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   2044篇
  免费   192篇
  2021年   16篇
  2018年   21篇
  2017年   19篇
  2016年   24篇
  2015年   50篇
  2014年   49篇
  2013年   72篇
  2012年   74篇
  2011年   74篇
  2010年   66篇
  2009年   55篇
  2008年   78篇
  2007年   84篇
  2006年   69篇
  2005年   74篇
  2004年   80篇
  2003年   71篇
  2002年   72篇
  2001年   21篇
  2000年   21篇
  1998年   21篇
  1997年   19篇
  1995年   20篇
  1993年   18篇
  1991年   21篇
  1990年   16篇
  1988年   17篇
  1987年   15篇
  1986年   17篇
  1984年   26篇
  1983年   22篇
  1982年   22篇
  1981年   26篇
  1980年   16篇
  1979年   18篇
  1978年   21篇
  1977年   26篇
  1976年   16篇
  1975年   21篇
  1974年   32篇
  1973年   27篇
  1972年   18篇
  1971年   14篇
  1970年   17篇
  1969年   18篇
  1965年   15篇
  1956年   16篇
  1910年   15篇
  1892年   16篇
  1889年   17篇
排序方式: 共有2236条查询结果,搜索用时 62 毫秒
1.
2.
Previous research has demonstrated that having more children decreases marital satisfaction among parents. However, the universality of these findings is limited since the vast majority of the studies have been conducted in Western countries. In the present study, 374 people from the Igbo ethnic group (Nigeria) were assessed for levels of marital satisfaction and the number of children. In contrast to almost all previous findings, we found a positive relationship between the number of children and marital satisfaction among parents. Number of children was the strongest predictor of marital satisfaction even when compared to other variables like wealth and education. Our results suggest that the negative relationship between the number of children and marital satisfaction is not culturally universal and probably only characterizes developed, individualistic Western countries. We discuss our findings from a sociocultural and evolutionary perspective.  相似文献   
3.
4.
5.
6.
7.
8.
A “double-blepharoplast” originates de novo in the spermatogenous cell of Ginkgo biloba L. Initially, the double-blepharoplast consists of two opposing hemispherical bodies comprised of densely staining material. The two blepharoplasts seemingly are pushed apart by the formation of densely packed fibrils which are oriented perpendicular to the distal, rounded edges of the two future blepharoplasts. As the latter move apart, each one develops lightly staining channels which are often organized in a hub and spoke configuration (procentrioles). Microtubules extend from the blepharoplasts as the latter move to their final position in the cell, and centrioles (probasal bodies) become organized at the periphery of each blepharoplast. Two large “osmiophilic globules,” conspicuous entities close to the nucleus of the mature spermatogenous cell, arise de novo. A fibrillogranular body in the cytoplasm, always closely associated with the nucleus, also arises de novo.  相似文献   
9.
Gifford , Ernest M., Jr . (U. California, Davis.) Incorporation of H3-thymidine into shoot and root apices of Ceratopteris thalictroides. Amer. Jour. Bot 47(10): 834–837. Illus. 1960.—The localization of tritiated thymidine in apical meristems of Ceratopteris thalictroides by the autoradiographic method is described. Intact, floating plants of the fern were placed in 1/2 strength Hoagland's inorganic nutrient solution containing H3-thymidine (10 μc/ml.) for 3 days. The material was killed, dehydrated and embedded in paraffin. Autoradiographic stripping film (AR 10 Kodak) was applied to serial sections. After an appropriate exposure period, the film was developed and the sections with the superimposed film were stained lightly with Harris' hematoxylin. The autoradiographs revealed the presence of the H3-thymidine in nuclei of the large, individualized apical cells of shoots and roots which is proof of DNA synthesis. In no instances were these nuclei unlabeled. If endomitotic reduplication is excluded the results of these studies lend support to the concept that apical cells actually do divide and perhaps at a higher rate than envisioned by other workers. Considerable cytoplasmic labeling occurred and its significance to general problems of DNA synthesis is discussed.  相似文献   
10.
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