Oology

Eggs of various birds, a [[turtle, various cartilaginous fish, a cuttlefish and various butterflies and moths. (Click on image for key)|right|thumb|link=File:Adolphe_Millot_oeufs-fixed.jpg#Key]]

Oviparous animals are animals that reproduce by depositing unfertilized egg cells or fertilized zygotes outside the body (i.e., by laying or spawning) in metabolically independent incubation organs (eggs), which nurture the embryo into moving offspring (hatchlings) with little or no embryonic development within the mother. This is the reproductive method used by most animal species, as opposed to viviparous animals that develop the embryos internally and metabolically dependent on the maternal circulation, until the mother gives birth to live juveniles.

thumb|alt=Old illustration, showing 23 birds' eggs of different size and colouring|Mid-19th century illustration showing the eggs of a number of bird species

thumb|Mantis ootheca An ootheca (: oothecae ) is a type of egg capsule made by any member of a variety of animal groups such as mollusks, mantises, and cockroaches. It is also referred to by its shortened nickname, "ooth".

The chalaza (; ; : chalazas or chalazae ) is a structure inside bird eggs and plant ovules. It attaches or suspends the yolk or nucellus within the larger structure.

grouping of eggs in a nest

thumb|right|A brooding white tern (Gygis alba). Broodiness is the action or behavioral tendency to sit on a clutch of eggs to incubate them, often requiring the non-expression of many other behaviors including feeding and drinking. Being broody has been defined as "Being in a state of readiness to brood eggs that is characterized by cessation of laying and by marked changes in behavior and physiology". Broodiness is usually associated with female birds, although males of some bird species become broody and some non-avian animals also show broodiness.

class of ways animals produce their young

thumb|Embryo of Zebrafish undergoing cleavage The blastodisc, also called the germinal disc, is the embryo-forming part on the yolk of the egg of an animal that undergoes discoidal meroblastic cleavage. Discoidal cleavage occurs in those animals with a large proportion of yolk in their eggs, and include insects, fish, reptiles and birds. The blastodisc is a small disc of cytoplasm that sits on top of the yolk. In birds, it is a small, circular, white spot (approximately 1.5-3 mm across) on the surface of the yellow yolk of an egg, at the animal pole.