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   13034篇
  免费   732篇
  国内免费   7篇
  2021年   125篇
  2020年   73篇
  2019年   86篇
  2018年   141篇
  2017年   126篇
  2016年   214篇
  2015年   314篇
  2014年   390篇
  2013年   1115篇
  2012年   626篇
  2011年   714篇
  2010年   417篇
  2009年   409篇
  2008年   676篇
  2007年   698篇
  2006年   705篇
  2005年   724篇
  2004年   679篇
  2003年   654篇
  2002年   720篇
  2001年   297篇
  2000年   292篇
  1999年   266篇
  1998年   193篇
  1997年   155篇
  1996年   139篇
  1995年   143篇
  1994年   137篇
  1993年   179篇
  1992年   226篇
  1991年   165篇
  1990年   154篇
  1989年   181篇
  1988年   141篇
  1987年   152篇
  1986年   122篇
  1985年   118篇
  1984年   119篇
  1983年   88篇
  1982年   103篇
  1981年   90篇
  1980年   81篇
  1979年   80篇
  1978年   63篇
  1977年   62篇
  1976年   54篇
  1975年   40篇
  1974年   43篇
  1973年   45篇
  1972年   37篇
排序方式: 共有10000条查询结果,搜索用时 531 毫秒
1.
The response of human newborn lymphocytes in autologous mixed lymphocyte culture was examined for specificity (by restimulation), responder cell phenotype, and responder cell frequency. When the newborn T cells were separated from non-T cells by rosetting with sheep erythrocytes (E) in fetal calf serum (FCS), approximately 1:20,000 T cells proliferated. These responders had specificity for E + FCS, were T4+, and were self-restricted. Significant numbers of responder T cells were not found when newborn T and non-T cells were separated by nylon wool. Responses in parallel autologous cultures of adult T cells showed that 1) adults had a higher frequency than newborns of E + FCS specific responders, 2) evidence for self specificity was lacking in restimulated cultures, and 3) occasional responses to antigen on the surface of monocytes could not be excluded.  相似文献   
2.
We examined whether actin filaments are involved in the cAMP-dependent activation of a high affinity sodium/glucose cotransporter (SGLT1) using epithelial expression systems. The expression of enhanced green fluorescent protein-tagged SGLT1 (EGFP-SGLT1) in Madin-Darby canine kidney (MDCK) cells was revealed by Western blotting and confocal laser microscopy. 8-Br-cAMP, a membrane permeable cAMP analog, enhanced [14C]-α-methyl glucopyranoside ([14C]-AMG) uptake. Both basal and 8-Br-cAMP-elicited [14C]-AMG uptakes were inhibited by N-(2{[3-(4-bromophenyl)-2-propenyl]-amino}-ethyl)-5-isoquinolinesulfonamide (H-89), a protein kinase A inhibitor, and cytochalasin D, an actin filament formation inhibitor. Furthermore, cytochalasin D inhibited the distribution of EGFP-SGLT1 at the apical surface. These results suggest that the EGFP-SGLT1 protein is functionally expressed in the apical membrane of MDCK cells, and is up-regulated by a cAMP-dependent pathway requiring intact actin filaments.  相似文献   
3.
The effects of delayed mating on mouse preimplantation embryos (78 ± 1 hours) were studied by setting up different mating periods in relation to the estimated time of spontaneous ovulation. Copulation occurred even in the late morning and early afternoon after the night of spontaneous ovulation. However, females mated in the early afternoon had no viable embryos at the time of laparotomy. Although embryonic development was not affected in the groups mated 6 or 10 hours after estimated ovulation, the percentage of degenerated embryos was increased in these groups. These results suggest that prolonged intervals between the estimated time of ovulation and mating have some deleterious effects on preimplantation embryos.  相似文献   
4.
A case of chromomycosis in which hyperthermia proved effective is reported. The patient was a 56-year-old male bean curd maker who, without any previous history of minor trauma, developed on the extensor side of the left upper arm an eczematous lesion that underwent gradual radial expansion. The lesion showed a well-defined, 7×10 cm infiltrated erythematous plaque with the central area healed and, at the upper and lower borders, adherent scales and crusts on the surface. Histological examination revealed granulomatous changes in the dermis, as well as sclerotic cells within giant cells and microabscesses. On culturing,Fonsecaea pedrosoi was isolated. The patient was treated with disposable chemical pocket warmers, which were secured over the lesion with a rather tight elastic bandage, so that they kept the affected area warm for 24 hours a day. After a month of such hyperthermic treatment, the erythema and infiltration had decreased considerably, and microscopic examination and culture of the crusts both yielded negative results. Examination of biopsy specimens of the lesion after the third month showed that it had cicatrized. The treatment was stopped after 4 months, and no relapse occurred. We also summarize the published results of local hyperthermic treatment of chromomycosis in Japan.  相似文献   
5.
