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   1973篇
  免费   118篇
  国内免费   21篇
  2112篇
  2024年   4篇
  2023年   22篇
  2022年   34篇
  2021年   38篇
  2020年   35篇
  2019年   62篇
  2018年   38篇
  2017年   41篇
  2016年   56篇
  2015年   55篇
  2014年   78篇
  2013年   96篇
  2012年   64篇
  2011年   85篇
  2010年   63篇
  2009年   106篇
  2008年   117篇
  2007年   108篇
  2006年   94篇
  2005年   78篇
  2004年   84篇
  2003年   68篇
  2002年   63篇
  2001年   48篇
  2000年   34篇
  1999年   35篇
  1998年   42篇
  1997年   37篇
  1996年   46篇
  1995年   31篇
  1994年   29篇
  1993年   31篇
  1992年   37篇
  1991年   34篇
  1990年   18篇
  1989年   29篇
  1988年   14篇
  1987年   26篇
  1986年   28篇
  1985年   20篇
  1984年   24篇
  1983年   11篇
  1982年   15篇
  1981年   6篇
  1980年   8篇
  1979年   4篇
  1978年   3篇
  1974年   2篇
  1973年   3篇
  1972年   2篇
排序方式: 共有2112条查询结果,搜索用时 0 毫秒
71.
本文对68例中晚期肺癌进行选择性支气管动脉插管采用转铁蛋白受体单克隆抗体与表阿霉素、顺伯等药物制成偶联物灌注。结果显示,肿瘤明显消退24例(35.2%),部分消退36例(52.5%),无变化8例(12.3%),总有效率为87.7%。通过对支气管动脉插管,导向灌注疗法结果显示,本疗法优于周围静脉供药,且无严重并发症,副作用轻,不失为一种可供选择的治疗方法。  相似文献   
72.
水洞沟第2地点是水洞沟遗址群的重要遗址之一, 自2003年来共进行了4次发掘, 揭露面积约100m2, 发现7个文化层, 包含用火遗迹、石制品、动物化石、串珠装饰品和磨制骨针等。本文观察和研究的石制品1万余件, 总体上显示中国北方石片石器工业的技术特征, 但最下部第7文化层出土1件具有水洞沟第1地点特征的石叶石核。动物化石比较破碎, 集中分布于火塘内部或周围, 反映了古人类围绕火塘进行肉类资源利用的行为。综合水洞沟第2地点的AMS14C及光释光测年数据,该地点主要文化层年龄集中在距今41ka-20ka BP之间。  相似文献   
73.
A variety of lines of evidence support the idea that neutral evolutionary processes (genetic drift, mutation) have been important in generating cranial differences between Neandertals and modern humans. But how do Neandertals and modern humans compare with other species? And how do these comparisons illuminate the evolutionary processes underlying cranial diversification? To address these questions, we used 27 standard cranial measurements collected on 2524 recent modern humans, 20 Neandertals and 237 common chimpanzees to estimate split times between Neandertals and modern humans, and between Pan troglodytes verus and two other subspecies of common chimpanzee. Consistent with a neutral divergence, the Neandertal versus modern human split-time estimates based on cranial measurements are similar to those based on DNA sequences. By contrast, the common chimpanzee cranial estimates are much lower than DNA-sequence estimates. Apparently, cranial evolution has been unconstrained in Neandertals and modern humans compared with common chimpanzees. Based on these and additional analyses, it appears that cranial differentiation in common chimpanzees has been restricted by stabilizing natural selection. Alternatively, this restriction could be due to genetic and/or developmental constraints on the amount of within-group variance (relative to effective population size) available for genetic drift to act on.  相似文献   
74.
Reconstructions of foraging behavior and diet are central to our understanding of fossil hominin ecology and evolution. Current hypotheses for the evolution of the genus Homo invoke a change in foraging behavior to include higher quality foods. Recent microwear texture analyses of fossil hominin teeth have suggested that the evolution of Homo erectus may have been marked by a transition to a more variable diet. In this study, we used microwear texture analysis to examine the occlusal surface of 2 molars from Dmanisi, a 1.8 million year old fossil hominin site in the Republic of Georgia. The Dmanisi molars were characterized by a moderate degree of surface complexity (Asfc), low textural fill volume (Tfv), and a relatively low scale of maximum complexity (Smc), similar to specimens of early African H. erectus. While caution must be used in drawing conclusions from this small sample (n = 2), these results are consistent with continuity in diet as H. erectus expanded into Eurasia.  相似文献   
75.
The gastropod fauna of the Upper Devonian Baggy and Pilton formations in south‐west England is revised and includes some 30 taxa. The topmost part of the Upper Famennian succession in Devon is represented by clastic near‐shore and shallow shelf sediments, indicating a short‐term transgressive phase (‘Strunian Transgression’). The sequence yields a highly diverse fauna dominated by brachiopods and ostracodes, locally supplemented by crinoids, bryozoans, trilobites and molluscs. The taxa ‘Patellostiumbritannicum sp. nov., Angyomphalus (Angyomphalus) junius sp. nov. and Dictyotomaria eurocapillaria sp. nov. are erected; a junior homonym is replaced by Macrochilina? piltonensis nom. nov. The gastropod fauna displays an independent character, where latest Devonian faunal elements overlap with Late Palaeozoic taxa expressing a transition similar to that of the bivalves, brachiopods, echinoderms and corals, without a sharp faunal break at the Devonian/Carboniferous boundary. Apart from the Caenogastropoda, all subclasses of gastropods are represented. Members of the bellerophontoids, pleurotomarioids and loxonematoids are most abundant, followed by murchisonioids, naticimorphs, euomphalomorphs and platyceratoids. The various gastropod groups represent different ecological demands and trophic categories, and together with the accompanying fauna indicate that nearly all habitats and niches were occupied in the shallow South Laurussian Shelf.  相似文献   
76.
Abstract

