THE EAR. 715 



longer differentiated into the special retinal elements, but joins 

 with the posterior to form a layer of columnar cells, the pars 

 ciliaris retinae. In front of this the anterior rim of the optic cup 

 passes forwards and lines the posterior surface of the iris, forming 

 the uvea of that organ, and terminates at the margin of the pupil. 



The rest of the substance of the iris is developed from the 

 mesoblast, from which also arise the choroid, the cornea, and the 

 sclerotic. 



The development of the eye may be thus briefly described. 

 An offshoot of nervous matter from the fore-brain forms the 

 retina and the uvea, and its stalk, or connection with the brain, 

 develops into the optic nerve. 



An involution of epiblast which grows into the nervous cup 

 forms the lens, whilst from the adjacent mesoblast the surround- 

 ing parts of the eye arise. The vitreous is produced by the meso- 

 blast growing through a fissure in the lower part of the optic cup 

 to fill its cavity. 



DEVELOPMENT OP THE EAR. 



The ear is developed chiefly from the epiblast, a special and 

 independent involution of which forms both its essential nervous 

 structures, and the general epithelium lining the membranous 

 labyrinth. The mesoblast supplies the surrounding firmer struc- 

 tures, such as the fibrous substance of the inner ear, and the bony 

 parts in which the organ lies. The auditory nerve grows as a 

 bud from the neural tissue forming the hind-brain, and connects 

 it to the delicate specialized auditory cells. 



The process begins by the appearance of a depression of the 

 general epiblast covering the head, which soon forms a tubular 

 diverticulum, lying in the mesoblast at the side of the hind-brain. 



This diverticulum becomes separated from the epiblast by the 

 obliteration of its outer extremity, which united it to the super- 

 ficial epiblast, and is then converted into a cavity and receives 

 the name of the otic vesicle. It soon becomes somewhat triangular 

 in shape, the base of the triangle lying upwards. 



From the lower angle arises a projection, which is the rudi- 



