ORGANS OF SPECIAL SENSE 281 
7. Choroid Fissure, Pecten, and Optic Nerve. The pecten of 
the hen’s eye 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 retinz 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 ceca 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 cca. 
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 
