412 



THE CHOROID FISSURE. 



loop is the only structure passing through the choroid slit. On this day 

 however a new structure appears, which remains permanently through life, 

 and is known as the pecten. It consists of a lamellar process of the 

 inesoblast 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 pecteii is 

 somewhat swollen, and sections through this part have a club-shaped torm. 

 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 one is placed near the ciliary edge of the slit, and is traversed by 

 the efferent branch of the above-mentioned vascular loop. This vessel soon 

 atrophies, and with it the distal opening in the choroid slit completely 

 vanishes. In some varieties of domestic Fowl (Lieberkiihn) the opening 

 however persists. The seam which marks the original site of the choroid 

 slit is at first conspicuous by the absence of pigment, and at a later period 

 by the deep colour of its pigment. Finally, a little after the ninth day, no 

 trace of it is to be seen. 



Up to the eighth day the pecten remains as a simple lamina; by 

 the tenth or twelfth day it begins to be folded or rather puckered, and 



by the seventeenth or eighteenth 

 fb day it is richly pigmeiited and 



thepuckerings have become nearly 

 as numerous as in the adult, there 

 being in all seventeen or eighteen. 

 The pecten is almost entirely com- 

 posed of vascular coils, which are 

 supported by a sparse pigmeiited 

 connective tissue ; and in the adult 

 the pecten is still extremely 

 vascular. The original artery 

 which became enveloped at the 

 formation of the pecten continues, 

 when the latter becomes vascular, 

 to supply it with blood. The 

 vein is practically a fresh de- 

 velopment after the atrophy of 

 the clibtal portion of the primitive 

 vascular loop of the vitreous 

 humour. 



There are no true retinal 

 blood-vessels. 



In the formation of the optic 

 cup the extreme peripheral part of 



FIG. 293. SECTION THROUGH THE FRONT 

 PART OF THK HEAD OF A LiEPIDOSTEUS EMBRYO 

 ON THE SEVENTH DAY AFTER IMPREGNATION. 



al. alimentary tract; fb. tlialamenceplia- 

 lon ; Z. lens of eye ; op.v. optic vesicle. The 

 mesoblast is not represented. 



the optic nerve, which is in immediate proximity with the artery of the 

 pecten, becomes folded. The permanent opening in the choroid fissure for 

 the pecteii is intimately related to the entrance of the optic nerve into the 

 eyeball ; the fibres of the optic nerve passing in at the inner border of the 

 pecten, coursing along its sides to its outer border, and radiating from it as 

 from a centre to all parts of the retina. 



