全文获取类型
收费全文 | 5738篇 |
免费 | 354篇 |
国内免费 | 989篇 |
出版年
2024年 | 22篇 |
2023年 | 87篇 |
2022年 | 131篇 |
2021年 | 138篇 |
2020年 | 120篇 |
2019年 | 174篇 |
2018年 | 218篇 |
2017年 | 145篇 |
2016年 | 171篇 |
2015年 | 168篇 |
2014年 | 421篇 |
2013年 | 466篇 |
2012年 | 286篇 |
2011年 | 277篇 |
2010年 | 272篇 |
2009年 | 331篇 |
2008年 | 382篇 |
2007年 | 421篇 |
2006年 | 317篇 |
2005年 | 336篇 |
2004年 | 264篇 |
2003年 | 248篇 |
2002年 | 217篇 |
2001年 | 161篇 |
2000年 | 136篇 |
1999年 | 139篇 |
1998年 | 130篇 |
1997年 | 96篇 |
1996年 | 94篇 |
1995年 | 82篇 |
1994年 | 77篇 |
1993年 | 70篇 |
1992年 | 49篇 |
1991年 | 62篇 |
1990年 | 52篇 |
1989年 | 45篇 |
1988年 | 31篇 |
1987年 | 37篇 |
1986年 | 29篇 |
1985年 | 30篇 |
1984年 | 32篇 |
1983年 | 17篇 |
1982年 | 19篇 |
1981年 | 18篇 |
1980年 | 16篇 |
1979年 | 14篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1972年 | 4篇 |
排序方式: 共有7081条查询结果,搜索用时 17 毫秒
41.
Functional morphology of the asterionic region in extant hominoids and fossil hominids 总被引:1,自引:0,他引:1
Asterionic sutural patterns in Plio-Pleistocene hominid crania have never been examined in detail. We present an analysis of this anatomical region in Australopithecus and Homo and relate different sutural patterns to functional changes in the masticatory apparatus. The great apes and A. afarensis share the common adult higher primate sutural pattern referred to as the "asterionic notch," which develops in response to the hypertrophy of posterior temporalis muscle fibers and the consequent formation of compound temporal/nuchal crests. This sutural configuration also appears to be present on the early Homo cranium KNM-ER 1805. In contrast, adult male A. boisei crania exhibit a unique pattern where the temporal squama overlaps the parietal which, in turn, overlaps the par mastoidea and the upper scale of the occipital bone. We relate this arrangement to the need to reinforce the rear of a thin-walled braincase against the net tensile forces exerted by the temporalis and nuchal muscles. The common juvenile hominoid edge-to-edge asterionic articulation is maintained in adult A. africanus, A. robustus, female A. boisei, and most Homo crania. We discuss the latter pattern in regard to anterior temporalis hypertrophy in A. africanus, A. robustus, and A. boisei and to craniofacial paedomorphosis in Homo. 相似文献
42.
43.
Biomass and production of Argyrodiaptomus furcatus,a tropical calanoid copepod in Broa Reservoir,southern Brazil 总被引:1,自引:1,他引:0
The biomass and the production of Argyrodiaptomus furcatus (Sars), the most abundant copepod in Broa Reservoir (São Carlos, São Paulo State), were estimated, determining in the laboratory the development time and the quantity of organic carbon and establishing the relationship between these two parameters. The daily production was calculated from P = B(1- egt) and the annual production was obtained by integrating daily production against time. The maximum production of Argyrodiaptomus furcatus in the reservoir depends on the region considered and on the period of the year. The maximum production was 45.15 mg C m–3d–1 in March, 1976 at station II, region of macrophytes and 6.74 mg C m–3d–1 at station IV, near the dam. The mean production for the year is 6.26 mg C m–3d–1 at station II and 1.43 mg C m–3d–1 at station IV. 相似文献
44.
