全文获取类型
收费全文 | 21229篇 |
免费 | 1543篇 |
国内免费 | 6篇 |
专业分类
22778篇 |
出版年
2023年 | 117篇 |
2022年 | 122篇 |
2021年 | 270篇 |
2020年 | 230篇 |
2019年 | 236篇 |
2018年 | 458篇 |
2017年 | 455篇 |
2016年 | 564篇 |
2015年 | 787篇 |
2014年 | 856篇 |
2013年 | 1222篇 |
2012年 | 1623篇 |
2011年 | 1676篇 |
2010年 | 946篇 |
2009年 | 711篇 |
2008年 | 1299篇 |
2007年 | 1348篇 |
2006年 | 1194篇 |
2005年 | 1090篇 |
2004年 | 1055篇 |
2003年 | 1005篇 |
2002年 | 880篇 |
2001年 | 492篇 |
2000年 | 466篇 |
1999年 | 297篇 |
1998年 | 191篇 |
1997年 | 162篇 |
1996年 | 159篇 |
1995年 | 143篇 |
1994年 | 131篇 |
1993年 | 131篇 |
1992年 | 145篇 |
1991年 | 134篇 |
1990年 | 139篇 |
1989年 | 115篇 |
1988年 | 110篇 |
1987年 | 90篇 |
1986年 | 100篇 |
1985年 | 117篇 |
1984年 | 114篇 |
1983年 | 104篇 |
1982年 | 103篇 |
1981年 | 111篇 |
1980年 | 95篇 |
1979年 | 115篇 |
1978年 | 113篇 |
1977年 | 87篇 |
1976年 | 80篇 |
1974年 | 77篇 |
1973年 | 62篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Background
Head and neck cancers (HNC) are relatively common and often very serious diseases in both dogs and humans. Neoplasms originating in the head and neck region are a heterogeneous group. HNC often has an unfavourable prognosis and the proximity of the tissue structures renders extirpation of tumours with sufficient margins almost incompatible with preservation of functionality. In humans oral malignant melanoma (OMM) is extremely rare, but represents a particular challenge since it is highly aggressive as is the canine counterpart, which thus may be of interest as a spontaneous animal model. 相似文献82.
Summary A protocol was developed for the preparation of Cucumis sativus var Straight 8 protoplasts that incorporates a two-step Ficoll® gradient and results in a high percentage of viable, debris-free protoplasts suitable for the transient expression of foreign genes. Polyethylene glycol and electroporation were compared for their effect on protoplast transfection with commonly used reporter genes. Using a polyethylene glycol method, cucumber protoplasts transfected with a plasmid containing the -glucuronidase gene showed high expression levels, while protoplasts transfected with a plasmid containing the chloramphenicol acetyl transferase gene showed levels of activity that were barely distinguishable from mock-transfected controls. Tomato ringspot virus genomic RNA was also transfected into the protoplasts, and the assembly of viral particles was confirmed. 相似文献
83.
84.
Célia Bordier Claire Saraux Vincent A. Viblanc Hélène Gachot-Neveu Magali Beaugey Yvon Le Maho Céline Le Bohec 《PloS one》2014,9(12)
As the number of breeding pairs depends on the adult sex ratio in a monogamous species with biparental care, investigating sex-ratio variability in natural populations is essential to understand population dynamics. Using 10 years of data (2000–2009) in a seasonally monogamous seabird, the king penguin (Aptenodytes patagonicus), we investigated the annual sex ratio at fledging, and the potential environmental causes for its variation. Over more than 4000 birds, the annual sex ratio at fledging was highly variable (ranging from 44.4% to 58.3% of males), and on average slightly biased towards males (51.6%). Yearly variation in sex-ratio bias was neither related to density within the colony, nor to global or local oceanographic conditions known to affect both the productivity and accessibility of penguin foraging areas. However, rising sea surface temperature coincided with an increase in fledging sex-ratio variability. Fledging sex ratio was also correlated with difference in body condition between male and female fledglings. When more males were produced in a given year, their body condition was higher (and reciprocally), suggesting that parents might adopt a sex-biased allocation strategy depending on yearly environmental conditions and/or that the effect of environmental parameters on chick condition and survival may be sex-dependent. The initial bias in sex ratio observed at the juvenile stage tended to return to 1∶1 equilibrium upon first breeding attempts, as would be expected from Fisher’s classic theory of offspring sex-ratio variation. 相似文献
85.
86.
87.
88.
Following the midcyclic gonadotropin surge, PACAP is transiently expressed for approximately 12 h in the cyclic adult rat ovary. PACAP is observed in granulosa/lutein cells of the large mature follicles destined to ovulate and is believed to be a regulator of acute progesterone production and luteinization in these follicles. PACAP is also observed in solitary theca cells of immature follicles and in interstitial glandular cells intimately surrounding immature follicles. To examine if PACAP could be involved in the process of cyclic recruitment of such immature follicles, we primed immature granulosa cells from prepubertal ovaries with PACAP (1 nM and 100 nM) for 12 h. The treatment significantly stimulated the subsequent 24 h FSH-induced estradiol production (2.2 and 2.4 fold, respectively). The response seemed to be caused by a stimulation of aromatase activity. Estradiol production induced by testosterone was increased 2.4 and 2.6 fold, respectively, whereas functional FSH-receptors (cAMP production following FSH stimulation) or spontaneous apoptosis (immunohistochemical detection of DNA fragments) was unaffected. We conclude that PACAP priming of immature rat granulosa cells for 12 h increases subsequent FSH induced estradiol production and that PACAP could be involved in the cyclic recruitment of immature follicles in the adult rat ovary. 相似文献
89.
Summary A new form of ectodermal dysplasia was observed in two siblings, offspring of healthy non-consanguineous parents. The main findings in both children are: hypodontia, abnormally shaped teeth, scalp hypotrichosis, pili annulati, follicular hyperkeratosis on the trunk and limbs, intensified delineation and reticular hyperpigmentation of the nape, and hyperopia; one of the siblings also has astigmatism. As both patients have normal nails and are euhidrotic, this is an ectodermal dysplasia of the pilodental subgroup. The cause is probably genetic and autosomal-recessive inheritance is most likely. 相似文献
90.
High resolution NMR for studying lipid hydrolysis and esterification in cod (Gadus morhua) gonads 总被引:2,自引:0,他引:2
High resolution NMR was applied to study biochemical changes of lipids in cod (Gadus morhua) gonads during 7 days storage at 4 degrees C. Changes were observed in the (13)C and (1)H resonances of cholesterol which were due to esterification of fatty acids at the hydroxyl position in roe and milt. Furthermore, the (13)C NMR spectra showed that the lipolytic changes in milt and roe where different. New resonances appeared during storage, due to formation of specific free fatty acids, with the corresponding changes in resonances of the esterified carbonyls and glycerols. The highly unsaturated n-3 fatty acids were hydrolysed from the sn-1 and sn-2 position of both phosphatidylcholine and phosphatidylethanolamine in milt. The lipolytical changes in roe were less prominent compared to the changes in milt, however significant levels of sn-1-lysophospholipids was detected both in roe and milt. The current data demonstrate that high resolution NMR may be a suitable method to non-destructively study hydrolysis and esterification reactions occurring in heterogeneous marine lipids in a one step procedure. 相似文献