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   625篇
  免费   38篇
  2021年   5篇
  2018年   4篇
  2016年   7篇
  2015年   19篇
  2014年   22篇
  2013年   32篇
  2012年   31篇
  2011年   27篇
  2010年   30篇
  2009年   31篇
  2008年   26篇
  2007年   23篇
  2006年   20篇
  2005年   29篇
  2004年   20篇
  2003年   8篇
  2002年   10篇
  2001年   11篇
  2000年   13篇
  1999年   6篇
  1998年   12篇
  1997年   12篇
  1996年   8篇
  1995年   10篇
  1994年   6篇
  1993年   7篇
  1992年   5篇
  1991年   7篇
  1990年   8篇
  1989年   12篇
  1988年   12篇
  1987年   4篇
  1986年   5篇
  1985年   5篇
  1984年   5篇
  1983年   5篇
  1982年   19篇
  1981年   4篇
  1979年   4篇
  1978年   10篇
  1977年   6篇
  1976年   6篇
  1975年   6篇
  1971年   7篇
  1969年   7篇
  1959年   6篇
  1957年   7篇
  1956年   6篇
  1949年   5篇
  1933年   4篇
排序方式: 共有663条查询结果,搜索用时 15 毫秒
1.
The ecology, abundance and diversity of galatheoid squat lobsters make them an ideal group to study deep-sea diversification processes. Here, we reconstructed the evolutionary and biogeographic history of Leiogalathea, a genus of circum-tropical deep-sea squat lobsters, in order to compare patterns and processes that have affected shallow-water and deep-sea squat lobster species. We first built a multilocus phylogeny and a calibrated species tree with a relaxed clock using StarBEAST2 to reconstruct evolutionary relationships and divergence times among Leiogalathea species. We used BioGeoBEARS and a DEC model, implemented in RevBayes, to reconstruct ancestral distribution ranges and the biogeographic history of the genus. Our results showed that Leiogalathea is monophyletic and comprises four main lineages; morphological homogeneity is common within and between clades, except in one; the reconstructed ancestral range of the genus is in the Atlantic and Indian oceans (Tethys). They also revealed the divergence of the Atlantic species around 25 million years ago (Ma), intense cladogenesis 15–25 Ma and low levels of speciation over the last 5 million years (Myr). The four Leiogalathea lineages showed similar patterns of speciation: allopatric speciation followed by range expansion and subsequent stasis. Leiogalathea started diversifying during the Oligocene, likely in the Tethyan. The Atlantic lineage then split from its Indo-Pacific sister group due to vicariance driven by closure of the Tethys Seaway. The Atlantic lineage is less speciose compared with the Indo-Pacific lineages, with the Tropical Southwestern Pacific being the current centre of diversity. Leiogalathea diversification coincided with cladogenetic peaks in shallow-water genera, indicating that historical biogeographic events similarly shaped the diversification and distribution of both deep-sea and shallow-water squat lobsters.  相似文献   
2.
3.
Summary Pre-implantation-stage embryos from rats, mice, and guinea-pigs were transferred to a non-uterine site — the anterior chamber of the eye — of female recipients. All 9 combinations of transfers were performed: 3 allogeneic (intraspecies) transfers as controls, and 6 xenogeneic (interspecies) transfers. Implantation, as judged by extravasation from blood vessels of the iris or ciliary body occurred with success rates of 90.4% per transfer in the control rat group, 76.9% in the control mouse group, and 81.8% in the control guinea-pig group. Significantly reduced implantation rates occurred in the rat to guinea-pig (0%), mouse to rat (46.9%), mouse to guinea-pig (6.7%), and guinea-pig to rat (0%) groups compared to controls. Reductions, although not significant, also occurred in the other 2 groups: rat to mouse (77.8%), and guinea-pig to mouse (44.4%). These results together with some ultrastructural and lightmicroscopical observations suggest a degree of species specificity involved in the vascular response to the implanting embryo. We propose that the peri-implantation embryo produces a signal(s) which is to some extent species specific and which in the normal allogeneic situation is responsible for the early vascular effects seen at implantation in most eutherian mammals.  相似文献   
4.
Protoscoleces from human, camel, cattle, sheep, goat (all from Kenya) and buffalo (from India) hydatid cysts were cultured under identical conditions in vitro using the diphasic culture system of Smyth (1979b). Organisms from all sources grew and segmented in culture. Genital anlagen developed in all cultured worms but further genital differentiation occurred only in cultures of cattle (testes) and camel (testes and genital pore) material. The possible significance of these results is discussed in relation to the general epidemiology of hydatid disease and the potential infectivity of the different strains to man.  相似文献   
5.
6.
7.
Two methods of semen collection were attempted on ten untrained dogs of several breeds. Manual manipulation as described proved most reliable for untrained dogs. Electro-ejaculation was attempted unsucessfully with twelve additional dogs.Two methods of insemination were compared. Equipment found most satisfactory is described and a modification suggested.  相似文献   
8.
9.
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