The genotoxicity of ptaquiloside (PT), recently isolated from bracken fern and shown to be carcinogenic, was examined by means of the hepatocyte primary culture/DNA-repair test. PT elicited clear unscheduled DNA synthesis with a dose-response effect. The result indicates that PT is a genotoxic carcinogen.  相似文献   
6.
The ethylene-induced synthesis and accumulation of proteinswere studied in the primary leaves of azuki bean (Vigna angularis).Seven different proteins, designated AZ17, 23, 27, 32, 35, 36,42 according to their molecular masses, were synthesized andaccumulated in response to ethylene. AZ27 and AZ42 were purifiedto homogeneity and characterized. AZ27 was identified as anacidic chitinase and accumulated in the extracellular space.The sequence of the 40 N-terminal amino acids of AZ27 showedno similarity to that of a basic chitinase from bean and tobacco,but it was highly homologous to that of a chitinase from virus-infectedcucumber leaves. AZ42 was identified as a glycoprotein thataccumulated intracellularly. A search for proteins with sequenceshomologous to an internal sequence of 18 amino acids in AZ42was unsuccessful. Immunochemical examination revealed that auxinand abscisic acid enhanced the ethylene-induced accumulationof AZ27 but not of AZ42. In contrast, levels of AZ42 were notaffected by auxin or abscisic acid, but cytokinin suppressedthe accumulation of one of the doublet bands of AZ42. TranslatablemRNAs coding for AZ27 and AZ42 were not present in leaves thathad not been treated with ethylene, but levels of these mRNAsincreased after such treatment. (Received March 1, 1991; Accepted May 8, 1991)  相似文献   
7.
8.
Anoplius eous Yasumatsu (Hymenoptera, Pompilidae) exhibits some outstanding nesting behavior. Females excavate a unicellular or multicellular nest in wet ground, or dig a single-celled nest in rotten wood, or build clustered mud cells in narrow spaces between walls of such substances as wood or vinyl sheets. The latter nest-type has been unknown within the subfamily Pompilinae. Females prepare a nest-cell before hunting, and construct it further after hunting, leaving the prey near the nest (behavior-formula: IVPTIOC). They transport their prey backward on the ground, grasping it in their mandibles by any part of the legs, and forward on the surface film of water or on the ground, grasping it by the middle part of the 1st or 2nd legs. Their only prey is the adult female semi-aquatic spider,Pardosa pseudoannulata (Lycosidae). These nesting and provisioning behavior patterns are compared with those of other pompilids. The position of the present species in the behavioral evolution of the Pompilidae is suggested.  相似文献   
9.
Methyl dl-12-homo-10-trans-juvenate (III) was synthesized from farnesol (IV) in eight steps involving stereoselective conversion of the trans-terminal methyl group to an ethyl group. The product (III) was less active than dl-C17-Cecropia juvenile hormone on both Tenebrio molitor and Galleria mellonella.  相似文献   
10.
When energetic particles irradiate matter, it becomes activated by nuclear reactions. Radioactivation induced cellular effects are not clearly understood, but it could be a part of bystander effects. This investigation is aimed at understanding the biological effects from radioactivation in solution induced by hadron radiation. Water or phosphate buffered saline was activated by being exposed to hadron radiation including protons, carbon- and iron-ions. 1 mL of radioactivated solution was transferred to flasks with Chinese hamster ovary (CHO) cells cultured in 5 mL of complete media. The induction of sister chromatid exchanges (SCE) was used to observe any increase in DNA damage responses. The energy spectrum and the half-lives of the radioactivation were analyzed by NaI scintillation detector in order to identify generated radionuclides. In the radioactivated solution, 511 keV gamma-rays were observed, and their half-lives were approximately 2 min, 10 min, and 20 min. They respectively correspond to the beta+ decay of 15O, 13N, and 11C. The SCE frequencies in CHO cells increased depending on the amount of radioactivation in the solution. These were suppressed with a 2-hour delayed solution transfer or pretreatment with dimethyl sulfoxide (DMSO). Our results suggest that the SCE induction by radioactivated solution was mediated by free radicals produced by the annihilated gamma-rays. Since the SCE induction and DMSO modulation are also reported in radiation-induced bystander effects, our results imply that radioactivation of the solution may have some contribution to the bystander effects from hadron radiation. Further investigations are required to assess if radioactivation effects would attribute an additional level of cancer risk of the hadron radiation therapy itself.  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