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   2567篇
  免费   223篇
  国内免费   1篇
  2023年   15篇
  2022年   44篇
  2021年   92篇
  2020年   47篇
  2019年   57篇
  2018年   65篇
  2017年   65篇
  2016年   139篇
  2015年   221篇
  2014年   207篇
  2013年   204篇
  2012年   314篇
  2011年   198篇
  2010年   126篇
  2009年   106篇
  2008年   122篇
  2007年   118篇
  2006年   108篇
  2005年   88篇
  2004年   67篇
  2003年   70篇
  2002年   51篇
  2001年   20篇
  2000年   11篇
  1999年   21篇
  1998年   17篇
  1997年   12篇
  1996年   12篇
  1995年   13篇
  1994年   6篇
  1993年   10篇
  1992年   15篇
  1991年   6篇
  1990年   4篇
  1989年   7篇
  1987年   5篇
  1986年   10篇
  1985年   7篇
  1984年   3篇
  1983年   11篇
  1982年   5篇
  1981年   6篇
  1979年   4篇
  1977年   4篇
  1976年   7篇
  1975年   9篇
  1973年   4篇
  1969年   5篇
  1968年   3篇
  1960年   3篇
排序方式: 共有2791条查询结果,搜索用时 15 毫秒
21.
22.
P. Licht    H. E. Hoyer    P. G. W. J. van  Oordt 《Journal of Zoology》1969,157(4):469-501
Annual testicular cycles in the lizards Lacerta sicula and L. muralis appear to be regulated by the interaction between seasonal changes in body temperature and an endogenous rhythmicity in thermal responsiveness. Photoperiodism does not appear to be an important factor; i.e. testicular activity does not appear to be regulated by daylength.
Following testicular regression in July, the lizards are refractory to sexual stimulation by high temperatures (i.e. normal preferred levels) for about five months. High temperatures accelerate gonadal regression and prevent recrudescence during late summer. Reduced temperatures stimulate testicular enlargement and spermatid formation during the autumn; this recrudescence can be blocked by treatment with testosterone. Very low temperatures suppress gonadal activity during mid-winter.
Maintenance of lizards at constant high temperatures (33°C) starting in July suppresses testicular recrudescence until December. Also, testicular collapse occurs in lizards transferred to high temperatures after recrudescence has started. The gonads are stimulated by exogenous gonadotropins at 33°C during the fall indicating that high temperatures reduce circulating levels of gonadotropins.
The refractoriness to high temperatures is "spontaneously" terminated during midwinter (December) under a wide range of photo-thermal conditions. Low temperatures may accelerate the termination of refractoriness. Thereafter, high temperatures stimulate, and are required for the final development of the testes and accessory sexual structures. Thus, the increase in body temperature following hibernation times the onset of breeding in the spring.
Temperature also has a marked influence on appetite and growth, independent of photo-period. Weight gains are greater at 33° than at 20°C. At 33° there is a tendency for abdominal fat bodies to enlarge but with little hepatic growth; whereas, the reverse occurs at 20°C.  相似文献   
23.
Of 100 children''s emergency admissions to hospital nearly half (49%) were admitted during the mother''s paramenstruum. There was a statistically significant association between the mother''s menstruation and the child''s admission both for accidents and for illnesses. The eldest child in the family appeared to be most affected.  相似文献   
24.
Zusammenfassung Die Kinetik der Ionenaufnahme durch junge und alte Sprosse von Mnium cuspidatum wurde untersucht. Die verschieden alten Sprosse unterscheiden sich vor allem durch das Vorhandensein einer aktiven Gipfelknospe bei den jungen Gametophyten, die bei den alten offenbar ihre Tätigkeit eingestellt hat. Im niedrigen Konzentrationsbereich (0–0,5 mM) haben jungen und alte Sprosse hyperbolische Isothermen der Ionenaufnahme, die sich etwas hinsichtlich der apparenten Michaeliskonstanten und der Maximalgeschwindigkeit unterscheiden. Im hohen Konzentrationsbereich (1–10 mM) ist der Unterschied qualitativ. Mit jungen Sprossen erhält man eine lineare oder exponentielle Isotherme, mit alten Sprossen eine hyperbolische Kurve. Der vermutete Einfluß der Gipfelknospe kann mit Effekten von Wuchsstoffen auf den Stofftransport zusammenhängen und läßt einen Einfluß dieser Regulationssysteme auf die Membranfunktion vermuten.
The kinetics of ion uptake by young and old branches of mnium cuspidatum
Summary Isotherms of K(Rb)-, Cl- and SO4-uptake by young and old branches of the moss Mnium cuspidatum were investigated. Old moss gametophytes from the 1966 vegetation period were collected in the forests surrounding Darmstadt from February to mid-April 1967 and from the 1967 season in late September 1967. Young plants were sampled from mid-April to the end of May 1967 and they were also grown by water culture of old plants.Both young and old branches have hyperbolic isotherms of ion uptake in the low concentration range (0–0.5 mM) (Fig. 1–3), which slightly differ in K mand V max (Table). Isotherms in the high range (1–10 mM), however, are drastically different, changing from linear or exponential with young moss branches to hyperbolic with old gametophytes (Figs. 1–3).The linear or exponential high-range isotherm obtained with young moss plants is compared with other examples reported in the literature (Fig. 4). As the leaflets of the moss plants, which constitute 2/3 of the fresh weight of the material used in the experiments, have well developed vacuoles, the correlation between hyperbolic isotherms and vacuolation does not apply here (Fig. 4a, Torii and Laties, 1966).The change in shape of the high-range moss isotherm with age resembles the change from exponential to hyperbolic kinetics in isolated potato discs during washing (Fig. 4b, Laties, Macdonald and Dainty, 1964). The events triggered by isolation of potato discs from the interior of the tuber may be similar to the changes in the moss material under the control of the terminal bud, which is only active in the young branches.The suggested influence of the active terminal bud of young moss plants on the ion absorption process of cells in the tissue may be related to effects of growth substances on translocation reported in the literature and may point to a direct effect of these regulatory systems on membrane function.In this respect the comparison of corn root stele and cortex is of interest. Isolated steles, both freshly isolated and after washing, have exponential isotherms in the high range (Fig. 4c), whereas cortex displays a hyperbolic isotherm which changes little with ageing (Lüttge and Laties, 1967). In contrast to the case in potato and moss materials, this phenomenon is not simply due to ageing but involves morphogenetic differences.Temperature is another factor which influences the shape of the high range isotherm. All examples discussed so far refer to experiments at room temperature. At low temperatures high-range isotherms for proximal root tissue or aged potato discs have an exponential shape (Torii and Laties, 1966; Laties, Macdonald and Dainty, 1964). It thus appears that the exponential isotherm of young moss branches indicates that as in freshly isolated potato discs or in corn root stele the metabolic high-range uptake system is not developed.
  相似文献   
25.
Eighteen filamentous fungi and six actinomycetes species were screened for their ability to metabolize bisoprolol, a \-blocking drug. All strains of Cunninghamella tested accumulated metabolite M4 = EMD 46193 ([4-(2-hydroxy-3-isopropylaminopropoxy)benzyloxy]ethanol). Among the strains investigated only Gliocladium deliquescens excreted the corresponding carbonic acid M1 = EMD 44025 into the culture medium. Biotransformation of bisoprolol by fungi occurred only during growth in complex medium or with resting cells after cultivation in complex medium. The screened Actinomycetes showed no biotransformation of the drug. Correspondence to: H. Schwartz  相似文献   
26.

