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.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.