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   109316篇
  免费   7428篇
  国内免费   16篇
  2023年   573篇
  2022年   436篇
  2021年   1052篇
  2020年   966篇
  2019年   975篇
  2018年   2618篇
  2017年   2325篇
  2016年   3268篇
  2015年   4911篇
  2014年   4970篇
  2013年   6692篇
  2012年   8197篇
  2011年   7667篇
  2010年   4888篇
  2009年   3608篇
  2008年   6260篇
  2007年   6190篇
  2006年   5645篇
  2005年   5282篇
  2004年   4943篇
  2003年   4564篇
  2002年   4188篇
  2001年   2182篇
  2000年   2149篇
  1999年   1864篇
  1998年   770篇
  1997年   608篇
  1996年   537篇
  1995年   565篇
  1994年   571篇
  1993年   444篇
  1992年   1281篇
  1991年   1193篇
  1990年   1063篇
  1989年   988篇
  1988年   927篇
  1987年   798篇
  1986年   728篇
  1985年   816篇
  1984年   705篇
  1983年   588篇
  1982年   464篇
  1981年   451篇
  1979年   597篇
  1978年   475篇
  1977年   426篇
  1976年   396篇
  1975年   461篇
  1974年   473篇
  1973年   474篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Summary The tips of the labial palps ofRhodogastria possess a pit housing uniform sensilla (Fig. 1), histologically characterized by wall-pores and receptor cells with lamellated outer dendrites (Fig. 2). The receptor cell axons project to glomeruli in the deutocerebrum (cf. Fig. 3) which are not innervated by antennal receptors. From their histology as well as from their central projection these sense organs are identical with palpal pit organs of other Lepidoptera (Lee et al. 1985; Kent et al. 1986; Lee and Altner 1986).Physiologically, the palp-pit receptors respond uniformly; they are most excitable by stimulation with carbon dioxide (Fig. 6) while they exhibit relatively moderate responses to various odorants (Fig. 4). The responses to CO2 (Fig. 7) show a steep dose-response characteristic. In ambient atmosphere (i.e., ca. 0.03% CO2) the cells are in an excited condition already; the seeming spontaneous activity exhibited in air is decreased if the preparation is kept under N2 or O2 or CO2-free air (Figs. 7, 10). There is hardly any adaptation of the responses to continuous or repeated stimulation (Fig. 8). Perhaps CO2 sensitivity is correlated with sensilla characterized by both wall-pores and lamellated dendrites. Pilot tests indicate that CO2 perception might be widespread in the Lepidoptera (cf. Fig. 12), but the biological significance remains obscure.  相似文献   
992.
Infertility associated with two accessory bisatellited chromosomes   总被引:1,自引:1,他引:0  
Summary Two extra bisatellited chromosome identified as inv dup (15) (pterq11.2::q11.2pter) were found in an oligoasthenospermic male. Analysis of Ag-staining in the proband and in one fertile brother with a normal karyotype revealed that nucleolar organizer region (NOR) activity was significantly increased in the patient. The frequency of satellite associations was also significantly higher in the index case, but no correlation was found between NOR activity and acrocentric associations. These results suggest that extra NOR activity and the elevated frequency of satellite associations could predispose to gametogenic impairment.  相似文献   
993.
Summary An anatomoclinical observation of agyria is reported. The karyotype revealed a partial deletion of the short arm of chromosome 17. The etiology of agyria is reviewed in the light of this chromosomal abnormality. In addition we describe the peculiar pattern of neurons in the cortex: Golgi stain demonstrated many inverted pyramidal cells in the superficial part of the cortical layer. The mechanism of this abnormality is discussed.  相似文献   
994.
Summary Lymphocyte-mediated lysis of autologous tumor cells (autologous lymphocyte cytotoxocity ALC) was tested at the time of surgery in 108 patients (46 squamous cell carcinomas of the lung, 25 adenocarcinomas of the lung, 19 soft tissue sarcomas and 18 osteosarcomas). The clinical course of these patients in relation to the test results has been published previously. The group was evaluated again after an extended observation time, now with a mean of 80.2 months (range 36–108). The test was rarely positive in patients with metastasis (2 out of 28 experiments).There was a correlation between the ALC results and the postsurgical clinical course for patients without detectable metastasis in that (1) a negative test was invariably a bad prognostic sign, i.e., all 32 patients with negative ALC died within 3 years (mean survival time 16.1 months). (2) The remission and survival times were longer for the ALC positive patients (p<0.001). (3) All 37 individuals who are alive at present without recurrence belong to the reactive group.The ALC results correlated with the clinical course in 88% of patients. The correlation was highest for the groups of soft tissue sarcoma and adenocarcinoma of the lung. There was no correlation between killing of K562 cells and ALC, or between lymphoproliferative response to PHA and ALC reactivity.  相似文献   
995.
