全文获取类型
收费全文 | 23198篇 |
免费 | 1611篇 |
国内免费 | 5篇 |
出版年
2023年 | 189篇 |
2022年 | 378篇 |
2021年 | 721篇 |
2020年 | 533篇 |
2019年 | 638篇 |
2018年 | 852篇 |
2017年 | 718篇 |
2016年 | 985篇 |
2015年 | 1335篇 |
2014年 | 1448篇 |
2013年 | 1746篇 |
2012年 | 2000篇 |
2011年 | 1826篇 |
2010年 | 1106篇 |
2009年 | 987篇 |
2008年 | 1241篇 |
2007年 | 1136篇 |
2006年 | 1068篇 |
2005年 | 907篇 |
2004年 | 791篇 |
2003年 | 751篇 |
2002年 | 706篇 |
2001年 | 327篇 |
2000年 | 299篇 |
1999年 | 268篇 |
1998年 | 157篇 |
1997年 | 143篇 |
1996年 | 103篇 |
1995年 | 85篇 |
1994年 | 87篇 |
1993年 | 75篇 |
1992年 | 133篇 |
1991年 | 95篇 |
1990年 | 80篇 |
1989年 | 90篇 |
1988年 | 74篇 |
1987年 | 76篇 |
1986年 | 57篇 |
1985年 | 60篇 |
1984年 | 65篇 |
1983年 | 36篇 |
1982年 | 39篇 |
1981年 | 36篇 |
1980年 | 29篇 |
1979年 | 46篇 |
1978年 | 25篇 |
1976年 | 25篇 |
1975年 | 31篇 |
1974年 | 22篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
121.
A study was undertaken to compare the susceptibility of laboratory-reared female Lutzomyia longipalpis to infection by different species or strains of New World Leishmania. The sand flies proved to be highly susceptible to infection by a strain of Le. guyanensis, with flagellates developing in all (18/18) of the specimens examined. A lower infection rate of 37% (10/27) was recorded in flies exposed to infection by a strain of Le. amazonensis. Flagellates developed in 13% (6/46) of the sand flies that blood fed on dogs in the early stage of experimental infection with an old laboratory strain of Le. chagasi. In contrast, promastigotes did not develop in sand flies that blood fed on dogs with naturally acquired Le. chagasi. The naturally infected dogs were in an advanced stage of disease. Flagellates developed in 9% (3/32) of the sand flies that blood fed on lesions of hamsters infected with a strain of Le. braziliensis and in 9% (3/34) of those that fed on hamsters with lesions due to a parasite of the mexicana complex (strain MHOM/BR/73/BH121). Sand flies did not develop flagellate infections after blood feeding on hamsters bearing lesions induced by strain MHOM/BR/71/BR49. Factors influencing the susceptibility of Lu. longipalpis to infection by New World species of Leishmania are discussed. 相似文献
122.
Nef genes of SIV 总被引:4,自引:0,他引:4
H W Kestler K Mori D P Silva T Kodama N W King M D Daniel R C Desrosiers 《Journal of medical primatology》1990,19(3-4):421-429
Molecular clones of SIVmac were constructed that differed only in sequences within the nef gene. DEAE-transfection of viral DNA containing an open from of nef yielded virus that replicated with similar kinetics and to a similar extent in macaque peripheral blood lymphocyte (PBL) cultures as virus with a deletion or stop codon within nef. Rhesus monkeys that received each kind of molecularly cloned virus became infected. Our results additionally suggest that mutant forms of virus are selected in vitro while open, functional forms are selected in vivo. In animals infected with virus containing a stop codon within nef, reversion of the stop codon to a coding codon was demonstrated in five of five clones analyzed. These results indicate that nef is playing some role crucial to the virus life cycle in vivo. 相似文献
123.
Silastic devices impregnated with oestradiol and blank devices were placed around both oviducts and around skeletal muscle bundles in the forelegs to attain local and systematic delivery, respectively. Another group of mice received an oestradiol-impregnated device around one oviduct and a blank device in the contralateral oviduct. Implantation of blank devices around the oviducts and in the forelegs did not alter ovum transport. Devices impregnated with oestradiol placed around both oviducts produced a dose-dependent delay of ovum transport, which was more pronounced than the effect of devices located in the forelegs. Oviducts receiving an oestradiol-loaded device had a larger retention of ova than did the contralateral oviducts receiving a blank device. These results demonstrate a direct action of oestradiol upon the oviduct to delay ovum transport in the mouse. 相似文献
124.
