THE ORGANS OF THE OUTER GERM-LAYER. 483 



f ormed the epithelial lining of the ciliary body ; from the thin portion 

 which surrounds the pupil and which remains smooth, the pigmented 

 epithelium (uvea) of the iris. Consequently there are now to be distin- 

 guished on the secondary optic cup three regions, as retinal, ciliary, 

 and iridal parts. To each of these territories the contiguous 

 connective tissue, and especially the part which becomes the middle 

 tunic of the eye, is adapted in a particular manner ; here it furnishes 

 the connective-tissue plate of the iris with its non-striated muscu- 

 lature, there the connective -tissue framework of the ciliary body 

 with the ciliary muscle, and in the third region the vascular choroidea 

 with the choriocapillaris and lamina fusca. 



In the development of the optic cup there arose on its lower wall 

 a fissure (fig. 265 aus), which marks the place at which the funda- 

 ment of the vitreous body grew into the interior of the cup. What 

 is the ultimate fate of this fissure, which is usually referred to in the 

 literature as choroid fissure 1 



It is for a time easily recognisable, after pigment has been 

 deposited in the outer lamella of the optic cup. It then appears on 

 the lower median side of the eyeball as a clear, unpigmented streak, 

 which reaches forward from the entrance of the optic nerve to the 

 margin of the pupil. 



The name choroid fissure takes its origin from this phenomenon. It was 

 given at a time when the formation of the optic cup was not adequately known, 

 when the pigmented epithelium was still referred to the choroidea. Therefore 

 in the absence of pigment along a clear streak on the under side of the eyeball 

 it was supposed that a defect of the choroidea a choroid fissure had been 

 observed. 



The clear streak afterwards disappears. The fissure of the eye is 

 closed by the fusion of its edges and the deposition of pigment in the 

 raphe. In the Chick this takes place on the ninth day, in Man 

 during the sixth or seventh week. 



In still another respect is the choroid fissure noteworthy. 



In many Vertebrates (Fishes, Reptiles, Birds) a highly vascular 

 process of the choroidea grows through the fissure, before its closure, 

 into the vitreous body and there forms a lamellar projection, which 

 extends from the optic nerve to the lens. In Birds it has received 

 the name "pecten," because it is folded into numerous parallel ridges. 

 It consists almost entirely of the walls of blood-vessels, which are 

 held together by a small amount of a black pigmented connective 

 tissue. 



In Mammals such a growth into the vitreous body is wanting 



