yolk-sac

Image: Gray459. png | Human embryo of about fourteen days, with yolk-sac.

During development, metatherians produce a yolk-sac placenta and give birth to " larval-like " offspring.

Image: Gray26. png | Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac.

Authors may regard the two terms as synonymous, or equate ovoviviparity only with aplacental yolk-sac viviparity, in which the embryos are solely sustained by yolk ( as opposed to secondary provisioning by their mother in the form of " uterine milk ", such as in the stingrays, or unfertilized eggs, such as in the mackerel sharks ; the latter is referred to as intrauterine oophagy ).

Initially it exists between the neural tube and the endoderm of the yolk-sac, but soon becomes separated from them by the mesoderm, which grows medially and surrounds it.

It is a remnant of the connection from the yolk-sac to the small intestine present during embryonic development.

The shortfin mako shark is a yolk-sac ovoviviparous shark, giving birth to live young.

Image: Gray26. png | Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac.

Image: Gray26. png | Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac.