12 THE EMBRYOLOGY OF THE EYE. 



lining of the labyrinth — the modified sensory 

 epithelium forming the percipient elements 

 of the organ of hearing. The auditory nerve 

 pushes out to it from the brain at a later period. 

 The neural epiblast undergoes a thickening 

 in its entire extent and forms the brain and 

 cord, the cavity remaining as the cerebral ven- 

 tricles and the spinal canal The first vesicle 

 forms the cerebral hemispheres, the corpus 

 callosum, lateral ventricles, etc.; the second 

 (thalamencephalon) gives rise to the retina 

 and optic nerves by means of its prolongation, 

 the optic vesicle, while its lateral walls thicken 

 to form the optic thalami, its roof to form the 

 pineal gland, its floor to form the pituitary 

 body, and the cavity remains as the third ven- 

 tricle. The early embryonic relations under- 

 go a considerable change, however, and the 

 optic tracts later connect the eye with the third 

 cerebral vesicle (mesencephalon), from which 

 arise the corpora quadrigemina. The fourth 

 vesicle gives rise to the cerebellum and pons, 

 and the fifth to the medulla oblongata. 



