ORGANS OF SPECIAL SENSE 275 
ment layer of the retina. About the middle of the fourth day 
pigment begins to develop in the outer layer and extends through- 
out it, even to the distal portion of the optic-stalk at first (Ucke, 
91). The histogenesis of the retina of the chick has been de- 
scribed by Weysse (1906). 
2. The Vitreous Humor (Corpus Vitreum). Until compara- 
tively recently embryologists have adhered to the view stated 
by Schoeler (1848) and Ixolliker (1861) that the vitreous body 
arises from mesenchymal cells that enter the eyeball through 
the choroid fissure. The fact that the embryonic vitreous humor 
of birds is almost entirely devoid of cells was a serious difficulty. 
The cells are in fact so scanty as to be absent in many entire 
sections. Moreover, in character they resemble embryonic 
blood-cells and not mesenchyme, and disappear entirely by the 
eighth day. It seems impossible that they should play any 
important part in the origin of the massive vitreous body. Re- 
searches of the last few years have demonstrated that the vitreous 
body is primarily of ectodermal origin, its fibers arising as processes 
of cells of the inner layer of the optic cup and the matrix as 
secretion. According to some the cells of the lens are responsible 
wholly (Lenhossek) or in part (Szili) for the fibers; this view, 
however, has been strongly combatted (Ix6lliker and Rabl) and 
requires further evidence to substantiate it. 
Both retinal and czecal parts of the cup take part in the forma- 
tion of the fibers of the vitreous body; the retinal part is at first 
the most important, and the primary vitreous body is almost 
entirely retinal in its origin. But after the cxeal part is differ- 
entiated the activity of the retinal part becomes less, and the 
greater part of the fibers of the vitreous body appears to be 
formed from cells of the cawcal part, that send out branching 
and anastomosing processes into the posterior chamber. There 
is no sharp boundary between the fibers that form the vitreous 
body and those that form the zonula; and the fibers of the latter 
may be regarded as homologous to those of the former. The 
matrix of the embryonic vitreous body may be regarded as a 
secretion of the walls of the optic cup. Later, the secretion 
appears to be confined to the ciliary processes. It is possible 
that the mesenchyme plays some part in the formation of the 
vitreous body after the formation of the pecten begins; but there 
is no evidence that it does so at first. 
