全文获取类型
收费全文 | 45423篇 |
免费 | 3283篇 |
国内免费 | 2246篇 |
专业分类
50952篇 |
出版年
2024年 | 108篇 |
2023年 | 848篇 |
2022年 | 1155篇 |
2021年 | 1572篇 |
2020年 | 1556篇 |
2019年 | 2052篇 |
2018年 | 1668篇 |
2017年 | 1174篇 |
2016年 | 1282篇 |
2015年 | 1641篇 |
2014年 | 2457篇 |
2013年 | 3219篇 |
2012年 | 1781篇 |
2011年 | 2235篇 |
2010年 | 1659篇 |
2009年 | 1949篇 |
2008年 | 1982篇 |
2007年 | 2103篇 |
2006年 | 1872篇 |
2005年 | 1766篇 |
2004年 | 1573篇 |
2003年 | 1377篇 |
2002年 | 1348篇 |
2001年 | 1143篇 |
2000年 | 965篇 |
1999年 | 877篇 |
1998年 | 823篇 |
1997年 | 772篇 |
1996年 | 722篇 |
1995年 | 681篇 |
1994年 | 636篇 |
1993年 | 581篇 |
1992年 | 571篇 |
1991年 | 529篇 |
1990年 | 407篇 |
1989年 | 407篇 |
1988年 | 378篇 |
1987年 | 298篇 |
1986年 | 257篇 |
1985年 | 317篇 |
1984年 | 408篇 |
1983年 | 226篇 |
1982年 | 304篇 |
1981年 | 291篇 |
1980年 | 229篇 |
1979年 | 207篇 |
1978年 | 149篇 |
1977年 | 104篇 |
1976年 | 111篇 |
1974年 | 47篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
111.
Michelle Lesimple Christian Dournon Charles Houillon 《Development genes and evolution》1990,198(7):420-429
Summary In urodele amphibians, the lack of a reliable germ cell marker restricts the experimental study of the germ lineage. In the present work, we conducted genetic and histological analyses in order to demonstrate that melanin from oocytes constitutes a germ cell marker available for intraspecific experiments in Ambystoma mexicanum. Then, using this marker, we implanted germ cells from undifferentiated gonads (stage 48) into the blastocoel of host embryos and investigated their fate and determined state. Our results show that, from this stage on, the donor cells do not differentiate into other cell types; therefore, they are restricted in developmental capacity and irreversibly determined as germ cells. On the other hand, exogenous germ cells were found in an isotopic position until the young tail-bud stage, and then were found in an ectopic position; these results suggest that, from the middle tail-bud stage on, an active process contributes to migration of primordial germ cells to the gonadal territory. 相似文献
112.
A total of 627 cattle representing seven breeds from south central Nebraska, USA were tested for 37 BoLA antigens which behave as products of 37 distinct alleles of the class I BoLA-A locus. Four antigens were absent from all breeds tested. The other antigens showed marked and statistically significant differences in breed distribution. There was no evidence for blank (null) alleles. The number of alleles in each breed ranged from 10 to 20. The Hereford and Simmental populations tested were less polymorphic than the Angus, Brown Swiss, Charolais, Gelbvieh and Limousin populations. 相似文献
113.
Sensitized guinea pigs produced specific IgG and IgE antibodies toward Cladosporium and Alternaria. In presence of fungal extracts, nasal mast cells degranulate. Ultrastructural modifications of the cells during degranulation have been established. The ciliary epithelium and the ciliary beating are not affected by fungal allergens. 相似文献
114.
Localized insertion of new S-layer during growth of Bacillus stearothermophilus strains 总被引:4,自引:0,他引:4
Bacillus stearothermophilus strains PV 72 and ATCC 12980 carry a crystalline surface layer (S-layer) with hexagonal (p6) and oblique (p2) symmetry, respectively. Sites of insertions of new subunits into the regular lattice during cell growth have been determined by the indirect fluorescent antibody technique and the protein A/colloidal gold technique.During S-layer growth on both bacillus strains the following common features were noted: 1. shedding of intact S-layer or turnover of individual subunits was not seen; 2. new S-layer was deposited in helically-arranged bands over the cylindrical surface of the cell at a pitch angle related to the orientation of the lattice vectors of the crystalline array; 3. little or no S-layer was inserted into pre-existing S-layer at the poles, and 4. septal regions and, subsequently, newly formed cell poles were covered with new S-layer protein. 相似文献
115.
G. Wiedemann R. Pabst T. Wagner F. Trepel 《Virchows Archiv. B, Cell pathology including molecular pathology》1988,56(1):407-412
Labelling index after one or repeated intravenous injections of 3H-thymidine was measured for various subpopulations of lymphatic
cells in different canine lymphoid compartments and correlated with cell morphology. High doses of tritiated thymidine were
injected and exposure times of up to 211 days were used. The labelling indices of lymphoid blasts were comparable in all tissues
investigated. Labelling index varied from 100% in immunoblasts to 4% in small-sized lymphocytes. Approximately 80% of immunoblasts
were labelled 1 h after 3H-thymidine application and 100% labelling was obtained after 12 h repetitive 3H-thymidine labelling.
