The fetuses of most mammals are situated similarly to the homo sapiens fetus within their mothers. 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.
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. 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.
Evolution of fetal stage
Many vertebrates have fetal stages, ranging from most mammals to many fish. In addition, some invertebrates bear live young, including some species of onychophoran and many arthropods. The prevalence of convergent evolution to the fetal stage shows that it is relatively easy to develop. Indeed, it presumably originates from a delay of egg release, with the eggs being hatched inside the parent before being laid. Over time, the robustness of the egg wall can be decreased until it becomes little more than a sac.
Etymology and spelling variations
Foetus is a variation of the Latin spelling, and has been in use since at least 1594, according to the Oxford English Dictionary, which describes it as "incorrectly written"; it arose as an erroneous hypercorrection, possibly first made by Saint Isidore of Seville in AD 620. The common English spelling in UK and Commonwealth countries, as well as in some other languages (e.g., French), is foetus. In the United States, in the medical/research community internationally, and in Latin, fetus is agreed-upon as the standard spelling.
Its correct plural is "fetuses", not "feti", as Latin fētus is fourth declension and its Latin plural is fētūs.
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