首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   13813篇
  免费   957篇
  国内免费   3篇
  2023年   135篇
  2022年   221篇
  2021年   368篇
  2020年   284篇
  2019年   360篇
  2018年   416篇
  2017年   406篇
  2016年   524篇
  2015年   776篇
  2014年   745篇
  2013年   1013篇
  2012年   1118篇
  2011年   1165篇
  2010年   736篇
  2009年   593篇
  2008年   794篇
  2007年   758篇
  2006年   647篇
  2005年   603篇
  2004年   563篇
  2003年   476篇
  2002年   446篇
  2001年   134篇
  2000年   106篇
  1999年   96篇
  1998年   125篇
  1997年   99篇
  1996年   76篇
  1995年   73篇
  1994年   59篇
  1993年   62篇
  1992年   64篇
  1991年   48篇
  1990年   51篇
  1989年   35篇
  1988年   43篇
  1987年   41篇
  1986年   33篇
  1985年   48篇
  1984年   39篇
  1983年   34篇
  1982年   33篇
  1981年   34篇
  1980年   35篇
  1979年   23篇
  1977年   23篇
  1976年   21篇
  1975年   23篇
  1974年   19篇
  1968年   22篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Summary Only Stephanopharynx compactus Fischoeder, 1901 is considered valid under the genus Stephanopharynx Fischoeder, 1901 and S. secundus Stunkard, 1929 and S. coilos Dollfus, 1963 are regarded as its synonyms. The species is redescribed and illustrated based on new material and scanning electron photomicrographs of its tegumental surfaces and the internal surface of its pharyngeal pouch are provided. Balanorchis anastrophus Fischoeder, 1901, the one and only species of the genus Balanorchis Fischoeder, 1901 is redescribed and illustrated. Photomicrographs of its tegumental surfaces as revealed by scanning electron microscopy are given. Additional information on Bilatorchis papillogenitalis Eduardo, 1980 is included. Part of a thesis approved by the University of London for the award of the Ph.D. degree. Part of a thesis approved by the University of London for the award of the Ph.D. degree.  相似文献   
52.
53.
Abstract: This study was designed to analyze possible differences in the binding of [3H]flunitrazepam ([3H]FNZP) and [3H]ethyl - β - carboline - 3 - carboxylate ([3H]β-CCE), to rat brain membranes, in various experimental conditions. In cerebral cortex, hippocampus, cerebellum, and orain stem the number of binding sites for [3H]β-CCE was higher than for [3H]FNZP; both were displaced by clonazepam. Until the 7th day of postnatal brain development the numbers of [3H]FNZP and [3H]β-CCE sites were equivalent; but later on, the β-carboline sites increased to a higher level. Noradrenergic denervation by 6-hydroxydopamine was followed in the hippocampal formation. Already after 2 days, there was a decrease in [3H]FNZP sites, which reached 70% of control after 14 days. Similar results were obtained with DSP-4 denervation. This change was only in Bmax and not in KD, In contrast, the [3H]β-CCE sites did not change with denervation. Neonatal injection of l - 2,4,5 - trihydroxyphenylalamine or DSP-4 produced in the adult a decrease in [3H]FNZP sites in the cerebral cortex, in parallel with the noradrenergic denervation. On the other hand, there was an increase in the cerebellum and brain stem, in correspondence with the hyperinnervation by sprouting. In these rats, the number of sites for [3H]β-CCE did not change in the different brain regions. With 0.1% Triton X-100, applied to synaptosomal membranes, [3H]FNZP binding was reduced by 35%, while that of [3H]β-CCE was not significantly changed. These results suggest that there is heterogeneity of binding sites for benzodiazepine receptors in rat brain. A tentative interpretation of the experiments involving noradrenergic denervation and hyperinnervation, as well as those with Triton X-100, is that [3H]FNZP binds to pre- and postsynaptic receptors, while [3H]β-CCE binds mainly to postsynaptic benzodiazepine receptors.  相似文献   
54.
An ELISA for the identification of mosquito bloodmeals (MBM) is described. The technique makes use of commercial antisera and allows to investigate at least three different animal sources on the same MBM. The efficiency of identification is 100% for up to 20 hrs-digested MBM.  相似文献   
55.
