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   18124篇
  免费   1269篇
  国内免费   3篇
  2023年   95篇
  2022年   202篇
  2021年   458篇
  2020年   347篇
  2019年   373篇
  2018年   453篇
  2017年   385篇
  2016年   616篇
  2015年   942篇
  2014年   1015篇
  2013年   1201篇
  2012年   1506篇
  2011年   1325篇
  2010年   861篇
  2009年   797篇
  2008年   1083篇
  2007年   997篇
  2006年   934篇
  2005年   875篇
  2004年   845篇
  2003年   751篇
  2002年   691篇
  2001年   241篇
  2000年   159篇
  1999年   216篇
  1998年   164篇
  1997年   156篇
  1996年   138篇
  1995年   120篇
  1994年   122篇
  1993年   85篇
  1992年   97篇
  1991年   111篇
  1990年   109篇
  1989年   90篇
  1988年   84篇
  1987年   69篇
  1986年   58篇
  1985年   67篇
  1984年   81篇
  1983年   38篇
  1982年   38篇
  1981年   40篇
  1980年   44篇
  1979年   33篇
  1978年   31篇
  1977年   34篇
  1976年   25篇
  1975年   23篇
  1969年   17篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
122.
123.
IfN(t) is the expected number of cells in a culture at timet, the corresponding time derivative, andf(t−τ)dt the probability that a cell of aget−τ at timet will divide in the succeeding time intervaldt, then according to Hirsch and Engelberg (this issue) there obtains the integral equation for describing the dynamics of the cell population. It is the purpose of this note to give two alternative derivations of this equation, one based on the age density equation of Von Foerster, and the other based on a generalized form of the Harris-Bellman equation describing the first moment of an age dependent, branching process. In addition, a probability model is posed from which the Von Foerster equation and, hence, the Hirsch-Engelberg equation readily follows.  相似文献   
124.
125.
126.
The ultrastructure of the mature spermatozoa and spermatogenesis of the bivalve Scrobicularia plana are described. Support cells extend from the basal lamina to the lumen of the testis and are laterally connected to the germinal epithelium. Germ cells present intercellular bridges and flagella since the spermatogonial stage. While spermatogonia and spermatocytes appear connected to support cells by desmosome-like junctions, elongated spermatids are held at the acrosomal region by support cell finger-like processes. During spermiogenesis, the acrosomal vesicle differentiates from a golgian saccule and then migrates to the nuclear apex. A microtubular manchette arising from centrioles surrounds the acrosomal vesicle, the nucleus, and the mitochondria at the time these three organelles start their elongation, disappearing after that. The mature spermatozoon of S. plana lacks a distinct midpiece because the mitochondria extend from the region of the pericentriolar complex along the nucleus anteriorly for approximately 1.4 μm. The features of this bivalve type of modified spermatozoon are compared with those of other animal groups having similar modifications.  相似文献   
127.
Recent studies have shown that IL-4 can affect lymphocyte responses to IL-2. To evaluate the effects of IL-4 on T cell responses to physiologically relevant stimuli, we studied normal human T cells cultured with a low concentration of anti-CD3 mAb and IL-2 in the presence and absence of added IL-4. The addition of IL-4 to cultures of T cells stimulated with anti-CD3 mAb and IL-2 reduced the proliferative response by 49 to 59%. The inhibitory effect was observed in 3-, 5-, and 7-day cultures. Inhibition was dose-dependent with maximal inhibition at concentrations greater than or equal to 5 to 10 U/ml IL-4. IL-4-mediated inhibition occurred early during the T cell response, inasmuch as addition of IL-4 after stimulation for 24 h did not result in significant inhibition. Phenotypic analyses of cells cultured in the presence of anti-CD3 mAb, IL-2, and IL-4 suggested that the mechanism of regulation by IL-4 involves the inhibition of IL-2R expression. The proportion of both CD4+ and CD8+ cells that expressed IL-2R in response to IL-2 was diminished in the presence of IL-4, although HLA-DR levels were unaffected. Soluble IL-2R was also reduced in supernatants of cultures stimulated with anti-CD3 mAb, IL-2, and IL-4 as compared to cultures stimulated with anti-CD3 mAb and IL-2. These findings indicate that when normal human T cells are stimulated in vitro in a manner that approximates a physiologic interaction with Ag in vivo, rIL-4 provides a potent inhibitory signal to IL-2 responsive cells that is likely mediated by IL-4-induced inhibition of IL-2R expression.  相似文献   
128.
129.
130.
Summary The pattern of inheritance of several polymorphic restriction sites associated with the -gene cluster, and spanning a region of 52 kb, demonstrates that a determinant for hereditary persistence of fetal hemoglobin (HPFH) segregates independently from the non- globin gene cluster, as we postulated several years ago on purely genetical grounds. This finding provides additional evidence for the existence of diffusible factors affecting -chain expression. Moreover, we have identified a private HinccII polymorphism, in the vicinity of the gene in the family studied.  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