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   406789篇
  免费   46293篇
  国内免费   173篇
  453255篇
  2018年   3504篇
  2017年   3493篇
  2016年   4678篇
  2015年   6236篇
  2014年   7261篇
  2013年   10842篇
  2012年   12138篇
  2011年   12277篇
  2010年   8213篇
  2009年   7662篇
  2008年   11159篇
  2007年   11304篇
  2006年   10886篇
  2005年   10288篇
  2004年   10180篇
  2003年   9868篇
  2002年   9624篇
  2001年   18928篇
  2000年   19125篇
  1999年   15532篇
  1998年   5448篇
  1997年   5699篇
  1996年   5514篇
  1995年   5008篇
  1994年   5131篇
  1993年   4991篇
  1992年   12943篇
  1991年   12368篇
  1990年   12302篇
  1989年   12255篇
  1988年   11172篇
  1987年   10617篇
  1986年   9838篇
  1985年   9876篇
  1984年   8020篇
  1983年   6976篇
  1982年   5312篇
  1981年   4672篇
  1980年   4559篇
  1979年   7678篇
  1978年   5939篇
  1977年   5309篇
  1976年   5129篇
  1975年   5544篇
  1974年   5793篇
  1973年   5718篇
  1972年   5171篇
  1971年   4580篇
  1970年   4039篇
  1969年   3759篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
942.
Transglutaminase (R-glutaminyl-peptide:amine alpha-glutamyl-yltransferase [EC 2.3.2.13]) has been purified to apparent homogeneity from extracts of rabbit liver. The enzyme is a single polypeptide chain of approximately 80 000 molecular weight containing one catalytic site per molecule. That the isolated enzyme is the rabbit counterpart of the well-characterized guinea pig liver transglutaminase is evidenced by the similarities in their amino acid compositions and in their enzymic activities toward several substrates, together with the fact that the isolated rabbit enzyme is immunologically distinct from both rabbit plasma and rabbit platelet blood coagulation factor XIII. A striking difference between the catalytic activities of the rabbit and guinea pig enzymes is the low activity of rabbit transglutaminase for hydroxylamine incorporation into benzyloxycarbonyl-L-glutaminylglycine, a reaction for which the guinea pig enzyme shows a high reactivity. This finding reveals the cause of error in an earlier report (Tyler, H.M., and Laki, K. (1967) Biochemistry 6, 3259) that rabbit liver contains little, if any, of the enzyme. Preparation of, and analytical data on, several glutamine-containing peptide derivatives used in this study are reported here.  相似文献   
943.
Induction of spherule formation in Physarum polycephalum by polyols   总被引:2,自引:1,他引:1       下载免费PDF全文
A method has been developed for inducing spherule formation (spherulation) in the myxomycete Physarum polycephalum by transferring the culture to synthetic medium containing 0.5 m mannitol or other polyols. This morphogenetic process occurred within 12 to 35 hr after the inducer was added. The mature spherules existed as distinct morphogenetic units, in contrast to the clusters of spherules formed during starvation. Ninety per cent of the spherules germinated by 24 hr in synthetic medium. The changes in the synthesis of ribonucleic acid (RNA), deoxyribonucleic acid (DNA), and protein during plasmodial growth, spherulation, and germination of spherules are described. When spherule formation was completed, RNA, protein, and DNA decreased, compared with the values at the beginning of the conversion. The incorporation of (3)H-uridine into trichloroacetic acid-insoluble material was different in each of these periods, and this incorporation was sensitive to actinomycin D. The amount of glycogen increased during growth, whereas it decreased during spherulation. (14)C-glucose could be taken up by the cells in the presence of the inducer, and mannitol could not replace glucose as a source of energy. The mode of action of mannitol and its mechanism of induction are discussed.  相似文献   
944.
945.
The bias due to incomplete matching   总被引:8,自引:0,他引:8  
Observational studies comparing groups of treated and control units are often used to estimate the effects caused by treatments. Matching is a method for sampling a large reservoir of potential controls to produce a control group of modest size that is ostensibly similar to the treated group. In practice, there is a trade-off between the desires to find matches for all treated units and to obtain matched treated-control pairs that are extremely similar to each other. We derive expressions for the bias in the average matched pair difference due to the failure to match all treated units--incomplete matching, and the failure to obtain exact matches--inexact matching. A practical example shows that the bias due to incomplete matching can be severe, and moreover, can be avoided entirely by using an appropriate multivariate nearest available matching algorithm, which, in the example, leaves only a small residual bias due to inexact matching.  相似文献   
946.
Seraspenide, a synthetic tetrapeptide, inhibits cell cycle entry of normal hematopoietic stem cells. In mice it protects hemopoiesis against the damage caused by cytarabine, cyclophosphamide and carboplatin. Seraspenide has been given to 53 cancer patients undergoing monochemotherapy with cytarabine and ifosfamide in a double-blind cross-over randomized study. A significant protection of peripheral blood cells has been observed. Seraspenide has been devoided of toxicity.  相似文献   
947.
24 patients with non-Hodgkin's lymphomas of low-grade malignancy (stage III and IV) were treated either with a single drug (prednimustine) or with combination chemotherapy (cyclophosphamide, vincristine and prednisone). Response to therapy was similar in both groups. Prednimustine induced complete remission in 6 from 13 patients, while in that group treated with combination chemotherapy a complete remission was recorded in 4 from 11 patients. Both regimens were well tolerated. Therapy with prednimustin has an advantage in oral administration enabling it to be used in out-patient practice.  相似文献   
948.
In the mesenterium of rats it was found that an overacidity of the blood flowing in the area of microcirculation and caused by irrigation with acid media may be objectively represented in the clearing of venols measurable by means of video technique. The cause of this clearing process is the swelling of erythrocytes setting in at lower pH-values. According to in-vitro findings this swelling of erythrocytes will lead to an increase of the apparent viscosity of the blood fluid or blood cell suspension respectively in conformity with the increase of hematocrit connected with it.  相似文献   
949.
950.
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