全文获取类型
收费全文 | 386篇 |
免费 | 44篇 |
专业分类
430篇 |
出版年
2023年 | 4篇 |
2019年 | 12篇 |
2017年 | 5篇 |
2016年 | 13篇 |
2015年 | 18篇 |
2014年 | 13篇 |
2013年 | 18篇 |
2012年 | 18篇 |
2011年 | 13篇 |
2010年 | 10篇 |
2009年 | 9篇 |
2008年 | 18篇 |
2007年 | 13篇 |
2006年 | 14篇 |
2005年 | 19篇 |
2004年 | 15篇 |
2003年 | 7篇 |
2002年 | 11篇 |
2001年 | 8篇 |
2000年 | 8篇 |
1999年 | 6篇 |
1998年 | 6篇 |
1997年 | 8篇 |
1996年 | 8篇 |
1995年 | 8篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1987年 | 8篇 |
1985年 | 8篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1974年 | 3篇 |
1973年 | 4篇 |
1966年 | 3篇 |
1965年 | 3篇 |
1962年 | 4篇 |
1958年 | 3篇 |
1950年 | 6篇 |
1948年 | 5篇 |
1946年 | 3篇 |
1945年 | 3篇 |
1939年 | 3篇 |
1935年 | 3篇 |
1933年 | 3篇 |
排序方式: 共有430条查询结果,搜索用时 15 毫秒
1.
John Bower 《Evolutionary anthropology》1995,4(4):130-139
The study of early food production in sub-Saharan Africa is at least as challenging as it is rewarding. Problems arise in large degree from the scarcity of relevant archeological material, particularly the remains of domesticated plants from prehistoric sites. This is attributable to several factors, including poor preservation, difficulties in recovering such material, and the limited amount of work so far invested in obtaining it. But, problems notwithstanding, fresh data and new methodological approaches have revealed aspects of early African food production that are interesting in themselves, as well as in global perspective. For example, contrary to what occurred in most other parts of the world, livestock herding in Africa often predated the earliest evidence of cultivation of domesticated plants. Moreover, the initial spread of food production throughout much of sub-Saharan Africa was accompanied by iron, rather than lithic, technology. This overview of current knowledge about early African food production is aimed at highlighting developmental patterns while also exposing limitations in our understanding of these patterns. Because of Africa's vast size, uniform coverage in reasonable depth of all parts of the continent is not possible. Thus, for example, I will not explicitly cover the complex neolithic record from Africa's Mediterranean region. Instead, I will generally concentrate on bodies of data and lines of investigation that characterize distinctive features of the African version of initial steps in raising crops and animals. 相似文献
2.
A second gene has been discovered at a previously studied Balbiani ring in Chironomus. Northern hybridizations demonstrated that cDNA clone pCt35 originated from a salivary gland specific 6.5-kilobase (kb) RNA that was abundant, nonribosomal, and apparently poly(A)+. pCt35 had a 120-base pair (bp) insert with 1.6 copies of a 75-bp sequence that contained two open reading frames. Southern hybridizations indicated that pCt35 was homologous to at least a 4-kb block of genomic DNA organized as a hierarchy of 150- and 300-bp tandem repeats. In situ hybridization localized these sequences to Balbiani ring 1. From these results we postulated that a 6.5-kb RNA gene may have evolved by stepwise duplication and amplification of a 75-bp ancestral sequence. 相似文献
3.
Chicken lens delta-crystallin gene expression and methylation in several non-lens tissues. 总被引:5,自引:2,他引:3 下载免费PDF全文
D J Bower L H Errington D N Cooper S Morris R M Clayton 《Nucleic acids research》1983,11(9):2513-2527
RNA sequences coding for the most abundant chicken lens proteins, delta-crystallin, were detected at very low levels in day old post hatch chick lung, heart, kidney and liver, and in 6 day embryo headless bodies. The pattern of cytosine methylation within the CCGG sequences of the delta-crystallin genes was also examined and shown to vary in several non-lens tissues, from several stages of development. Embryonic neural retina, which expresses a higher level of delta-crystallin RNA than the above tissues, is no less methylated in the sites studied than the tissues which have no association with the eye, and is actually more heavily methylated than the kidney. Thus no obvious correlation was found between undermethylation and gene expression. 相似文献
4.
5.
Inhibition of mitosis in fertilized sea urchin eggs by inhibition of the cyclic AMP-dependent protein kinase 总被引:1,自引:0,他引:1
Inhibition of cAMP-dependent protein kinase activity by microinjection of a specific physiologic protein inhibitor into sea urchin eggs inhibits the first cleavage after fertilization. Inhibition apparently occurs at some time prior to or during formation of the mitotic spindle. Measurement of the total protein kinase activity of sea urchin egg homogenates after fertilization showed that cAMP-dependent phosphorylation increases after fertilization and then declines prior to or at the time of the first cleavage. It is concluded that a cAMP-dependent phosphorylation plays a significant role in events leading to regulation of mitotic spindle assembly. 相似文献
6.
Contradictory reports in the literature provoked the investigation of a possible oxytocic effect of boar semen on in vitro rat uterus preparations. When fresh boar seminal plasma (SP), dialyzed seminal plasma (DSP) or a filtrate of acetone-extracted seminal plasma (AE) were added to uterine preparations, both the frequency and force of spontaneous contractile activity tended to be depressed. When the uterine preparations were primed with oxytocin (1.6 USP units/100 ml bathing solution), treatment with SP and AE resulted in an increased frequency of contraction but no increase in force. DSP increased neither the frequency nor force of contractions. A solution of salts and organic compounds that approximated the small molecular or dialyzable components of boar semen ("synthetic boar seminal plasma" or BS) affected contractility in a manner similar to SP and AE. When the rat uterus was bathed in a low calcium physiological salt solution, none of the seminal treatments (SP, DSP, AE, or BS) were capable of initiating a contraction. Further, the force of carbachol-induced contractions in these uterine preparations was markedly depressed by these seminal treatments. In contrast, treatment with oxytocin nearly doubled the contractile force. The depressant effect of SP, DSP and AE on both spontaneous and carbachol-induced contractions appeared to be irreversible. Mineral analysis of SP, DSP, AE, BS and the low calcium salt solution showed the SP, AE and BS contained amounts of Ca(2+) and Mg(2+) which could have altered the uterine contractility. In addition, a non-dial-yzable factor in SP appeared to have a similar effect. 相似文献
7.
8.
9.
During a 12-year period 50 patients with measles encephalitis were treated with typhoid vaccine—45 of them not until they were in a vegetative state. There were no deaths in the treated cases. All except one were clinically normal after treatment was ended. Late in the series it became apparent that after the clinical normal is attained and the electroencephalogram becomes normal, treatment should still be continued until the cortex has been “challenged” repeatedly and the electroencephalogram shown to remain normal.The “shock” element in the treatment with typhoid vaccine was prevented by anticipating and circumventing the unfavorable reactions to the vaccine.It may take 20 to 50 treatments or more with typhoid vaccine to return a patient to normality. Each individual is different and responds in a different period of time. 相似文献
10.
Leonard H Glasson E Nassar N Whitehouse A Bebbington A Bourke J Jacoby P Dixon G Malacova E Bower C Stanley F 《PloS one》2011,6(3):e17875