Motivation

Home range is a common measure of use of space by animals because it provides ecological information that is useful for conservation applications. In macroecological studies, values are typically aggregated to species means to examine general patterns of use of space by animals. However, this ignores the environmental context in which the home range was estimated and does not account for intraspecific variation in home range size. In addition, the focus of macroecological studies on home ranges has historically been biased towards terrestrial mammals. The use of aggregated numbers and the terrestrial focus limit our ability to examine home-range patterns across different environments, their variation in time and variation between different levels of organization. Here, we introduce HomeRange, a global database with 75,611 home-range values across 960 different species of mammals, including terrestrial, aquatic and aerial species.

Main types of variables contained

The dataset contains estimates of home ranges of mammals, species names, methodological information on data collection, method of home-range estimation, period of data collection, study coordinates and name of location, in addition to species traits derived from the studies, such as body mass, life stage, reproductive status and locomotor habit.

Spatial location and grain

The collected data are distributed globally. Across studies, the spatial accuracy varies, with the coarsest resolution being 1°.

Time period and grain

The data represent information published between 1939 and 2022. Across studies, the temporal accuracy varies; some studies report start and end dates specific to the day, whereas for other studies only the month or year is reported.

Major taxa and level of measurement

Mammalian species from 24 of the 27 different taxonomic orders. Home-range estimates range from individual-level values to population-level averages.

Software format

Data are supplied as a comma-delimited text file (.csv) and can be loaded directly into R using the “HomeRange” R package ( https://github.com/SHoeks/HomeRange ).  相似文献   
27.
Newly hatched young of the benthic, coastal-living octopod, Octopus vulgaris , enter the plankton and remain there for perhaps eight weeks. At hatching the arms are short and bear a few, large, primary suckers. The buccal mass is relatively large in proportion to the size of the animal. The eyes are large. The central nervous system has fairly well-defined lobes, some of which develop earlier than others. We shall follow the development of several features of O. vulgaris from hatching, through its life in the plankton until settlement and correlate them with changes in the brain and behaviour.  相似文献   
28.
29.
Summary Crypts of the lingual tonsil were investigated in 10 male and female Macaca fascicularis by use of correlated light and scanning-electron microscopy. Counting of crypt openings provided an estimate of the total number of respective crypto-lymphatic units, which were found to range from 20 to 39. Crypt openings appeared in three distinct morphological varieties, i.e. circular, oval or slit-like. Tonsillar units existed individually or were arranged in a rosary fashion below a slit-like mucosal fold serving as a common exit. Although the crypt epithelium was generally non-keratinized, individual cells showing a surface pattern similar to that of the keratinized cells could be encountered. The crypt epithelium was frequently fragmented and showed heavy mononuclear cell infiltration and surface discontinuities, with lymphoid cells coming in contact with luminal contents. The crypt lumen either appeared as a simple epithelial invagination or existed as a complex, cavernous pouch with many blind-ending diverticula. The lumen contained a mixture of exfoliated epithelial cells, leucocytes and bacteria. The secretory ducts of the posterior lingual glands opened occasionally at various levels into the crypt lumina or independently to the exterior.  相似文献   
30.
Morphological investigations on follicular atresia in canine ovaries   总被引:1,自引:0,他引:1  
Summary Morphologically, canine ovaries show two types of regression pattern in secondary follicles. In type A necrotic changes of the oocytes and the zona pellucida dominate, whereas in type B degeneration, necrobiosis and necrosis of the granulosa prevail. The atretic course of type B regression results in a pseudoantrum and leads, by pseudogrowth, to a structure imitating tertiary follicles. In both type A and type B regression, four consecutive stages of atresia are distinguished by light- and electron microscopy.True tertiary follicles display only one regression pattern which resembles type B of secondary follicles. Early, advanced, late, and terminal stages of atresia are again described.  相似文献   
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