CH. LVIII.J 



DEVELOPMENT OF THE EYE. 



849 



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

 and consequently no arteria centralis retinae exists in them. But 

 they possess an important permanent relic of the original protru- 

 sion of the mesoblast through the choroidal fissure, in 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 

 get into apposition with the pigment layer of the retina. The 

 inner segments of the rods are the first formed, then the outer. 

 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 



Fig. 6c9. Blood-vetutela of the capsulo-pupillary membrane of a new-bom kitfc 

 magnified. The drawinm is taken from a preparation injected by Tiersch, and sho 

 in the central part the convergence of the net-work of vessels in the pupilla 



new-bom kitten, 



IW8 



ipillary 



posterior wall. The epithelium of the cornea is developed from 

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

 mesoblast which intervenes between the epiblast and the primi- 

 tive lens which was originally continuous with it. The sclerotic 

 coat is developed round the eye-ball from the general mesoblast in 

 which it is embedded. The choroid is developed from the meso- 

 blast on the outside of the optic cup, and the iris by the growing 

 forwards of the anterior edge of the optic cup. The ciliary 

 processes arise from the hypertrophy of the edge of the optic 

 cup which forms folds into which the choroidal mesoblast grows, 

 nd in which blood-vessels and pigment-cells develop. 



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

 inwards, from the fore part of the choroid, between the lens and 



K.P. i 



