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The class Mammalia is divided into two subclasses based on reproductive techniques: egg-laying mammals (yinotherians or monotremes - see also Australosphenida), and mammals which give live birth . The latter subclass is divided into two infraclasses: pouched mammals ( metatherians or marsupials ), and placental mammals ( eutherians , for which ...
Monotremes (/ ˈ m ɒ n ə t r iː m z /) are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified by structural differences in their brains, jaws, digestive tract ...
Among mammals, monotremes (four species of echidna, and the platypus) are uniquely oviparous. In all but special cases of both ovuliparity and oviparity, the overwhelming source of nourishment for the embryo is the nutrients stored in the yolk, pre-deposited in the egg by the reproductive system of the mother (the vitellogenesis).
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 ...
Oviparity, as in most invertebrates and reptiles, monotremes, dinosaurs and all birds which lay eggs that continue to develop after being laid, and hatch later. [19] Viviparity, as in almost all mammals (such as whales, kangaroos and humans) which bear their young live. The developing young spend proportionately more time within the female's ...
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.
Oviparity: fertilisation is internal, but the female lays zygotes as eggs with a substantial quantity of yolk to feed the embryo while it remains in the egg. The egg is not retained in the body, or only for a limited time. [1] Oviparity is found in insects, birds. Among mammals, the monotremes are oviparous.
Monotremes are the only mammals (apart from the Guiana dolphin) [55] known to have a sense of electroreception, and the platypus's electroreception is the most sensitive of any monotreme. [ 56 ] [ 54 ] Feeding by neither sight nor smell, [ 57 ] the platypus closes its eyes, ears, and nose when it dives. [ 58 ]