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With closure of the prosencephalic part of the neural tube, the optic groove evaginates to form the optic vesicle. The vesicle is attached to the rostral part of the diencephalon by the optic stalk. The lumen within the vesicle and stalk is continuous with the prosocoele.

The floor of the diencephalon where the optic stalks join is called the chiasmal part of the hypothalamus.

Ectoderm adjacent to the optic vesicle thickens to form the lens placode.



Placode VIII (otic) invaginates to produce the hollow, egg shaped otocyst from which the membranous labyrinth of the internal ear develops.

The acoustic part (VIII) of the facioacoustic neural crest terminates on the rostral surface of the otocyst.


The dorsal part of the first pharyngeal pouch comes into contact with ectoderm lining the dorsal part of the first branchial groove. The area of contact is the closing membrane or primitive tympanic membrane.

The auditory tube is derived from the proximal part of the first pharyngeal pouch, whereas the distal (lateral) part becomes the primitive middle ear cavity.


The external auditory meatus arises from the dorsal part of the first branchial groove.

The auricle develops from the tissue surrounding the groove in the first and second branchial arches.

Source: Atlas of Human Embryos.