DEVELOPMENT OF THE EYE 



1081 



From the optic cup is formed the whole of the retinal or nervous tunic. It will be noticed 

 that this tunic is composed of two layers, with a narrow sht-like interval between them, but that 

 the layers are continuous with one another at the margin of the cup. This margin is afterward 

 found, in the fully developed eye, at the pupillary margin of the iris. The outer investing layer 

 forms the pigment layer, and the inner inverted layer gives rise to the other parts of the retina, 

 viz., the pars optica, over the bottom of the cup, the pars ciharis, in the ciliary region, and the 

 pars iridica, near the margin of the original cup, including the dilatator and sphincter pupillae 

 muscles of the iris. 



The lens is formed as a hollow invagination from the surface epithelium, which sinks into 

 the hoUow of the optic cup. The margins meet and fuse, enclosing a cavity, and the lens mass 

 sinking more deeply in, loses its connection with the surface, and a layer of mesoderm passes 

 in between them. 



The anlages of the lens and the primitive retina are at first in contact with one another. 

 They gradually draw apart, and the intervening space is filled by the vitreous humour. The 

 origin of the vitreous humour is not yet fully understood, but it appears to be developed from the 

 adjacent ectoderm of the optic cup, and in part from the surrounding mesoderm. 



Figs. 



823, 824, 825 and 826. — Sections Representing Four Successh^e Stages in the 

 Origin of the' Optic Vesicle and the Development of the Eyeball. 



.-V 



-i--^:^"-'^:; 



The optic cup and the lens are surrounded by mesoderm and from it are formed the struc- 

 tures of the tunica vasculosa (middle coat) in its different parts, viz., chorioid, ciliary body 

 and iris, and also the sclera and cornea (fibrous portion). 



fill J^^- t^^^^J^^ chamber is formed by cleavage of the mesoderm, a space appearing in it, 

 mied with fluid. The mesoderm surrounding this space forms the endothelium lining the 

 anterior chamber. The mesoderm also forms a vascular covering for the front of the lens, 

 termed the capsula vasculosa lentis, or pupillary membrane, which disappears from the sur- 

 lace of the lens in the later months of development. 



The eyelids and conjunctiva are formed from the integumentary covering of the eye. 



Ihe former are mostly skin folds, which, at first separate, meet and fuse with one another along 



l'^ ^^'"g^"- Subsequently they become undermined by the ingrowth of epithelium from a 



central horizontal sht, the rima palpebrarum; the central part of the invading epithelium breaks 



down, and the free folds are formed. 



The lacrimal gland is developed from a series of tubular outgrowths from the conjunctival sac. 



