全文获取类型
收费全文 | 20603篇 |
免费 | 1555篇 |
出版年
2023年 | 144篇 |
2022年 | 252篇 |
2021年 | 541篇 |
2020年 | 328篇 |
2019年 | 430篇 |
2018年 | 536篇 |
2017年 | 493篇 |
2016年 | 724篇 |
2015年 | 1143篇 |
2014年 | 1169篇 |
2013年 | 1497篇 |
2012年 | 1736篇 |
2011年 | 1612篇 |
2010年 | 1030篇 |
2009年 | 1006篇 |
2008年 | 1210篇 |
2007年 | 1114篇 |
2006年 | 1064篇 |
2005年 | 982篇 |
2004年 | 922篇 |
2003年 | 815篇 |
2002年 | 786篇 |
2001年 | 247篇 |
2000年 | 172篇 |
1999年 | 215篇 |
1998年 | 228篇 |
1997年 | 157篇 |
1996年 | 136篇 |
1995年 | 116篇 |
1994年 | 114篇 |
1993年 | 109篇 |
1992年 | 94篇 |
1991年 | 74篇 |
1990年 | 81篇 |
1989年 | 62篇 |
1988年 | 60篇 |
1987年 | 64篇 |
1986年 | 39篇 |
1985年 | 61篇 |
1984年 | 68篇 |
1983年 | 57篇 |
1982年 | 53篇 |
1981年 | 58篇 |
1980年 | 34篇 |
1979年 | 30篇 |
1978年 | 24篇 |
1977年 | 34篇 |
1975年 | 29篇 |
1974年 | 24篇 |
1973年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
Verónica Téllez Ariel Ahumada Juan Muro Soledad Sepúlveda Luis Izquierdo 《Development genes and evolution》1988,197(6):360-365
Summary Two-cell mouse ova, which were centrifuged for l h at 70 000–90 000xg, showed a precise stratification of the cytoplasm and an elongation of the nucleus. The ova were fixed at different times and observed by light and electron microscopy using cytochemical methods and detergent extractions. Within 40 min after centrifugation the normal-looking morphology was recovered except for the persisting lipid caps at the centripetal poles of the blastomeres. Cleavage, compaction and blastulation were not prevented by centrifugation. Treatments with colcemid or cytochalasin D delayed but did not impair recovery. These results suggest that a resilient cytoskeletal structure may be involved in this kind of embryonic regulation. 相似文献
62.
Summary Phenotypic analyses of genetic combinations involving the gene extramacrochaetae (emc) reveal its participation in the differentiation of both sensory elements and wing veins. The study of near-amorphic alleles of emc in mitotitc recombination clones indicates that it also affects cell proliferation. These clones show abnormal sizes, shapes and spatial distribution. They differentiate extra sensory elements as well as extra veins. A gain of function mutation in the gene causes opposite phenotypes in both differentiation systems. The effects of the mutant on proliferation and patterning are consistent with the emc gene being involved in the transfer of information between neighbouring cells, which leads to the spatial expression of the achaetescute gene complex and genes involved in vein formation. 相似文献
63.
J. L. Luis Hernandez 《BioControl》1988,33(2):163-171
Résumé Cinquante deux souches deB. thuringiensis appartenant à 13 sérovars ont été testées sur des chenilles néonates deSpodoptera frugiperda en contaminant la surface du milieu semi-synthétique d'élevage. Deux souches du sérovarkenyae et une autre du sérovartolworthi provoquent le plus de mortalité, suivies par les souches des sérovarsaizawai etkurstaki. Les souches les moins actives appartiennent aux sérotypesalesti, dendrolimus, sotto etcolmeri. L'action des souches sur le développement larvaire a aussi été abordée. Les souches des sérovarskenyae, aizawai etkurstaki ont ralenti le développement des chenilles, tandis que les souches des sérovarsalesti, sotto etcolmeri n'ont eu aucun effet.
相似文献
64.
C López-García A Molowny F J Martínez-Guijarro J M Blasco-Ibá?ez J A Luis de la Iglesia A Bernabeu J M García-Verdugo 《Histology and histopathology》1992,7(4):725-746
The cerebral cortex of Squamate reptiles (lizards and snakes) may be regarded as an archicortex or "reptilian hippocampus". In lizards, one cortical area, the medial cortex, may be considered as a true "fascia dentata" on grounds of its anatomy, connectivity and cyto- chemo-architectonics of its main zinc-rich axonal projection. Moreover, its late ontogenesis and postnatal development support this view. In normal conditions, it shows delayed postnatal neurogenesis and growth during the lizard's life span. Remnant neuroblasts in the medial cortical ependyma of adult lizards seasonally proliferate. The late-produced immature neurocytes migrate to the medial cortex cell layer where they differentiate and give off zinc-containing axons directed to the rest of cortical areas. This results in a continuous growth of the medial cortex and its zinc-rich axonal projection. Perhaps the most important characteristic of the lizard medial cortex is that it can regenerate after having been almost completely destroyed. Recent experiments in our laboratory have shown that chemical lesion of its neurons (up to 95%) results in a cascade of events; first, those related with massive neuronal death and axonal-dendritic retraction and, secondly, those related with a triggered neuroblast proliferation and subsequent neo-histogenesis, and the regeneration of an almost new medial cortex that shows itself undistinguishable from a normal undamaged one. This is the only report to our knowledge that an amniote central nervous centre may regenerate by new neuron production and neo-histogenesis. Perhaps the medial cortex of lizards may be used as a model for neuronal regeneration and/or transplant experiments in mammals or even in primates. 相似文献
65.
