DEVELOPMENT. 701 



mals projects, not only into the secondary optic vesicle, but also into the 

 pedicle of the primary optic vesicle invaginating it for some distance 

 from beneath, and thus carrying up the arteria centralis retince into its 

 permanent position in the centre of the optic nerve. 



This invagination of the optic nerve does not occur in birds, and con- 

 sequently no arteria centralis retinae exists in them. But they possess 

 an important permanent relic of the original protrusion of the meso- 

 blast through the choroidal fissure, forming the pecten, while a remnant 

 of the same fissure sometimes occurs in man under the name coloboma 

 iridis. The cavity of the primary optic vesicle becomes completely 

 obliterated, and the rods and cones come into apposition with the pig- 

 ment layer of the retina. The cavity of its pedicle disappears and the 

 solid optic nerve is formed. Meanwhile the cavity which existed in the 

 centre of the primitive lens becomes filled up by the growth of fibres 

 from its posterior wall. The epithelium of the cornea is developed from 



FIG. 486. FIG. 487. 



FIG. 486. Diagrammatic sketch of a vertical longitudinal section through the eyeball of a 

 human foetus of four weeks. The section is a little to the side, so as to avoid passing through the 

 ocular cleft; c, the cuticle where it becomes later the cornea! epithelium; I, the lens; op, optic 

 nerve formed by the pedicle of the primary optic vesicle; vp, primary medullary cavity or optic 

 vesicle; p, the pigment layer of the retina; r, the inner waif forming the retina proper; vs, second- 

 ary optic vesicle containing the rudiment of the vitreous humor, x 100. (Kolliker. ) 



FIG. 487. Transverse vertical section of the eyeball of a human embryo of four weeks. The 

 anterior half of the section is represented: or, the remains of the cavity of the primary optic vesi- 

 cle; p, the inner part of the outer layer forming the retinal pigment; r, the thickened inner part 

 giving rise to the columnar and other structures of the retina; v, the commencing vitreous humor 

 within the secondary optic vesicle; v', the ocular cleft through which the loop of the central blood- 

 vessel, a, projects from below; I, the lens with a central cavity, x 100. (.Kolliker.) 



the epiblast, while the corneal tissue proper is derived from the meso- 

 blast which intervenes between the epiblast and the primitive lens which 

 was originally continuous with it. The sclerotic coat is developed round 

 the eye-ball from the general mesoblast in which it is imbedded. 



The iris is formed rather late, as a circular septum projecting in- 

 wards, from the fore part of the choroid, between the lens and the cor- 

 nea. In the eye of the foetus of Mammalia, the pupil is closed by a 

 delicate membrane, the membrana pupillaris, which forms the front por- 

 tion of a highly vascular membrane that, in the foetus, surrounds the 

 lens, and is named the membrana capsulo-pupillaris (Fig. 488). It is 



