848 



DEVELOPMENT. 



[en. LVIII. 



hollow ball the crystalline lens is developed. The way in which 

 this occurs has been described in a previous chapter under the 

 head of structure of the lens (see p. 747). By the in-growth of 

 the lens the anterior wall of the primary optic vesicle is forced 

 back nearly into contact with the posterior, and thus the primary 

 optic vesicle is almost obliterated. The cells in the anterior wall 

 are much longer than those of the posterior wall; from the 



Fig. 667. 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 epithe- 

 lium ; I, the lens ; op, optic nerve formed 

 by the pedicle of the primary optic vesicle ; 

 vp, primary medullary cavity or optic vesi- 

 cle; p, the pigment layer of the retina; 

 r, the inner wall forming the remaining 

 layers of the retina ; vs, secondary optic 

 vesicle containing the rudiment of the 

 vitreous humour, x 100. (Kolliker.) 



Fig. 668. 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 ; p, the inner 

 part of the outer layer forming the 

 retinal pigment ; r, the thickened 

 inner part giving rise to the 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, x 100. 

 (Kolliker.) 



former all the layers of the retina are developed, except the layer 

 of pigment cells which is formed from the latter. 



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

 termed the secondary optic vesicle : its walls grow up all round, 

 leaving, however, a slit below where it meets the lens. 



Through this slit, termed the choroidal fissure, a process 

 of mesoblast containing numerous blood-vessels projects, and 

 occupies the cavity of the secondary optic vesicle behind the lens, 

 filling it with vitreous humour 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 

 retinae into its permanent position in the centre of the optic nerve. 



