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A3,A4: Trigeminal ganglion, dorsal end of 1st pharyngeal arch, vestibulocochlear ganglion (rostral to otocyst), superior glossopharyngeal ganglion.
A4: Large head vein lateral to otocyst. Mesencephalon
DEVELOPMENT OF THE NERVOUS SYSTEM
Al-7: Midbrain and hindbrain. The isthmus can be observed on A3-4 as a constriction between the mid- and hind-brain. The hindbrain is already subdivided into two parts, the slim myelencephalon and the wider metencephalon. The boundary between these two parts is roughly at the level of the otic vesicles A3-4.
Development of the Auditory System
A4: Indentation in rostral margin of otocyst = anlage of utriculosaccular canal. Note alignment of superior cardinal vein (LS) in relation to the nerve trunks (XS). Trigeminal ganglion. Dorsal end of 1st pharyngeal arch and groove. Note variation in height of cells of L otocyst wall.
