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A fetus (sometimes spelled foetus) is a stage in the development of viviparous organisms. This stage lies between the embryonic stage and birth. The fetuses of most mammals are situated similarly to the ''homo sapiens'' fetus within their mothers.〔ZFIN, (Pharyngula Period (24-48 h) ). Modified from: Kimmel et al., 1995. Developmental Dynamics 203:253-310. Downloaded 5 March 2007.〕 However, the anatomy of the area surrounding a fetus is different in litter-bearing animals compared to humans: each fetus of a litter-bearing animal is surrounded by placental tissue and is lodged along one of two long uteri instead of the single uterus found in a human female. Development at birth varies considerably among animals, and even among mammals. Altricial species are relatively helpless at birth and require considerable parental care and protection. In contrast, precocial animals are born with open eyes, have hair or down, have large brains, and are immediately mobile and somewhat able to flee from, or defend themselves against, predators. Primates are precocial at birth, with the exception of humans.〔Lewin, Roger. (Human Evolution ), page 78 (Blackwell 2004).〕 The duration of gestation in placental mammals (i.e. mammals other than monotremes and marsupials) varies from 18 days in jumping mice to 23 months in elephants.〔Sumich, James and Dudley, Gordon. (Laboratory and Field Investigations in Marine Life ), page 320 (Jones & Bartlett 2008).〕 Generally speaking, fetuses of larger land mammals require longer gestation periods.〔 ==Benefits of a fetal stage== A fetal stage means that young are more developed when they are born. Therefore, they may need less parental care and may be better able to fend for themselves. However, carrying fetuses exerts costs on the mother, who must take on extra food to fuel the growth of her offspring, and whose mobility and comfort may be affected (especially toward the end of the fetal stage). In some instances, the presence of a fetal stage may allow organisms to time the birth of their offspring to a favorable season.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fetus (biology)」の詳細全文を読む スポンサード リンク
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