全文获取类型
收费全文 | 782551篇 |
免费 | 93105篇 |
国内免费 | 900篇 |
出版年
2018年 | 7009篇 |
2016年 | 9471篇 |
2015年 | 13565篇 |
2014年 | 15774篇 |
2013年 | 22076篇 |
2012年 | 25263篇 |
2011年 | 25977篇 |
2010年 | 17384篇 |
2009年 | 16044篇 |
2008年 | 22907篇 |
2007年 | 23550篇 |
2006年 | 22060篇 |
2005年 | 21204篇 |
2004年 | 20960篇 |
2003年 | 20109篇 |
2002年 | 19493篇 |
2001年 | 33742篇 |
2000年 | 33657篇 |
1999年 | 26863篇 |
1998年 | 10071篇 |
1997年 | 10421篇 |
1996年 | 9922篇 |
1995年 | 9363篇 |
1994年 | 9043篇 |
1993年 | 9218篇 |
1992年 | 22694篇 |
1991年 | 22534篇 |
1990年 | 21843篇 |
1989年 | 21294篇 |
1988年 | 19587篇 |
1987年 | 18837篇 |
1986年 | 17702篇 |
1985年 | 17689篇 |
1984年 | 14779篇 |
1983年 | 12796篇 |
1982年 | 9780篇 |
1981年 | 8855篇 |
1980年 | 8353篇 |
1979年 | 13938篇 |
1978年 | 11275篇 |
1977年 | 10137篇 |
1976年 | 9576篇 |
1975年 | 10777篇 |
1974年 | 11522篇 |
1973年 | 11332篇 |
1972年 | 10169篇 |
1971年 | 9399篇 |
1970年 | 7879篇 |
1969年 | 7688篇 |
1968年 | 6974篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
221.
222.
BHK cells either untreated or infected with Semliki Forest virus have been fractionated on sucrose density gradients. Virus infection caused an increase in density of a membrane fraction enriched in sphingomyelin (SM), cholesterol, SM synthase and sialyltransferase activity. This increase in density was related to incorporation of viral proteins into this fraction, which is likely to contain trans-Golgi network (TGN) membranes. In contrast, glucosylceramide synthase and galactosyltransferase activities (markers for cis/medial and trans-Golgi respectively) underwent no density shift and alkaline phosphodiesterase, a plasma membrane marker, was only slightly density-shifted in infected cells. When cells were incubated with NBD-ceramide to enable them to synthesise NBD-SM and then washed with albumin to remove surface label, fluorescence in untreated cells was concentrated in a single juxtanuclear spot but in infected cells this region of bright fluorescence was larger and extended around the nucleus. After fractionation of these cells, NBD-SM (but only a small proportion of the NBD-ceramide) was found to be shifted into the higher density fraction in infected cells. This work provides further evidence that SM synthase is not mainly localised in the early Golgi cisternae as previously thought, but is associated more with a cholesterol-rich compartment which could be the TGN. 相似文献
223.
The insulin receptor substrates (IRSs)-1-4 play important roles in signal transduction emanating from the insulin and insulin-like growth factor (IGF)-I receptors. IRS-4 is the most recently characterized member, which has been found primarily in human cells and tissues. It interacts with SH2-containing proteins such as phosphatidylinositol 3'-kinase (PI3K), Grb2, Crk-II, and CrkL. In this study, we transfected IRS-4 in mouse NIH-3T3 cells that overexpress IGF-I receptors. Clones expressing IRS-4 showed enhanced cellular proliferation when cells were cultured in 1% fetal bovine serum without added IGF-I. Addition of IGF-I enhanced cellular proliferation in cells overexpressing the IGF-I receptor alone but had an even greater proliferative effect in cells overexpressing both the IGF-I receptors and IRS-4. When etoposide and methylmethane sulfonate (MMS), both DNA damaging agents, were added to the cells, they uniformly induced cell cycle arrest. Fluorescence-activated cell sorter analysis demonstrated that the arrest of the cell cycle occurred at the G(1) checkpoint, and furthermore no significant degree of apoptosis was demonstrated with the use of either agent. In cells, overexpressing IGF-I receptors alone, IGF-I addition enhanced cellular proliferation, even in the presence of etoposide and MMS. In cells overexpressing IGF-I receptors and IRS-4, the effect of IGF-I in overcoming the cell cycle arrest was even more pronounced. These results suggest that IRS-4 is implicated in the IGF-I receptor mitogenic signaling pathway. 相似文献
224.