The sperm head of the plains mouse, Pseudomys australis, has three curved hooks projecting from its anterior margin. The two ventral hooks have previously been shown to consist largely of an extension of the subacrosomal material. To characterize further the structure and composition of the ventral hooks, we have examined their formation during spermiogenesis using transmission electron microscopy, silver staining, and actin localization with NBD-phallacidin. The ventral hooks develop as an extension of the perinuclear space and postacrosomal dense lamina on the anteroventral margin of the sperm head. Bundles of 6-nm-thick filaments appear in the core of each hook; these are probably actin filaments based on staining of the hooks with NBD-phallacidin. Just prior to spermiation, electron-dense material condenses in the core of the ventral hooks and concurrently in the perinuclear space in the remainder of the sperm head. The two ventral hooks thus appear to consist of a core of perinuclear material and actin filaments, which is enclosed by a continuation of the postacrosomal dense lamina. 相似文献
45.
46.
47.
48.
Intra- and interspecific competition between laboratory populations of four green leafhoppers, Nephotettix spp. was studied in the laboratory under three different temperature regimes of 24°C, 27°C and 30°C. For the single-species population of the three tropical species, the equilibrium density increased as the temperature increased. On the other hand, for the temperature species N. cincticeps, the highest equilibrium density was at the intermediate temperature and the lowest at high temperature. Interspecific interactions between two tropical (N. virescens vs. N. nigropictus), a tropical and a temperature (N. virescens vs. N. cincticeps) and a rice-feeding and a grass-inhabiting (N. virescens vs. N. malayanus) Nephotettix species were also studied in the laboratory at the three temperature regimes. Temperature differentially affected the outcome of competition between two Nephotettix species. Between N. virescens and N. nigropictus, the latter was more successful over the former at low and intermediate temperatures, while the former was more successful at high temperature. Between N. virescens and N. cincticeps, the temperate species inhibited the growth of the tropical species at low temperature while the tropical species inhibited the growth of the temperate species at high temperature. At intermediate temperature, the population of N. virescens persisted at a slightly higher density over the population of N. cincticeps. Between the rice-feeding N. virescens and the grass-inhabiting N. malayanus, regardless of temperature the population density of the latter was greatly reduced and later became extinct while the population of the former continued its growth. These consequences of competition between two Nephotettix species conformed fairly well to those predicted by theLotka-Volterra model using demographic parameters specified for each species. 相似文献
49.
Monique Monnerot Michel Solignac David R. Wolstenholme 《Journal of molecular evolution》1990,30(6):500-508
Summary Restriction sites were compared in the mitochondrial DNA (mtDNA) molecules from representatives of two closely related species of fruit flies: nine strains ofDrosophila teissieri and eight strains ofDrosophila yakuba. Nucleotide diversities amongD. teissieri strains and amongD. yakuba strains were 0.07% and 0.03%, respectively, and the nucleotide distance between the species was 0.22%. Also determined was the nucleotide sequence of a 2305-nucleotide pari (ntp) segment of the mtDNA molecule ofD. teissieri that contains the noncoding adenine+thymine (A+T)-rich region (1091 ntp) as well as the genes for the mitochondrial small-subunit rRNA, tRNAf-met, tRNAgln, and tRNAile, and portions of the ND2 and tRNAval genes. This sequence differs from the corresponding segment of theD. yakuba mtDNA by base substitutions at 0.1% and 0.8% of the positions in the coding and noncoding regions, respectively. The higher divergence due to base substitutions in the A+T-rich region is accompanied by a greater number of insertions/deletions than in the coding regions. From alignment of theD. teissieri A+T-rich sequence with those ofD. yakuba andDrosophila virilis, it appears that the 40% of this sequence that lies adjacent to the tRNAile gene has been highly conserved. Divergence between the entireD. teissieri andD. yakuba mtDNA molecules, estimated from the sequences, was 0.3%; this value is close to the value (0.22%) obtained from the restriction analysis, but 10 times lower than the value estimated from published DNA hybridization results. From consideration of the relationships of mitochondrial nucleotide distance and allozyme genetic distance found among seven species of theDrosophila melanogaster subgroup, the mitochondrial nucleotide distance observed forD. teissieri andD. yakuba is anomalously low in relation to the nuclear genetic distance. 相似文献
50.
What causes the aphid 28S rRNA to lack the hidden break? 总被引:1,自引:0,他引:1