ORGANS OF SPECIAL SENSE 281 



7. Choroid Fissure, Peclen, and Optic Nerve. The pecten of 

 the hen's e3^e is a pigmented vascular plate inserted in the depres- 

 sion occupying the center of the elongated blind spot, or entrance 

 of the optic nerve, which extends meridionally from the fundus 

 nearly to the ora serrata. The pecten projects a considerable 

 distance into the posterior chamber and its free edge is much 

 longer than its base, being consequently folded like a fan; hence 

 the name. The optic nerve runs along the base of the pecten, 

 its fibers passing off on either side into the retina; thus it con- 

 tinually diminishes in size until it disappears. The pecten is 

 consequently separated from the choroid coat by the optic nerve. 

 It is supposed to function as a nutrient organ for the layers of 

 the retina, by means of lymph channels that pass off from its 

 base into the retina. There is no arteria centralis retinae in the 

 bird's eye. 



These structures develop in connection with the choroid 

 fissure as follows: On the fourth day the choroid fissure has be- 

 come a very narrow slit, and by the middle of the day its edges 

 are in apposition in the pars cceca of the bulbus. Proximally, 

 however, the meeting of the lips of the fissure is prevented by the 

 mesoblast, in which the basal blood-vessel runs along the entire 

 length of the open portion of the fissure. During the fourth 

 day this blood-vessel enters the posterior chamber with its en- 

 veloping mesenchyme along the entire length of the open portion 

 of the choroid fissure, and forms a low mesenchymal ridge con- 

 nected by a narrow neck of mesenchyme in the fissure with the 

 mesenchyme outside. During the fifth day the ridge becomes 

 higher and keel-shaped, and a thickening appears along part of 

 its free edge above the blood-vessel. During this day also fusion 

 of the lips of the choroid fissure has taken place in the pars cseca. 

 At the same time an important change begins in the proximal 

 portion of the choroid fissure that leads to the formation of the 

 pecten proper. This is an involution of the lips of the optic cup 

 bounding the choroid fissure on each side of the mesodermal 

 keel, and their continuous ingrowth until they meet over the 

 keel and fuse above it in a mass in which the outer and inner 

 layers of the retina are indistinguishably fused. Thus the proxi- 

 mal portion of the mesodermal keel is enclosed in a kind of tunnel 

 composed of the involuted edges of the optic cup. The forma- 

 tion of this tunnel progresses gradually from the fundus towards 



