Search results
Results from the WOW.Com Content Network
Most mammals are viviparous, giving birth to live young. [1] However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. [2] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian ...
The egg cell is generally asymmetric, having an animal pole (future ectoderm). It is covered with protective envelopes, with different layers. The first envelope – the one in contact with the membrane of the egg – is made of glycoproteins and is known as the vitelline membrane (zona pellucida in mammals).
The young are born at a very early stage, and can be classified as a "larva" in the biological sense. [18] In placental mammals, the egg itself is void of yolk, but develops an umbilical cord from structures that in reptiles would form the yolk sac. Receiving nutrients from the mother, the fetus completes the development while inside the uterus.
The evolutionary reason for such a change likely was that the advanced and structurally complex brain, characteristic of mammals, requires more time to develop, but the maximum time spent in utero is limited by other factors, such as relative size of the final fetus to the mother (ability of the fetus to pass mother's genital tract to be born ...
The mammals included are only viviparous (marsupials and placentals) as some mammals, which are monotremes (including platypuses and echidnas) lay their eggs. A marsupial has a short gestation period, typically shorter than placental. For more information on how these estimates were ascertained, see Wikipedia's articles on gestational age.
In the case of whales, dolphins and porpoises, the single calf is normally born tail first which minimizes the risk of drowning. [7] The mother encourages the newborn calf to rise to the surface of the water to breathe. [8] Large mammals which give birth to twins is much more rare, but it does occur occasionally even for mammals as large as ...
The egg is not retained in the body for most of the period of development of the embryo within the egg, which is the main distinction between oviparity and ovoviviparity. [1] Oviparity occurs in all birds, most reptiles, some fishes, and most arthropods. Among mammals, monotremes (four species of echidna, and the platypus) are uniquely oviparous.
Mammals reproduce by internal fertilisation [120] and are solely gonochoric (an animal is born with either male or female genitalia, as opposed to hermaphrodites where there is no such schism). [121] Male mammals inseminate females during copulation and ejaculate semen through a penis, which may be contained in a prepuce when not erect.