The Neogene Fortuna Basin (Murcia Region, SE Spain) is rich in microvertebrate sites. Its continental sections include localities extended from the middle Turolian (MN12) to the early Ruscinian (MN14). However, there are few works dealing with the taxonomy of these fossil assemblages. In this paper we provide a complete taxonomic study of the rodents and insectivores from three levels (ROM-2B, ROM-2C and ROM-3A) within the Romerales section. We infer a late Turolian age (late Messinian) for these levels, among which the richest and most diverse level is ROM-C, including at least 11 different taxa. In addition, the paleoecological analysis of these fossil assemblages suggests the dominance of open herbaceous meadows under temperate climate during the formation of these sites, with a slight decrease in temperature and humidity from ROM-2B to ROM-2C.  相似文献   
77.
The cranial endocast of Pristerodon mackayi is described, which has been virtually reconstructed on the basis of neutron tomographic data. The gross anatomy of the cranial endocast of Pristerodon resembles other nonmammalian synapsids such as Thrinaxodon liorhinus in having a narrow, tubular forebrain, well developed olfactory bulbs, a large parietal foramen and unossified zone. As it is the case in cynodonts the hindbrain of Pristerodon is broader than the mid‐ and forebrain. Large paraflocculi are developed. The medulla oblongata can be well distinguished from the pons. The pons is divided by a median ridge into two portions. There is no evidence for a neocortex, which seems to be also reflected in the low encephalization quotient of 0.18 estimated according to the method of Eisenberg. J. Morphol. 276:1089–1099, 2015. © 2015 Wiley Periodicals, Inc.  相似文献   
78.
Embryos of the brine shrimp, Artemia franciscana, are genetically programmed to develop either ovoviparously or oviparously depending on environmental conditions. Shortly upon their release from the female, oviparous embryos enter diapause during which time they undergo major metabolic rate depression while simultaneously synthesize proteins that permit them to tolerate a wide range of stressful environmental events including prolonged periods of desiccation, freezing, and anoxia. Among the known stress-related proteins that accumulate in embryos entering diapause are the late embryogenesis abundant (LEA) proteins. This large group of intrinsically disordered proteins has been proposed to act as molecular shields or chaperones of macromolecules which are otherwise intolerant to harsh conditions associated with diapause. In this research, we used two model systems to study the potential function of the group 1 LEA proteins from Artemia. Expression of the Artemia group 1 gene (AfrLEA-1) in Escherichia coli inhibited growth in proportion to the number of 20-mer amino acid motifs expressed. As well, clones of E. coli, transformed with the AfrLEA-1 gene, expressed multiple bands of LEA proteins, either intrinsically or upon induction with isopropyl-β-thiogalactoside (IPTG), in a vector-specific manner. Expression of AfrLEA-1 in E. coli did not overcome the inhibitory effects of high concentrations of NaCl and KCl but modulated growth inhibition resulting from high concentrations of sorbitol in the growth medium. In contrast, expression of the AfrLEA-1 gene in Saccharomyces cerevisiae did not alter the growth kinetics or permit yeast to tolerate high concentrations of NaCl, KCl, or sorbitol. However, expression of AfrLEA-1 in yeast improved its tolerance to drying (desiccation) and freezing. Under our experimental conditions, both E. coli and S. cerevisiae appear to be potentially suitable hosts to study the function of Artemia group 1 LEA proteins under environmentally stressful conditions.

Electronic supplementary material

The online version of this article (doi:10.1007/s12192-015-0647-3) contains supplementary material, which is available to authorized users.  相似文献   
79.
Two tooth morphotypes corresponding to one or two tetrapod species from the Late Jurassic or Earliest Cretaceous locality of Ksar Metlili (KM), Anoual Syncline (eastern Morocco), are reported and described. These teeth cannot be related to any of the identified vertebrate major groups of this site. They are tricusped and uniradiculate, with a high and large main central cusp mesio-distally surrounded by two smaller accessory cusps. Their morphology is reminiscent of several taxa such as pterosaurs, notosuchians and mammals, with which they are compared here. These morphotypes are tentatively referred to cf. Cynodontia indet. They would be the most recent non-mammaliaform cynodonts reported in Africa and among the latest described. The KM specimens display remarkable plesiomorphic dental features with respect to known contemporaneous non-mammaliaform cynodonts. They might indicate the survival of a relict lineage in a North African refugium.  相似文献   
80.
It has been proposed that a multiregional model could describe how Homo sapiens evolved in Africa beginning 300,000 years ago. Multiregionalism would require enduring morphological or behavioral differences among African regions and morphological or behavioral continuity within each. African fossils, archeology, and genetics do not comply with either requirement and are unlikely to, because climatic change periodically disrupted continuity and reshuffled populations. As an alternative to multiregionalism, I suggest that reshuffling produced novel gene constellations, including one in which the additive or cumulative effect of newly associated genes enhanced cognitive or communicative potential. Eventual fixation of such a constellation in the lineage leading to modern H. sapiens would explain the abrupt appearance of the African Later Stone Age 50–45 thousand years ago, its nearly simultaneous expansion to Eurasia in the form of the Upper Paleolithic, and the ability of fully modern Upper Paleolithic people to swamp or replace non‐modern Eurasians.  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