Most
mammals are
viviparous
Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...
, giving birth to live young. However, the five species of monotreme, the
platypus
The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or monot ...
es and the
echidna
Echidnas (), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the family Tachyglossidae . The four extant species of echidnas and the platypus are the only living mammals that lay eggs and t ...
s, lay eggs. The monotremes have a
sex determination system different from that of most other mammals. In particular, the
sex chromosome
A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical ...
s of a platypus are more like those of a chicken than those of a therian mammal.
The
mammary gland
A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in pr ...
s of mammals are specialized to produce milk, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the
teat
A teat is the projection from the mammary glands of mammals from which milk flows or is ejected for the purpose of feeding young. In many mammals the teat projects from the udder. The number of teats varies by mammalian species and often corr ...
s seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.
Viviparous mammals are in the subclass
Theria
Theria (; Greek: , wild beast