全文获取类型
收费全文 | 30348篇 |
免费 | 96篇 |
国内免费 | 119篇 |
专业分类
系统科学 | 139篇 |
丛书文集 | 224篇 |
教育与普及 | 70篇 |
理论与方法论 | 94篇 |
现状及发展 | 12258篇 |
研究方法 | 1287篇 |
综合类 | 15999篇 |
自然研究 | 492篇 |
出版年
2013年 | 273篇 |
2012年 | 427篇 |
2011年 | 915篇 |
2010年 | 190篇 |
2008年 | 521篇 |
2007年 | 635篇 |
2006年 | 621篇 |
2005年 | 603篇 |
2004年 | 559篇 |
2003年 | 566篇 |
2002年 | 524篇 |
2001年 | 1150篇 |
2000年 | 1114篇 |
1999年 | 647篇 |
1994年 | 343篇 |
1992年 | 622篇 |
1991年 | 473篇 |
1990年 | 542篇 |
1989年 | 521篇 |
1988年 | 472篇 |
1987年 | 490篇 |
1986年 | 529篇 |
1985年 | 614篇 |
1984年 | 484篇 |
1983年 | 447篇 |
1982年 | 404篇 |
1981年 | 385篇 |
1980年 | 396篇 |
1979年 | 1003篇 |
1978年 | 756篇 |
1977年 | 697篇 |
1976年 | 598篇 |
1975年 | 638篇 |
1974年 | 838篇 |
1973年 | 694篇 |
1972年 | 677篇 |
1971年 | 838篇 |
1970年 | 1096篇 |
1969年 | 782篇 |
1968年 | 833篇 |
1967年 | 736篇 |
1966年 | 716篇 |
1965年 | 513篇 |
1959年 | 265篇 |
1958年 | 447篇 |
1957年 | 302篇 |
1956年 | 254篇 |
1955年 | 241篇 |
1954年 | 236篇 |
1948年 | 204篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
There are many examples of patterned developing systems which are size invariant: if the total size of the system is reduced, then the pattern responds by changing its scale in such a way that the number of pattern elements remains constant. This phenomenon is one of the bases which underaly the formulation of the concept of positional information, one of the great unifying ideas in developmental biology. However, there are less common examples of patterns which are size dependent. In these, alterations of overall size lead to a reduction in pattern elements. Such size-dependent patterns are therefore of theoretical interest. Here we describe how the number of feather germs along the wing bud of the developing chick embryo responds to shortening of the limb, and consider the implications of these observations. 相似文献
942.
Flavins generate oxidants during metabolism and when exposed to light. Here we report that the photoreceptor layer of retinas from black-eyed rats is reduced in size by a dietary regime containing excess riboflavin. The effect of excess riboflavin was dose-dependent and was manifested by a decrease in photoreceptor length. This decrease was due in part to a reduction in the thickness of the outer nuclear layer, a structure formed from stacked photoreceptor nuclei. These changes were accompanied by an increase in photoreceptor outer segment autofluorescence following illumination at 328 nm, a wavelength that corresponds to the excitation maxima of oxidized lipopigments of the retinal pigment epithelium. 相似文献
943.
R. J. Capon K. Elsbury M. S. Butler C. C. Lu J. N. A. Hooper J. A. P. Rostas K. J. O'Brien L. -M. Mudge A. T. R. Sim 《Cellular and molecular life sciences : CMLS》1993,49(3):263-264
The Antarctic marine spongeTedania charcoti has been shown to contain extraordinarily high natural concentrations of cadmium and zinc, which have in turn been correlated to the ability of the crude ethanol extract to modulate protein phosphorylation in chicken forebrain and to inhibit the growth of several test bacteria. 相似文献
944.
945.
The temporal requirement for endothelin receptor-B signalling during neural crest development 总被引:12,自引:0,他引:12
Endothelin receptor B (EDNRB) is a G-protein-coupled receptor with seven transmembrane domains which is required for the development of melanocytes and enteric neurons. Mice that are homozygous for a null mutation in the Ednrb gene are almost completely white and die as juveniles from megacolon. To determine when EDNRB signalling is required during embryogenesis, we have exploited the tetracycline-inducible system to generate strains of mice in which the endogenous Ednrb locus is under the control of the tetracycline-dependant transactivators tTa or rtTA. By using this system to express Ednrb at different stages of embryogenesis, we have determined that EDNRB is required during a restricted period of neural crest development between embryonic days 10 and 12.5. Moreover, our results imply that EDNRB is required for the migration of both melanoblasts and enteric neuroblasts. 相似文献
946.
Distribution of spatial and nonspatial information in dorsal hippocampus 总被引:10,自引:0,他引:10
The hippocampus in the mammalian brain is required for the encoding of current and the retention of past experience. Previous studies have shown that the hippocampus contains neurons that encode information required to perform spatial and nonspatial short-term memory tasks. A more detailed understanding of the functional anatomy of the hippocampus would provide important insight into how such encoding occurs. Here we show that hippocampal neurons in the rat are distributed anatomically in distinct segments along the length of the hippocampus. Each longitudinal segment contains clusters of neurons that become active when the animal performs a task with spatial attributes. Within these same segments are ordered arrangements of neurons that encode the nonspatial aspects of the task appropriate to those spatial features. Thus, anatomical segregation of spatial information, together with the interleaved representation of nonspatial information, represents a structural framework that may help to resolve conflicting views of hippocampal function. 相似文献
947.
It is usually assumed that, after construction of basic network architecture in embryos, immature networks undergo progressive maturation to acquire their adult properties. We examine this assumption in the context of the lobster stomatogastric nervous system. In the lobster, the neuronal population that will form this system is at first orgnanized into a single embryonic network that generates a single rhythmic pattern. The system then splits into different functional adult networks controlled by central descending systems; these adult networks produce multiple motor programmes, distinctively different from the single output of the embryonic network. We show here that the single embryonic network can produce multiple adult-like programmes. This occurs after the embryonic network is silenced by removal of central inputs, then pharmacologically stimulated to restore rhythmicity. Furthermore, restoration of the flow of descending information reversed the adult-like pattern to an embryonic pattern. This indicates that the embryonic network possesses the ability to express adult-like network characteristics, but descending information prevents it from doing so. Functional adult networks may therefore not necessarily be derived from progressive ontogenetic changes in networks themselves, but may result from maturation of descending systems that unmask preexisting adult networks in an embryonic system. 相似文献
948.
949.
Identification of in vivo substrates of the chaperonin GroEL 总被引:22,自引:0,他引:22
950.