The carbon-substrate utlization profile of 234 wild strains of thermophilic campylobacters originating from different animal sources and different part of the world was studied using a microgallery as well as the profile of 25 type strains ofCampylobacter species and reference strains ofCampylobacter-like organisms. Among the 98 substrates tested, succinate, fumarate,d-l-lactate,l-malate, pyruvate,l-glutamate,l-aspartate, andl-serine (with one exception for the last two) were always utilized by the wild strains, and acetate, propionate,d-malate, 2-cetoglutarate, itaconate, citrate, andl-proline by some of the strains. A strong association was found between assimilation ofd-malate and a positive hippurate test.  相似文献   
996.
The dynamics of changing dimensions of "no reflow" area following reperfusion after 30 min-1 h-long ischemia is characterized by three basic phases. The reperfusion following and hour-long ischemia altered considerably the character of phases of "no reflow" phenomenon. The data obtained suggest that the therapy of transitory ischemia must be directed not only to ischemia itself, but also to postischemia reperfusion-induced "no reflow" phenomenon.  相似文献   
997.
Abstract Using plasma glycoprotein glycans, a correlation was established between their inhibitory capacity of sheep mannose-resistant haemagglutination (MRHA) properties of bovine enterotoxigenic Escherichia coli (ETEC) and their monosaccharide content. Sialic acid seems to be the major component of the inhibitors of adherence of calf ETEC.  相似文献   
998.
N Wu  J C C?té  R Wu 《Gene》1986,50(1-3):271-278
The rice (Oryza sativa L. var. Labelle) chloroplast (cp) gene encoding cytochrome f has been isolated and sequenced. The coding region of this rice gene displays 95.1%, 85.3% and 85.2% nucleotide sequence homology with that of wheat, pea and spinach, respectively. To examine the cpDNA sequence variation in rice, cpDNA from Labelle and its parents, Belle Patna and Dawn was compared. Using the cytochrome f gene as the probe for hybridization, we found several differences in the size and number of restriction fragments in the cp genome of three rice varieties. An additional restriction fragment found in the Belle Patna cp suggests that this cp genome is either heterogeneous or contains two copies of cytochrome f gene per cpDNA.  相似文献   
999.
A cDNA library of erythrocyte mRNAs was established from immature red blood cells of the adult amphibian, Pleurodeles waltlii (urodele; salamander). The cDNA clones corresponding to the four adult globin chains were first identified and characterized by positive selection and the cDNAs derived from the two (major and minor) alpha-globin chains sequenced. The sequences presented contain both the complete 3'-noncoding region and the coding region of both chains, with the exception of the first nine codons of the minor alpha-chain, and a portion of the 5'-noncoding region of the major chain. The amino acid sequences of the encoded alpha-globin polypeptides have been deduced and compared with those of Xenopus laevis and of man. These comparative studies suggest that the alpha-globins of Pleurodeles waltlii and Xenopus laevis may have diverged from a common ancestral gene at the time when mammalian and amphibian lines diverged, and that they then evolved separately. Duplication of the alpha-gene, which is responsible for the polypeptide heterogeneity, appears to have occurred earlier in Pleurodeles waltlii than in Xenopus laevis.  相似文献   
1000.
Effects of septic shock by repeated inoculations with Escherichia coli on the ultrastructure of the folliculo-stellate cells and cavities of the adenohypophysis of the chicken were investigated in order to determine the function of these cavities. The principal morphological modifications were dilation of the Golgi apparatus, endoplasmic reticulum and autophagic vacuoles, and necrosis phenomena in the stellate cells. The follicular cavities showed dilation, and there was heterogeneous dense material and granular elements in the follicular lumen. Based on results reported in the literature, the observations reported here are evidence of a "cleaning-role", for the removal of cell debris, when there is endocrine disfunction.  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