480 EMBRYOLOGY. 



whereas the inner (ib) remains unpigmented even later and is 

 composed of cylindrical cells. 



Subsequently the ciliary processes become greatly thickened through 

 increase of the very vascular connective-tissue framework, and 

 acquire a firm union with the capsule of the lens through the 

 formation of the zonula Zinnii. In Man the latter is formed, 

 according to KOLLIKER'S account, during the fourth month, in a 

 manner that here, as well as in other Mammals, is still incompletely 

 explained. 



LiEBEEKtrKN remarks that the zonula is distinctly recognisable in eyes 

 which have attained half their definite size. If one takes out of an eye the 

 vitreous body together with the lens, and then removes the latter by opening 

 the capsule on the front side, the margin of the capsule appears surrounded 

 oy blood-vessels which pass from the posterior over on to the anterior surface. 



" At the places where the processus ciliares are entirely removed, tufts of 

 fine fibres are to be seen which correspond to, and fill up, the depressions 

 between the ciliary processes ; but between these tufts is also to be seen a 

 thin layer of the same kind of finely striate masses, which must have lain at 

 the same level as the ciliary processes." Furthermore LiEBERKttKN states 

 that " there lie within this striated tissue numerous cell-bodies of the same 

 appearance as those that are found elsewhere in the embryonic vitreous body 

 at a later period." 



ANGELUCCT believes that the zonula arises from the anterior part of the 

 vitreous body ; at the time when iris and ciliary processes are developed he 

 finds the vitreous body traversed by fine fibres, which extend from the ora 

 serrata to the margin of the lens. He describes as lying between the fibres 

 sparse migratory cells, which are maintained, however, to have no share in 

 the formation of the fibres. 



The fundus of the optic cup (figs. 266, 267, 270) furnishes the 

 most important part of the eye the retina. The inner lamella of 

 the cup (r) becomes greatly thickened, and, in consequence of its 

 cells being elongated into spindles and overlapping one another in 

 several layers, acquires an appearance similar to that of the wall of 

 the embryonic brain. Subsequently it becomes marked off by an 

 indented line, the ora serrata (at the place indicated by a star in 

 fig. 270), from the adjoining attenuated part of the optic vesicle, 

 which furnishes the ciliary folds. It also early acquires at its two 

 surfaces a sharp limitation through the secretion of two delicate 

 membranes : on the side toward the fundament of the vitreous body 

 it is bounded by the membrana limitans interna; on that toward the 

 outer lamella, which becomes pigmented epithelium, by the membrana 

 limitans externa. 



In the course of development its cells, all of which are at first 



