DEVELOPMENT. gH 



cle is almost obliterated. The cells in the anterior wall are much longer 

 than those of the posterior wall ; from the former the retina proper is 

 developed, from the latter the retinal pigment. 



The cup-shaped hollow in which the lens is now lodged is termed 

 the secondary optic vesicle: its walls grow up all round, leaving, how- 

 ever, a slit at the lower part. 



Choroidal Fissure. Through this slit (fig. 506), often termed the 

 choroidal fissure, a process of mesohlast containing numerous blood- 



Fig. 505. Fig. 506. 



Fig. 505. 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 corneal 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 wall forming the retina proper ; 

 vs, secondary optic vesicle containing the rudiment of the vitreous humor. X 100. (Kolliker.) 



Fig. 506. Transverse vertical section of the eyeball of a human embryo of four weeks. The 

 anterior half of the section is represented : pr, the remains of the cavity of the primary optic 

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

 f >art 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.) 



vessels projects, and occupies the cavity of the secondary optic vesicle 

 behind the lens, filling it with vitreous humor and furnishing the lens 

 capsule and the capsulo-pupillary membrane. This' process in mammals 

 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 

 consequently no arteria centralis retinae exists in them. But they pos- 

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

 blast through the choroidal fissure, in the pecten, while a remnant of 

 the same fissure sometimes occurs in man under the name coloboma iri- 

 dis. The cavity of the primary optic vesicle becomes completely obliter- 

 ated, and the rods and cones growing up from the external limiting 

 membrane, get into apposition with the pigment layer of the retina. 



