The hypophysis, or pituitary gland, develops from two completely different parts.
An ectodermal outpocketing of the stomodeum immediately in front of the buccopharyngeal membrane known as Rathke's pouch.
A downward extension of the diencephalon, the infundibulum. When the embryo is approximately 3 weeks old, Rathke's pouch appears as an evagination of the oral cavity and subsequently grows dorsally toward the infundibulum.
By the end of the second month, it loses its connection with the oral cavity and is then in close contact with the infundibulum.
During further development, cells in the anterior wall of Rathke's pouch increase rapidly in number and form the anterior lobe of the hypophysis, or adenohypophysis.
A small extension of this lobe, the Pars tuberalis, grows along the stalk of the infundibulum and eventually surrounds it.
The posterior wall of Rathke's pouch develops into the pars intermedia, which in humans seems to have little significance.
The infundibulum gives rise to the stalk, or posterior lobe of the hypophysis (neurohypophysis).
