Development of the Third Day 179 



the optic stalk the lips do not increase in 

 height, and a cleft or fissure, the choroid 

 fissure (Fig. 64, O H), is left at that place. 

 The exact method of formation of the cho- 

 roid fissure is not easy to determine. The 

 invagination of the optic vesicle to form the 

 optic cup may be partly caused by the me- 

 chanical pushing inward of the lens, but it is 

 probably also caused by the unequal growth 

 of the walls of the vesicle. In a similar man- 

 ner the unequal growth of the lips of the cup 

 may cause the formation of the choroid fissure ; 

 but there is some evidence to show that the 

 fissure may be, in part, the result of the growth 

 of the fibres of the optic nerve. The choroid 

 fissure is a very transient structure ; by the 

 sixth day its edges have met, and shortly after- 

 wards they fuse, so that by the ninth day no 

 trace of the fissure remains. 



The inner wall of the cup, which is the 

 thicker almost from the first, by the third day 

 consists of elongated, nucleated cells arranged 

 in a single row perpendicular to the surface of 

 the cup. The thickness of this inner wall con- 

 tinues to increase, and by a process of histo- 

 logical differentiation that is difficult to follow, 

 it is converted into the retina. The outer 



