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   684462篇
  免费   76402篇
  国内免费   339篇
  2018年   5909篇
  2016年   7915篇
  2015年   10699篇
  2014年   12463篇
  2013年   18204篇
  2012年   20514篇
  2011年   20567篇
  2010年   13898篇
  2009年   12786篇
  2008年   18508篇
  2007年   19084篇
  2006年   17958篇
  2005年   17208篇
  2004年   16814篇
  2003年   16309篇
  2002年   16154篇
  2001年   33374篇
  2000年   33736篇
  1999年   26746篇
  1998年   8880篇
  1997年   9351篇
  1996年   8949篇
  1995年   8287篇
  1994年   8352篇
  1993年   8054篇
  1992年   22310篇
  1991年   21573篇
  1990年   21020篇
  1989年   20729篇
  1988年   18948篇
  1987年   17912篇
  1986年   16485篇
  1985年   16509篇
  1984年   13324篇
  1983年   11660篇
  1982年   8763篇
  1981年   7731篇
  1980年   7363篇
  1979年   12832篇
  1978年   9809篇
  1977年   8842篇
  1976年   8291篇
  1975年   9208篇
  1974年   9680篇
  1973年   9515篇
  1972年   8703篇
  1971年   7598篇
  1970年   6831篇
  1969年   6349篇
  1968年   5787篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
831.
The seminal receptacle or spermatheca of Portunus sanguinolentus consists of two parts--an anterior glandular and a posterior chitinous part. The chitinous part continues as the oviduct, which opens on the sternite of the sixth thoracic segment. Significant morphological and histological differences were observed between the spermatheca, as well as the oviduct, of mated and unmated crabs. In mated crabs the spermatheca is much more bulging, owing to receipt of a copious supply of seminal products, and its cells are hyperactive. Further stages of ovarian development were observed as indicators of sequential changes in the spermatheca. The secretory cells gradually disintegrate by way of holocrine secretion; this results in cellular stratification and the formation of distinct furrows in the chitinous posterior part.  相似文献   
832.
833.
Prairie voles, Microtus ochrogaster, were infected with Eimeria ochrogasteri and exposed to 2 environmental temperatures, 5 and 22 C. Dry weights of the small and large intestines increased by 33% and 19%, respectively, in infected animals. Infected animals also exhibited a 14% decrease in cecal length compared to uninfected animals. The interaction between temperature and infection affected the length of the small intestine. Infected animals maintained at 5 C had longer small intestines than both infected animals housed at 22 C, and uninfected animals at 22 or 5 C. Furthermore, the dry weight of the small intestine was affected by a 3-way interaction (infection, temperature, and sex). Temperature affected stomach and liver dry weights, as well as lengths of the small intestine and cecum. Stomach and liver dry weights, as well as small intestine lengths, were greater in those animals held at 5 C, whereas cecum lengths decreased. Prepatency, patency, and total oocyst production were not affected by temperature; however, infected animals held at 5 C exhibited diarrhea during the patent period.  相似文献   
834.
835.
Fire has historically been an important ecological component of forests in the Intermountain Region of the northwestern United States. This study is set in a small biogeographically disjunct mountain range. Our research objectives were to (1) investigate the historical frequency, severity, size, and spatial pattern of fire; (2) determine if and how fire regimes have changed since Euro-American settlement; and (3) compare how fire regimes of a small isolated range compare to nearby, but considerably larger, mountain agglomerations. Our findings suggest that this mountain range has historically supported fires typified by small size and high frequency, resulting in a high degree of spatial pattern complexity compared to mountain agglomerations. We also found disparity in size and burn severity solely within the study area based on the bisecting Continental Divide. Since the advent of Euro-American settlement in the 1870s, fire frequency and sizes of individual fires in the West Big Hole Range have significantly decreased resulting in an estimated 87% reduction in area burned. We discuss potential relationships of mountain range isolation and fire regimes in the Intermountain Region. Furthermore, we suggest that the relative small size of this mountain range predisposes it to greater anthropogenic effects upon fire occurrence.  相似文献   
836.
The hyperinsulinemic euglycemic clamp technique was used to investigate the effect on insulin sensitivity of 2 different diets used in practical cattle feeding in calves. Ten 4 to 5-month-old heifer calves were allocated to 2 feeding groups, LO or HI, to obtain growth rates of 400 g/day or 900 g/day. The heifers were fed and housed individually for 5 weeks. Growth rates close to calculated rates were obtained with the diets used. Weekly blood samples were collected from the jugular vein for analysis of glucose, insulin, cortisol, total serum protein, urea, cholesterol and nonesterified fatty acids. During week 5, insulin sensitivity was estimated using the hyperinsulinemic euglycemic clamp technique. Insulin sensitivity did not differ between the groups, but the plasma glucose levels were higher during weeks 3 and 4 for the HI group compared to the LO group. It may be concluded that the amount of concentrate in the diet was too low to induce changes in either the basal plasma insulin levels or the insulin sensitivity in the HI group.  相似文献   
837.
838.
We describe the characterization of the zebrafish homologue of the human gene DLG3. The zebrafish dlg3 gene encodes a membrane-associated guanylate kinase containing a single PDZ domain. This gene was cloned using a gene-trap construct inserted in the gene's first intron. The insertion co-segregates with a viable mutation called humpback (hmp), which leads to formation of ankylotic vertebrae in adult fishes. Insertion and mutation have both been mapped to chromosome 12, in a segment which is syntenic with region p12 to q12 of human chromosome 17. The hmp mutant phenotype, however, appears to be due to two point mutations in the guanylate kinase domain rather than to the transgene insertion itself. The results of this study are discussed in the light of the possible function of the guanylate kinase domain.  相似文献   
839.
Actin participates in the structure of liver intermediate filaments   总被引:1,自引:0,他引:1  
A dominating protein fraction (p45) having molecular weight of 45000 and pI 5.45 was found in the intermediate filaments pellet obtained from rat liver besides the present cytokeratins. Peptide mapping and radioimmunological assays with antibodies against this protein and muscle actin proved that the p45 protein belongs to the actin group. Immunoelectron microscopy revealed that this protein is located on the liver intermediate filaments. By melting of the cytokeratin complexes in urea it was established that p45 protein is complexed with the low molecular weight cytokeratin.  相似文献   
840.
A strain of 1,507 typised donors enables 15.97 average donors to be selected for a thrombocyte transfusion in one of 75 patients selected at random with HLA antibodies being previously determined (1 to 66 donors per patient). HLA compatible donors were found for 72 patients (97.33 per cent). More than 5 HLA compatible donors could be found for 58 patients. The high number of compatible donors is based on the fact that among 1,507 donors there were 156 HLA homozygotes and 556 donors with 3 HLA-A or B-antigens respectively. Compatibility in the ABO-system was not taken into account.  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