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   420335篇
  免费   43785篇
  国内免费   198篇
  2016年   4522篇
  2015年   5959篇
  2014年   7064篇
  2013年   10632篇
  2012年   11501篇
  2011年   11983篇
  2010年   8160篇
  2009年   7486篇
  2008年   10704篇
  2007年   11351篇
  2006年   10693篇
  2005年   10169篇
  2004年   10263篇
  2003年   10033篇
  2002年   9840篇
  2001年   17368篇
  2000年   17412篇
  1999年   14066篇
  1998年   4984篇
  1997年   5290篇
  1996年   5015篇
  1995年   4731篇
  1994年   4640篇
  1993年   4631篇
  1992年   12275篇
  1991年   12086篇
  1990年   12090篇
  1989年   11863篇
  1988年   11168篇
  1987年   10505篇
  1986年   9827篇
  1985年   10241篇
  1984年   8443篇
  1983年   7278篇
  1982年   5615篇
  1981年   5025篇
  1980年   4701篇
  1979年   8090篇
  1978年   6304篇
  1977年   5948篇
  1976年   5675篇
  1975年   6134篇
  1974年   6705篇
  1973年   6571篇
  1972年   6123篇
  1971年   5538篇
  1970年   4768篇
  1969年   4785篇
  1968年   4454篇
  1967年   3810篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Mutants with impaired biosynthesis of unsaturated fatty acids or altered metabolism of the phospholipids were isolated at a rather high frequency from a set of temperature-sensitive lysis mutants. It is suggested that preselection for the lysis phenotype makes it possible to isolate several kinds of mutants affected in the integrity of the cytoplasmic membrane.  相似文献   
972.
973.
974.
975.
Sporadic (less than regular weekly) coital activity is associated with a particular type of infertility—the short hyperthermic (luteal) phase. This preliminary report is the first suggestion that a reduced frequency of human sexual activity might, itself, be associated with subfertile reproductive function.  相似文献   
976.
977.
A morphometric study of subcellular structures in human primary breast tumors (of the ductal infiltrating type) and their synchronous lymph node metastases was carried out using planimetry and stereology. Each primary neoplasm was compared separately with its axillary metastatic growth in order to detect differences in the quantitative estimates of their subcellular components. A set of 24 morphometric parameters was used to quantitatively describe each neoplastic cell profile. Although some subcellular parameters showed significant differences between the cells of a primary tumor and its metastases, no parameter was found to be consistently altered in all cases. No consistent increase or decrease in the values of the affected parameters was observed in the metastases. More than being due to the expression of a biologically variant (more-or-less differentiated) metastatic phenotype, the differences in stereologic parameters detected in metastatic cells appear to reflect the existence of different functional states of the same cell type.  相似文献   
978.
979.
We report a new mutation in the sterol 27-hydroxylase (CYP 27) gene in a Dutch family with cerebrotendinous xanthomatosis: a G→A transition in the splice donor site in intron 4. This mutation leads to skipping of exon 4, resulting in a loss of 66 amino acids in the CYP 27 enzyme molecule. Received: 15 March 1997 / Accepted: 26 March 1997  相似文献   
980.
Duodenal active calcium transport and longitudinal bone growth rate have been shown previously to be regulated in parallel by alteration of gonadal hormone status in sexually maturing female rats. The present study was designed to extend these observations to the sexually maturing male rat. Male rats were orchidectomized (ORX) and given Silastic implants containing either testosterone or estradiol at 6 weeks of age. At 9 weeks of age, duodenal active calcium transport was measured by the everted gut sac method and longitudinal bone growth rate was determined by tetracycline labeling. Decreases in body weight, longitudinal bone growth rate, duodenal calcium transport, and serum Ca and P were exhibited by ORX animals as compared with age-matched control animals. Testosterone administration to ORX animals resulted in an increase in body weight, longitudinal bone growth rate, duodenal calcium transport, and serum Ca and P as compared with ORX animals to a level not significantly different from that of age-matched control animals. Estradiol administration to ORX animals resulted in an additional decrease in body weight, although no significant effect on duodenal calcium transport, serum Ca, or P was noted as compared with ORX animals. There were no statistically significant alterations in the circulating levels of 1,25-dihydroxyvitamin D, parathyroid hormone, or osteocalcin in response to any of the experimental manipulations of gonadal status. These results indicate that, as in the female, gonadal hormone status affects intestinal calcium transport in sexually maturing male rats in parallel with changes in bone growth rate by mechanisms that are independent of circulating levels of 1,25-dihydroxyvitamin D.  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