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   763篇
  免费   65篇
  国内免费   198篇
  1026篇
  2024年   9篇
  2023年   9篇
  2022年   14篇
  2021年   25篇
  2020年   16篇
  2019年   23篇
  2018年   16篇
  2017年   17篇
  2016年   22篇
  2015年   28篇
  2014年   34篇
  2013年   35篇
  2012年   72篇
  2011年   61篇
  2010年   53篇
  2009年   47篇
  2008年   50篇
  2007年   46篇
  2006年   36篇
  2005年   33篇
  2004年   26篇
  2003年   29篇
  2002年   37篇
  2001年   38篇
  2000年   21篇
  1999年   25篇
  1998年   18篇
  1997年   6篇
  1996年   7篇
  1995年   5篇
  1994年   9篇
  1993年   12篇
  1992年   11篇
  1991年   9篇
  1990年   6篇
  1989年   17篇
  1988年   9篇
  1987年   8篇
  1986年   8篇
  1985年   8篇
  1984年   11篇
  1983年   7篇
  1982年   9篇
  1981年   7篇
  1976年   4篇
  1974年   4篇
  1973年   3篇
  1972年   3篇
  1967年   3篇
  1956年   3篇
排序方式: 共有1026条查询结果,搜索用时 15 毫秒
1.
2.
Consort intrusion behavior has been proposed to largely function as a means of preventing or suppressing the reproductive activity of subordinate males. In the Japanese macaques of Arashiyama B troop this behavior was found to be ineffective in influencing female mate choice and affecting differential reproduction. Mainly directed towards the female of the pair intrusion resembled sexual solicitation and appeared to be an extension of consort behavior. Although the intruder male was almost always dominant over the other male, intrusion failed to provide priority of access to the estrous female as she most frequently chose to remain with her current partner. Copulations which were intruded upon were as likely to end in ejaculation as those which were undisturbed. Only a small proportion of intruder male-female pairs mated. Pairs that did mate were likely to have been in consort before or during the period in which intrusion occurred. No significant correlation was found between male rank order and number of possible females impregnated. Individual young, middle to lower ranking males were estimated to have been reproductively the most successful.  相似文献   
3.
Metacercariae of Sphaeridiotrema globulus (Trematoda) were obtained from naturally infected Goniobasis virginica (Pleuroceridae). Excysted metacercariae were placed individually on blood-agar plates. After 8 hr of incubation at 42 C the plates exhibited beta-hemolysis. The reaction zone was approximately 1.5 mm in diameter. Laboratory-reared mallard ducks (Anas platyrhynchos) were infected with S. globulus. Mallards developed fatal sphaeridiotremiasis and demonstrated increased prothrombin time on days 3 and 6 postinfection.  相似文献   
4.
The feasibility of radioisotope-fueled circulatory support systems depends on the ability of the body to dissipate the reject heat from the power source driving the blood pump as well as to tolerate chronic intracorporeal radiation. Our studies have focused on the use of the circulating blood as a heat sink. Initial in vivo heat transfer studies utilized straight tube heat exchangers (electrically and radioisotope energized) to replace a segment of the descending aorta. More recent studies have used a left ventricular assist pump as a blood-cooled heat exchanger. This approach minimizes trauma, does not increase the area of prosthetic interface with the blood, and minimizes system volume. Heat rejected from the thermal engine (vapor or gas cycle) is transported from the nuclear power source in the abdomen to the pump in the thoracic cavity via hydraulic lines. Adjacent tissue is protected from the fuel capsule temperature (900 to 1200 degrees F) by vacuum foil insulation and polyurethane foam. The in vivo thermal management problems have been studied using a simulated thermal system (STS) which approximates the heat rejection and thermal transport mechanisms of the nuclear circulatory support systems under development by NHLI. Electric heaters simulate the reject heat from the thermal engines. These studies have been essential in establishing the location, suspension, surgical procedures, and postoperative care for implanting prototype nuclear heart assist systems in calves. The pump has a thermal impedance of 0.12 degrees C/watt. Analysis of the STS data in terms of an electrical analog model implies a heat transfer coefficient of 4.7 x 10(-3) watt/cm(2) degrees C in the abdomen compared to a value of 14.9 x 10(-3) watt/cm(2) degrees C from the heat exchanger plenum into the diaphragm.  相似文献   
5.

Research Notes on Avian Biology 1994: Selected Contributions from the 21st International Ornithological CongressMorphology and Physiology: Endocrinology

Subject: Endocrinology and chronobiology  相似文献   
6.
本文报道在湛江市附近海域海鸟体内获得的两种吸虫,经鉴定为新种,命名为巨口类茎吸虫,新种Microphalloides macrostonrs sp.nov.,珊瑚多黄吸虫,新种Multivitellus coralius sp.nov.  相似文献   
7.
Based on a sample of 237 live births recorded over a period of 30 years, a tendency for longer interbirth intervals following the birth of daughters than sons was recognized, in the provisioned Arashiyama troop of Japanese macaques. This may indicate that female infants were more costly to produce than male infants. This tendency seemed to be independent of a mother’s rank.  相似文献   
8.
9.
In order to assess the impact of nutritional status on the onset of menarche and the association between age at menarche and age at marriage, a survey of 1155 girls, ages 10 through 20, was conducted in a rural area of Bangladesh in March 1976. In order to obtain an estimated mean of age of menarche, probit analysis was used. The mean age of menarche using this technique is estimated at 15.65 for Muslims and 15.91 for Hindus. It was learned that in recent years the age of menarche has increased in a rural area. This increase seems to be associated with malnutrition caused by the war, postwar inflation, floods and famines during the 1971-75 period. When age is controlled for, the prominent effect of weight on menstrual status is evident. 98% of the girls whose weights were 88 pounds or greater had reached menarche compared to only 1% of those weighing less than 66 pounds. Body weight appears to be 1 of the most important factors for the determination of onset of menarche. There exists a seasonality of onset of menarche with a peak in winter. Age of marriage among this rural population has increased and may be associated with the increasing age of menarche. Since both age of menarche and age of marriage have increased, fertility among females age 15-19 may be expected to decrease in the future if this pattern continues.  相似文献   
10.
以化学纯饲料饲养北京的桃蚜   总被引:4,自引:0,他引:4  
应用修改后的Dadd和Mitter(1966)全纯饲料配方配制成人工饲料饲养定居在北京温室烟草上的桃蚜 Myzus persicae可完成生活史井连续饲养3代。本文描述饲料配制、饲养和取食量测定的方法。这3代初羽化无翅孤雌胎生雌蚜的平均体重分别为:440±90.7μg,264±104.9μg和312±127.9μg。用放射性同位素稀释法测定取食量的结果得悉若虫期的总取食量每蚜约为1.74μg,相当于1.16μl。  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