as rapidly as for extracellular records. When propagation was blocked by heat, the propagation potential slowly decayed. There was no ripple pattern during the decay. In a volume conductor, electrodes contacting the worm did not show the typical propagation potential, but electrodes located a few centimeters away from the worm did. Simple core-conductor models based on classical action potential theory did not reproduce the propagation potential. More complex, modified core-conductor models were needed to accurately simulate it. The results suggest that long, slowly conducting fibers can contribute to the scalp-recorded EEG.  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