In contrast with mediumsized and large lymphocytes, immunoblasts seem to be rapidly proliferating cells in the dog with almost
no Go cells.
Supported by the Deutsche Forschungsgemeinschaft DFG Grant SFB 112 相似文献
116.
我们以前的工作指出,生长抑素可以预防链佐霉素对大鼠胰岛B细胞的损伤。本工作测定给药前后大鼠血清和组织中Zn、Cu含量,以进一步观察链佐霉素破坏大鼠胰岛B细胞后生长抑素预防效应的可能机制。结果为:链佐霉素(STZ)(35mg/kg,ip)注射后24h的大鼠,血清Zn浓度下降,Cu浓度升高,Zn/Cu比值倒置;用生长抑素作预防性注射后,血清Zn浓度与正常对照水平相同,Cu浓度虽稍有升高,但Zn/Cu比值正常。保护组的血清Zn与损伤组比较,差异极为显著。两组的Zn/Cu比值同样也具有显著差异。在给药后72h,血清Zn/Cu比值在各组之间仍呈上述关系,但差异减小。在大鼠胰腺,注射链佐霉素后胰腺组织Zn浓度未见改变,但生长抑素作预防性注射可使组织锌含量增加。上述结果提示,在生长抑素对胰岛B细胞的保护机制中可能有Zn和Cu的参与。预防性注射生长抑素所引起的高Zn状态可能有利于机体细胞含Zn酶类的活性,增强已损伤细胞的修复。 相似文献
117.
Y Murayama T Ishida M Minezawa A Noguchi O Takenaka 《Journal of medical primatology》1988,17(1):31-40
A cell line (JAMH17+) resistant to 8-azaguanine was established from a human T-cell leukemia virus type 1 related virus (simian T-cell leukemia virus-1) positive Japanese macaque cell line. Lymphoblastic cell lines were established from the peripheral blood mononuclear cells of humans, hominoids, and several species of macaques by coculture with JAMH17+ in hypoxanthine-aminopterin-thymidine medium. HTLV-1 specific antigen was detected in some of the established cell lines. Phenotypic analysis showed that several cell lines of crab-eating macaques expressed Leu11a antigen, which is a marker of human natural killer cells. 相似文献
118.
Robert V. Masterson K. Biagi J. G. Wheeler J. Stadler D. W. Morris 《Plant molecular biology》1988,10(3):273-279
The maize inbred line A188 is popularly used for the production of embryogenic cell lines. A188, maintained at the University of Minnesota, was found upon molecular analysis to contain 2 to 4 copies of a DNA sequence very similar in structure to transposable Mu1 elements, which have been implicated in Robertson's Mutator system. These Mu1-like elements are in the same chromosomal locations in sibling plants and in A188 cell cultures derived from them. This suggests that the elements are in an inactive state and do not undergo transposition. However, we have observed that they are not modified at the target sites for certain restriction endonucleases. Possible causes for the apparent lack of transposition of these Mu1-like elements in these A188 lines are discussed. Inasmuch as the elements do not transpose, they must be maintained in this line as homozygous Mendelian elements by self-pollination.Journal paper no. J-12269 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa 50011. Project 2707. 相似文献
119.
In vitro enzyme activities of glycolysis, pentose-phosphate pathway and dark CO2 fixation were assayed in batch cultures of heterotrophic Catharanthus roseus cells under various gassing rates and partial pressures of carbon dioxide. Detrimental effects of low pCO2 culture conditions on the growth characteristics could be linked to marked changes in levels of enzymes of primary metabolism during growth. The enzyme levels observed during the early stages of growth were found to be more stable when a constant pCO2 (20 mbar) was maintained and enabled exponential growth to be reached more rapidly.The importance of carbon dioxide as a conditioning factor of the culture medium is discussed. 相似文献
120.
Peter Alliger Wolfgang Traut Eric Carstens Ellen Fanning 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1988,951(2-3)
A monkey cell factor that interacts specifically with double- and single-stranded DNA sequences in the early domain of the simian virus 40 (SV40) core origin of replication was identified using gel-retention assays. The protein was enriched over 1200-fold using ion-exchange and affinity chromatography on single-strand DNA cellulose. Binding of protein to mutant origin DNA restriction fragments was correlated with replication activity of the mutant DNAs. Exonuclease footprint experiments on single-stranded DNA revealed prominent pause sites in the early domain of the core origin. The results suggest that this cellular protein may be involved in SV40 DNA replication. 相似文献