Structure-function analysis of hepatocyte growth factor: identification of variants that lack mitogenic activity yet retain high affinity receptor binding. 总被引:11,自引:0,他引:11 下载免费PDF全文
N A Lokker M R Mark E A Luis G L Bennett K A Robbins J B Baker P J Godowski 《The EMBO journal》1992,11(7):2503-2510
Hepatocyte growth factor (HGF) is a potent mitogen for parenchymal liver, epithelial and endothelial cells. Structurally, it has similarities to kringle-containing serine proteases, although it does not possess proteolytic activity. A structure-activity relationship study of human HGF was performed by functional analysis of HGF substitution and deletion variants. Analysis of HGF variants was accomplished by defining their ability to induce DNA synthesis on hepatocytes in primary culture and to compete with wild-type HGF for binding to a soluble form of the HGF receptor. Three groups of variants were made: (i) substitutions at the cleavage site, (ii) substitutions within the protease-like domain and (iii) deletions of the beta-chain and/or kringle domains. Our results show that: (i) single-chain HGF is a zymogen-like promitogen in that cleavage into a two-chain form is required for biological activity, however, the single chain form of HGF still retains substantial receptor binding capacity; (ii) certain mutations in the protease-like domain result in variants that are completely defective for mitogenic activity, yet exhibit apparent receptor binding affinities similar to wild-type HGF (Kd approximately 50-70 pM); and (iii) a variant containing the N-terminal 272 residues of mature HGF showed only a 4-fold increase in Kd when compared with wild-type HGF indicating that a primary receptor binding determinant is located within this sequence. 相似文献
66.
Eloi Garí Manuel Ollé Isidre Gibert Montserrat Llagostera Dr. Jordi Barbé 《Current microbiology》1992,24(3):151-157
A new virulent bacteriophage, designated RZ1, was isolated from a local pond on the facultative phototrophic bacteriumRhodobacter sphaeroides ZZ101. Electron microscopic studies revealed that, in general morphology, phage RZ1 resembles the bacteriophage ofEscherichia coli. The host range of phage RZ1 is limited to some strains ofR. sphaeroides. The phage genome consists of double-stranded DNA of about 44 kb lacking cohesive ends and seems to present terminal redundancy and cyclic permutation. RZ1 phage may carry out a lytic cycle only in recombination-defective mutants ofR. sphaeroides. Nevertheless, a derivative of the RZ1 phage, termed RW1, able to grow in recombination-proficient strains ofR. sphaeroides, has also been obtained. In vitro restriction analysis of both RZ1 and RW1 phages shows the presence of a rearrangement in their DNA. Generalized transduction of Strr and Rifr chromosomal markers has not been detected with either RZ1 or RW1 phages. 相似文献
67.
Nacyra Assad-García Neftali Ochoa-Alejo Enrique García-Hernández Luis Herrera-Estrella June Simpson 《Plant cell reports》1992,11(11):558-562
A protocol for the Agrobacterium-mediated transformation of tomatillo was developed. Up to 40 transgenic plants could be obtained in experiments using 60 cotyledon expiants. The transformed nature of the regenerated plants was confirmed by NPT II and Southern blot hybridization analysis. Using the b-glucuronidase system the tissue specific and developmental patterns of expression of the Cauliflower Mosaic Virus 35S promoter were determined in transgenic tomatillo plants. It was found that this promoter is developmentally regulated during fruit and seed formation. 相似文献
68.
José L. García-Martínez Manuel Martí Teresa Sabater Amparo Maldonado Yolanda Vercher 《Physiologia plantarum》1991,83(3):411-416
The histological development of fertilized ovules during fruit-set and development in pea ( Pisum sativum L. cv. Alaska) has been investigated. Killing the ovules on day 0 (anthesis) or day 1 prevented fruit-set and resulted in ovary degeneration. When the ovules were destroyed at later stages the ovaries developed, though the rate of growth of the pod was reduced significantly. Pollination in pea occurs normally the day before anthesis, and fertilization of the egg cell 32 to 48 h later. The first divisions of the zygote and endosperm nuclei started simultaneously (ca 48 h after pollination) but the endosperm developed more rapidly than the embryo; the embryo sac cavity was lined with free endosperm nuclei at the time of beginning suspensor elongation. Extracts of endosperm and ovule coats from ovules at day 7 after anthesis showed fruit-set activity in pea, the latter material having about 3 times more activity than the former per ovule basis. These results indicate that fertilization of the ovule is necessary for fruit-set in pea, and that compounds which induce fruit-set are probably synthesized in the ovules following fertilization. 相似文献
69.
70.
Gerardo Argüello Enrique García-Hernández Mireya Sánchez Patricio Gariglio Luis Herrera-Estrella June Simpson 《The Plant journal : for cell and molecular biology》1992,2(3):301-309
Two protein factors binding to the regulatory region of the pea chlorophyl a/b binding protein gene AB80 have been identified. One of these factors is found only in green tissue but not in etiolated or root tissue. The second factor (denominated ABF-2) binds to a DNA sequence element that contains a direct heptamer repeat TCTCAAA. It was found that presence of both of the repeats is essential for binding. ABF-2 is present in both green and etiolated tissue and in roots and factors analogous to ABF-2 are present in several plant species. Computer analysis showed that the TCTCAAA motif is present in the regulatory region of several plant genes. 相似文献