Final answer:
Oxytocin is the hormone that stimulates the alveoli of the breasts to eject milk into the ducts.
Step-by-step explanation:
The hormone that stimulates the alveoli of the breasts to eject milk into the ducts is oxytocin. Oxytocin is released from the pituitary gland in response to suckling stimulation and triggers the contraction of the muscle cells surrounding the alveoli, causing the milk to be ejected into the ducts. This process is known as the let-down reflex.
Learn more about Hormones and breast milk production