File talk:Streeter1906 fig04.jpg

Diagram representing the growth and stages of differentiation of the human membranous labyrinth. At 3.5 weeks (6-7 mm.) the ear vesicle consists of two simple pouches, into the upper of which opens the endolymphatic appendage. At 4 weeks (9 mm.) there is at the base of the vestibular pouch an atrium, the space destined to form the utricle and saccule. At 5 weeks (12 mm.) this space is circumscribed from the cochlear pouch below by a constriction corresponding to the ductus reuniens, and above from the rest of the vestibular pouch by the formation of the semicircular canals. At 6 weeks (20 mm.) an ingrowth of the wall of the atrium divides it into an upper part (utricle) and lower part (saccule). At 10 weeks (30 mm.) this partition between the utricle and saccule is complete and extends inward in such a way as to split the orifice of the endolymphatic duct.