DEVELOPMENT OF THE EYEBALL. 



vesicle, 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. 236). Subsequently there is 



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 being to the 

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

 vesicle and vitreous humour. 



* Fig. 236. Diagrammatic sketch of a vertical longitudinal section 

 through the eyeball of a human fo3tus of four weeks (after Kblliker) ~. 

 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 ; ?, the 

 lens ; 0,%), optic nerve formed by the pedicle of the primary optic vesicle ; 

 vp, 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. 237. Transverse vertical section of the eyeball of a human 

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

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

 optic vesicle ; p, 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 secondaiy optic vesicle ; tf t the ocular cleft through which 

 the loop of the central blood-vessel, a, projects from below ; I, the lens 

 with a central cavity. 



3 D 2 



