VI. J THE CHOROID FISSURE. 147 



to be complete. The fibres arise in the remainder of 

 the nerves somewhat later. At first the optic nerve 

 is equally continuous with both walls of the optic cup ; 

 as must of necessity be the case, since the interval 

 which primarily exists between the two walls is con- 

 tinuous with the cavity of the stalk. When the cavity 

 within the optic nerve vanishes, and the fibres of the 

 optic nerve appear, aU connection between the outer 

 wall of the optic cup and the optic nerve disappears, 

 and the optic nerve simply perforates the outer wall, 

 remaining continuous with the inner one. 



The choroid fissure. During the third day of incu- 

 bation there passes in through the choroid slit a vas- 

 cular loop, which no doubt supplies the transuded 

 material for the growth of the vitreous humour. Up to 

 the fifth day this vascular loop is the only structure 

 passing through the choroid slit. On this day however 

 a new structure appears, which remains permanently 

 through Hfe, and is known as the pecten. It consists 

 of a lamellar process of the mesoblast cells round the 

 eye, passing through the choroid slit near the optic 

 nerve, and enveloping part of the afferent branch of 

 the vascular loop above mentioned. The proximal part 

 of the free edge of the pecten is somewhat swollen, and 

 sections through this part have a club-shaped form. 

 On the sixth day the choroid slit becomes rapidly 

 closed, so that at the end of the sixth day it is reduced 

 to a mere seam. There are however two parts of this 

 seam where the edges of the optic cup have not 

 coalesced. The proximal of these adjoins the optic 

 nerve, and permits the passage of the pecten, and at a 

 later period of the optic nerve ; and the second or distal 



10—2 



