THE EAR. 711 



cells of the lens substance, or as originating from a thin layer of 

 mesoblast, which forms not only the lens capsule, but also the 

 hyaloid membrane, which is continuous with it. 



The optic cup gives origin to the retina. The inner or anterior 

 layer of the cup becomes thickened, and from it are differenti- 

 ated the various layers of the retina, except the layer of pigment 

 cells which lies next to the choroid. The posterior layer de- 

 velops this layer of pigment cells, which, from their intimate 

 connection to the choroid, were formerly considered as part of 

 that membrane. 



The thickening of the inner or anterior layer of the optic cup 

 ceases at the ora serrata. The outer layer with its contiguous 

 choroid is thrown into a number of folds the ciliary processes 

 and, passing in front of the lens, helps to form the iris. 



In front of the ora serrata the anterior layer of the cup is no 

 longer differentiated into the special retinal elements, but joins 

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

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

 passes forward and lines the posterior surface of the iris, form- 

 ing 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 

 mesoblast growing through a fissure in the lower part of the optic 

 cup to fill its cavity. 



DEVELOPMENT OF THE EAR. 



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

 independent involution of which forms both its essential nervous 



