DEVELOPMENT OF THE EYEBALL. 



773 



?, is a pouch of the common integument, which sub- 

 sequently becomes a shut sac, and in which is developed 

 the lens and its capsule (fig. 229), Subsequently there is 

 Fig. 230.* Fig. 231^ 



protruded from below upwards, between the lens in front 

 and the primary optic vesicle behind, another process or 



closed in B and C ; I, the lens follicle and lens ; pr, the primary optic 

 vesicle ; in A and B, the pedicle is shown ; in C, the section heing to the 

 side of the pedicle, the latter is not shown ; v, the secondary ocular 

 vesicle and vitreous humour. 



* Fig. 230. Diagrammatic sketch of a vertical longitudinal section 

 through the eyeball of a human ftetus of four weeks (after Kb'lliker) ^. 

 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 ; I, the 

 lens ; o, p, optic nerve formed by the pedicle of the primary optic vesicle ; 

 v p, primary medullary cavity or optic vesicle ; p, the pigment layer of 

 the choroid coat of the outer wall ; r, the inner wall forming the retina ; 

 v s, secondary optic vesicle containing the rudiment of the vitreous 

 humour. 



f Fig. 231. Transverse vertical section of the eyeball of a human 

 embryo of four weeks (from Kolliker) ~. The anterior half of the 

 section is represented : p, r y the remains ot the cavity of the primary 

 optic vesicle ; j9, the inner part of the outer layer forming the choroidal 

 pigment; r, the thickened inner part giving rise to the columnar and 

 other structures of the retina ; v, the commencing vitreous humour 

 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. 