We studied the impulse activity of neurons of the basal and lateral amygdalar nuclei generated when experimental animals (rats) performed fast stereotyped food-procuring movements by the forelimb. Within the basolateral amygdala, there are neurons whose activity is related to different stages of getting off the food, and according to the characteristics of their spiking these neurons should be divided into a number of subpopulations. Activation forestalling the movement initiation by 0.5-1.0 sec was observed in most neurons of the basolateral amygdala; this is considered a manifestation of excitation related to a motivation component of the food-procuring behavior. Activation of amygdalar neurons following movement initiation can result from generation in this structure of additional excitation necessary for successful performance of a complete food-procuring motor cycle. 相似文献
225.
Richard D. Gregory 《Ecography》1998,21(1):92-96
226.
227.
228.
It has been proposed that amplification of genes for esterase that provide resistance to insecticides may originate from
transposition events. To test this hypothesis, we have constructed a minigene coding for a soluble acetylcholinesterase under
the control of a nontissue-specific promoter (hsp70). When introduced into Drosophila, the gene is expressed in all tissues and the extra acetylcholinesterase produced confers a low level of insecticide resistance
(twofold). The minigene was mobilized by crossing the initial transformant with a strain providing a source of P-element transposase.
After 34 generations of exposure to the organophosphate parathion, we obtained a strain with a higher resistance (fivefold).
This strain had only one extra Ace gene, which overexpressed acetylcholinesterase. Thus, following transposition, resistance resulted from the overexpression
of a single copy of the gene and not from gene amplification.
Received: 9 August 1996 / Accepted: 27 May 1997 相似文献
229.
Field observations of two sympatric pitheciine species reveal that the positional repertoire of the white-faced saki, Pithecia pithecia, is dominated by leaping behaviors, whereas the bearded saki, Chiropotes satanas, is predominantly quadrupedal. Examination and comparison of the postcranial skeletal morphologies and limb proportions of these species display numerous features associated with their respective locomotor behaviors. These observations accord with associations found in other primate and mammalian groups and with predictions based on theoretical and experimental biomechanics. Preliminary observations of the skeletal morphology of Cacajao calvus demonstrate a marked similarity to that of Chiropotes. The fossil platyrrhine Cebupithecia sarmientoi displays greater similarity to Pithecia, suggesting that its positional repertoire also included significant leaping and clinging behaviors. 相似文献
230.
D. Tab Rasmussen 《International journal of primatology》1985,6(5):501-517
Breeding records of 11 taxa of captive lemurs housed at the Duke University Primate Center (DUPC), North Carolina, were analyzed
for differences in the timing of births, for the relationship between breeding season and photoperiod, and for differences
in litter size. At DUPC there are significant differences in the timing of births among certain taxa, including differences
among some subspecies of Lemur fulvus.However, changes in latitude result in changes in the timing of the breeding season. Lemurs moved to higher latitudes mate
at lower light-dark ratios than on Madagascar. The data presented here are consistent with the following model: a photoperiodic
cue initiating reproductive activity, presumably a light-dark threshold, precedes the actual mating season by approximately
2 months, with an intervening period of physiological and social preparation. On Madagascar, selection may have favored births
that coincide with the end of dry seasons and the beginning of wet seasons, which results in lactation and weaning during
times of resource abundance. Taxa from the north and east have the highest mean litter sizes; those from the west have the
lowest. 相似文献