484 



EMBRYOLOGY. 



The closure of the choroid fissure takes place at an early period and 

 completely. 



Occasionally in Man the normal course of development is inter- 

 rupted, so that the margins of the choroid fissure remain apart. The 

 usual consequence of this is a defective development of the vascular 

 tunic of the eye at the corresponding place an indication of the 

 extent to which the development of the connective-tissue envelope is 

 dependent on the formative processes of the two epithelial layers, as 

 has already been stated. Both retinal and choroidal pigment are 

 therefore wanting along a streak which begins at the optic nerve, so 

 that the white sclera of the eye shows through to the inside and can 

 be recognised in examinations with the ophthalmoscope. When the 

 defect reaches forward to the margin of the pupil, a fissure is formed 

 in the iris which is easily recognised upon external observation of the 

 eye. The two structures resulting from this interrupted develop- 

 ment are distinguished from each other as choroidal and iridal fissures 

 (coloboma choroideaB and coloboma iridis). 



(d) The Development of the Optic Nerve. 



The stalk of the optic vesicle (fig. 272), by which the vesicle is 

 united with the between-brain, is in direct connection with both 

 lamellse of the optic cup, the primary 

 optic vesicle having been infolded from 

 below by the fundament of the vitreous 

 body to form the cup. Its dorsal wall 

 is continuous with the outer lamella or 

 pigment-epithelium of the retina ; its 

 ventral wall is prolonged into the inner 

 lamella, which becomes the retina. 

 Thus, aside from the formation of the 

 vitreous body, the development of a 

 choroid fissure also has a significance 

 in view of the persistence of the direct 

 connection between retina and optic 

 nerve. For if we conceive the optic 

 vesicle invaginated merely at its an- 

 terior face by the lens, the wall of the 

 optic nerve would be continued into 

 the outer, uninvaginated lamella only; direct connection with the-- 

 retina itself, or the invaginated part, would be wanting. 



Fig. 272. Plastic representation of 

 the optic cup with lens and 

 vitreous body. 



ab, Outer wall of the cup ; ib, its 

 inner wall ; h, space between the 

 two walls, which afterwards en- 

 tirely disappears ; Sn, fundament 

 of the optic nerve (stalk of the 

 optic vesicle with groove-for- 

 mation along its lower face) ; 

 aus, choroid fissure ; gl, vitreous 

 body ; I, lens. 



