The fact that imprinting is most certain to occur if the opportunity for imprinting is presented to the organism at a very specific age indicates the existence of

a critical period

Explanation:

In developmental psychology and developmental biology, a critical period is a maturational stage in the lifespan of an organism during which the nervous system is especially sensitive to certain environmental stimuli(Wikipedia). It is a time during early postnatal life when the development and maturation of functional properties of the brain, its 'plasticity', is strongly dependent on experience or environmental influences(Sengpiel, 2007).

The fact that imprinting is most certain to occur if the opportunity for imprinting is presented to the organism at a very specific age indicates the existence of a critical period in the life of organisms.

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.

a symbiotic relationship with the mitochondria