The protective effect of zinc against slight teratogenical action, exerted by low cadmium concentrations, was evaluated inBufo arenarum embryos treated simultaneously with both cations or preincubated with Zn before Cd treatment. Data on survival, malformations, and delay in development pointed out that Zn could prevent the-deleterous effects of Cd in previous and simultaneous treatments with that heavy metal. 相似文献
125.
126.
Concentration dependence of the subunit association of oligomers and viruses and the modification of the latter by urea binding. 总被引:2,自引:1,他引:1
A theoretical model is presented that accounts for the facilitation of the pressure dissociation of R17 phage, and for the partial restoration of the concentration dependence of the dissociation, by the presence of subdenaturing concentrations of urea. As an indifferent osmolyte urea should promote the stability of the protein aggregates under pressure, and the decrease in pressure stability with urea concentration demonstrates that such indirect solvent effects are not significant for this case, and that the progressive destabilization is the result of direct protein-urea interactions. By acting as a "homogenizer" of the properties of the phage particles, urea addition converts the pressure-induced deterministic dissociation of the phage into a limited stochastic equilibrium. The model establishes the origin of the uniform progression from the stochastic equilibrium of dimers, to the temperature-dependent and partially concentration-dependent association of tetramers, to the fully deterministic equilibrium observed in many multimers and in the virus capsids. 相似文献
127.
Chromosomal polymorphism regarding number of NOR sites in the cyprinid fish Chondrostoma lusitanicum was examined using C-banding, silver-staining (Ag), and fluorescent staining with chromomycin A3 (CMA3). The analysis of heterochromatic regions allowed a more precise identification of the centromeric regions and the proposal of a revised haploid chromosome formula (7M: 15S: 3A). We describe variability in the number of NOR regions per genome, number of active NOR sites per cell, and relative size of individual NORs. Individuals expressed two or four NOR-bearing chromosomes. Polymorphism was detected in all the populations studied and sex-related differences were not found. The observed chromosomal NOR phenotypes suggest the occurrence of structural rearrangements during the evolutionary process of this diploid leuciscine cyprinid. 相似文献
128.
Cristina de Oliveira Dias 《Hydrobiologia》1996,324(3):253-256
Seven monstrilloid species of the genera Thaumaleus and Monstrilla (Copepoda) are reported from the Brazilian coast. Occurrence, distribution and ecological information is given for: T. longispinosum, T. rigidum, T. quadridens, M. recticulata, M. grandis, M. dubia and M. rugosa. 相似文献
129.
By means of gel electrophoresis of tissue extracts we have studied protein polymorphism inEligmodontia typus. The analysis was performed on specimens from five population samples collected at different sites in Patagonia (Argentina). Mean heterozygosity (\-h) and proportion of polymorphic loci (P) were determined on the basis of 19 loci. Considering all individuals as one sample, \-h gave a value of 0.16 and P was 70%. Although these values are much higher than those reported for most rodent species, they are very similar to those obtained by us for four species of the genusCalomys and forGraomys griseoflavus. There is a striking genetic identity (IN=0.99) among populations from regions with different environmental conditions, indicating that the species possesses a common genic pool. Genetic distance with other species of the Phyllotini was estimated. DN was lower betweenE. typus andCalomys (mean DN=0.88) than betweenE. typus andGraomys griseoflavus (DN=1.01). The high morphological similarity between these last two species, especially regarding those characters related to desert life adaptation, could be assigned, at least in part, to convergent evolution. 相似文献
130.
Macrothrix mexicanus sp. nov. is described from central México, a transition zone between the nearctic and neotropics. All localities where it was found are over 1800 meters above sea level. It shows many resemblances with M. laticornis, M. camjatae and M. rosea but is characterized by a persistent dorsal tooth on the valve keel, a spinous papilla on the basipodite of the antenna, the second thoracic limb with a long conical sensillum between scraper 1 and the gnathobase, the endopod of trunk limb IV having two setae; the postabdomen with the dorsal margin bilobed, and the distal segment of the seta natatoria which is unusually long.Abbreviations used on figures EN
Endopodite
- EP
Epipodite
- EX
Exopodite
- IDL
Inner distal lobe
- ODL
Outer distal lobe
- GT
Gnatobase
- E1
Endite 1
- E2
Endite 2
- E3
Endite 3 相似文献