Summary The genus Calicophoron N?smark, 1937 is redefined. Only the following species originally assigned by N?smark to the genus are considered valid: C. calicophorum (Fischoeder, 1901) N?smark, 1937 (type species); C. papillosum (Stiles & Goldberger, 1910) N?smark, 1937; and C. raja N?smark, 1937. The following species previously assigned to the genus Paramphistomum Fischoeder, 1901 are considered valid but moved to the genus Calicophoron N?smark, 1937 in new combinations: C. bothriophoron (Braun, 1892); C. microbothrium (Fischoeder, 1901); C. papilligerum (Stiles & Goldberger, 1910); C. clavula (N?smark, 1937); C. microbothrioides (Price & McIntosh, 1944); C. sukari (Dinnik, 1954); C. phillerouxi (Dinnik, 1961); C. daubneyi (Dinnik, 1962) and C. sukumum (Dinnik, 1964). The genus Bothriophoron (Stiles & Goldberger, 1910) Grétillat 1958 is synoymized with Calicophoron N?smark, 1937. Paramphistomum crassum Stiles & Goldberger, 1910, P. cauliorchis Stiles & Goldberger, 1910, P. ijimai Fukui, 1922, P. (Cauliorchis) skrjabini Popova, 1937, P. erschovi Davydova, 1959, Calicophoron orientalis Mukherjee, 1966, C. wuchengense Wang, 1979 and Cotylophoron skrjabini Mitskevich, 1958 in part are considered synonyms of Calicophoron calicophorum (Fischoeder, 1901) N?smark, 1937. C. zhejiangense Wang, 1979 is regarded a synonym of C. papillosum (Stiles & Goldberger, 1910) N?smark, 1937. P. vangrembergeni Van Strydonck, 1970 and P. togolense Albaret, Bayssade-Dufour, Guilhon, Kulo & Picot, 1978 are considered synonyms of Calicophoron phillerouxi (Dinnik, 1961) n. comb. and Ceylonocotyle petrovi Davydova, 1961 is synonymized with Calicophoron microbothrioides (Price & McIntosh, 1944) n. comb. The species considered valid under the genus are redescribed and illustrated and scanning electron photomicrographs of the tegumental surfaces of some species are provided. A key to the species is given. Part of a thesis approved by the University of London for the award of the Ph.D. degree.  相似文献   
56.
Bacteriophage PBS2 replication is unaffected by rifampicin and other rifamycin derivatives, which are potent inhibitors of Bacillus subtilis RNA synthesis. Extracts of gently-lysed infected cells contain a DNA-dependent RNA polymerase activity which is specific for uracil-containing PBS2 DNA. The PBS2-induced RNA polymerase is insensitive to rifamycin derivatives which inhibit the host's RNA polymerase.  相似文献   
57.
58.
This paper refers to a case of polymorphism in the desmid genus Xanthidium Ehr. It is based on material from Lake Dais Irmaios, the main body of water in the Zoological and Botanical Garden in Recife, Pernambuco, northeastern Brazil, collected at 4 different times of the year during 1967 and 1968. A detailed examination of almost 1300 specimens showed an enormous variety in form of Xanthidium regulare Nordst., X. fragile Borge, and X. pseudoregulare Borge, thus allowing the authors to draw the following conclusions: (1) the name X. regulare Nordst. should be retained until further and more detailed studies on form variation within the species are available; (2) the names X. regulare Nordst. var. asteptum Nordst. in Borge, X. regulare Nordst. var. sexangulare Grönbl., X. regulare Nordst. var. sexangulare Grönbl. f. robustior Grönbl., X. fragile Borge, X. fragile Borge forma, and X. fragile Borge var. depauperatum Borge should be considered synonymous, all referring to a single variety of X. regulare Nordst., var. asteptum Nordst. in Borge emend. C. Bic. & L. M. Carv.; (3) X. pseudoregulare Borge must be treated as a variety of X. regulare Nordst. and must be called X. regulare Nordst. var. pseudoregulare (Barge) C. Bic. & L. M. Carv. Finally, a key is given to the 3 varieties of X. regulare Nordst. proposed in the present paper.  相似文献   
59.
Summary In the toad Bufo arenarum Hensel the following regions of the hypothalamic — neurohypophyseal system were studied under the electronmicroscope: preoptic and paraventricular nuclei, median eminence and infundibular process of the neurohypophysis.Neuronal perikarya of the preoptic nucleus are loaded with typical neurosecretory granules of peptidergic nature having a mean diameter of 1660 Å. While most neurons of the winter toad are in a storage stage a few show signs of a more active synthetic activity. A distinctive feature of preoptic neurons is the presence of large lipid droplets. The paraventricular nucleus contains small neurons containing granulated vesicles with a mean diameter of 800-1000 Å. In the region extending between these two nuclei and the median eminence axons containing either neurosecretory elementary granules or granulated vesicles are observed.The inner zone of the median eminence is occupied by axons of the preoptic neurohypophyseal tract; two types of axons, according to the size and density of the neurosecretory granules, may be recognized. The outer zone of the median eminence contains mainly axons and nerve terminals containing granulated vesicles of probable monoaminergic nature and only a few with granules of peptidergic type.The neurohypophysis contains two kinds of axons: one with more dense granules of 1800 Å and the other with granules of lesser electron density and 2100 Å. At the ending proper small clear vesicles of synaptic type are found.A progressive increase in volume of the peptidergic granules along the axon is demonstrated. This is of the order of 218% from the preoptic perikarya down to the infundibular process. The physiological significance of the two neurosecretory systems — i.e. the monoaminergic and the peptidergic — and the probable nature of the two types of peptidergic axons is discussed.Supported by grants from the Consejo Nacional de Investigaciones Científicas y Técnicas and by the Air Force Office of Scientific Research (AF-AFOSR 963-67).The authors want to express their gratitude to Mrs. Defilippi-Novoa and Mr. Alberto Sáenz for their skillful assistance.  相似文献   
60.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号